A Novel Thin Adipose Compartment at the Colonic Mesentery–Perirenal Fat Interface: Histological and Three-Dimensional Morphological Studies

A Novel Thin Adipose Compartment at the Colonic Mesentery–Perirenal Fat Interface: Histological and Three-Dimensional Morphological Studies | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A Novel Thin Adipose Compartment at the Colonic Mesentery–Perirenal Fat Interface: Histological and Three-Dimensional Morphological Studies Satoru Muro, Atsuhiko Ochi, Sho Mitsumaru, Yuki Tajika, Akimoto Nimura, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7369510/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Objective: To elucidate the anatomical characteristics and three-dimensional continuity of a previously unrecognized thin adipose compartment between the colonic mesentery and retroperitoneum, using correlative microscopy and block-face imaging (CoMBI). Summary Background Data: The interface between the colonic mesentery and retroperitoneum has traditionally been considered a fusion fascia (e.g., Toldt’s fascia). However, emerging evidence suggests more complex fascial remodeling, with previous studies lacking comprehensive spatial context. Methods: Seven adult cadavers were examined. Histological analysis was conducted on six specimens using paraffin sections stained with Elastica van Gieson and Masson’s trichrome. One cadaver underwent three-dimensional morphological analysis using CoMBI. Serial block-face images of the perirenal region were captured at 100 μm intervals, and three-dimensional reconstruction segmentation was performed. Results: A distinct thin adipose compartment (0.3–2.0 mm thick) was consistently observed between the colonic mesentery and perirenal fat, enclosed by dense connective tissue and containing small vessels. Similar compartments were also found between the perirenal fat and pararenal fat, and beneath the peritoneum along the abdominal wall. These compartments extended in three directions from the peritoneal reflection and demonstrated craniocaudal continuity, laterally, these form a triad-like junction. Conclusions: The thin adipose compartment represents a structurally organized anatomical unit rather than amorphous filler. Its consistent continuity and integration with adjacent structures support a compartment-based framework of intra-abdominal anatomy, with implications for surgical navigation. Surgery colonic mesentery retroperitoneum Toldt’s fascia histology three-dimensional reconstruction intra-abdominal anatomy surgical navigation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 INTRODUCTION The anatomical relationship between the colonic mesentery and retroperitoneal structures in the abdomen has been reevaluated in parallel with advances in surgical techniques. Particularly, the ascending and descending mesocolons are thought to fuse with the parietal peritoneum during fetal development, forming a fusion (Toldt’s) fascia, which has traditionally been considered the boundary between the mesocolon and retroperitoneum. 1 This fascial interface is reportedly not directly penetrated by vessels or nerves. Instead, nerves traverse from the retroperitoneal side into the mesocolon, avoiding this layer. 2 Such connective tissue planes are typically avascular and used as natural dissection layers during surgery. 3 Some reports have challenged the concept of Toldt’s fascia as a true fusion fascia, proposing that it represents a retracted extraperitoneal fascia formed during peritoneal remodeling. 4 Regardless of the interpretation, these connective tissue layers serve as critical anatomical interfaces between the colonic mesentery and retroperitoneum, and a precise understanding of their structure is essential for evaluating tumor invasion and ensuring safe surgical dissection. Previous studies examining the interface between the ascending and descending mesocolons and retroperitoneal space primarily relied on histological observations from limited anatomical regions, hindering comprehensive understanding of the spatial continuity and three-dimensional architecture of these structures. Conventional anatomical approaches are insufficient to integrate macroscopic observations across extensive areas with microscopic analysis of fine structural features. To address this limitation, the present study employed correlative microscopy and block-face imaging (CoMBI), a method that enables integration of high-resolution histological and three-dimensional morphological analyses. 5 – 7 Using this method, we aimed to elucidate the structural continuity and spatial characteristics of the adipose compartment located between the colonic mesentery and perirenal fat (PeRF), along with both the ascending and descending colons, in a three-dimensional and systematic manner. In an earlier study, we demonstrated a previously unrecognized thin adipose compartment (TAC) in the region posterior to the kidney and reported its anatomical relationships with the posterior renal fascia and the lateral conal fascia. 8 In the present study, we identified a TAC located anterior to the kidney, between the colonic mesentery and PeRF, and demonstrated its anatomical relationships with the fusion fascia. We aimed to define the anatomical characteristics and spatial extent of this adipose compartment, and to evaluate its continuity with adjacent structures, including the subperitoneal fat and posterior abdominal wall. METHODS Preparation of Cadaveric Specimens Seven adult cadavers (six male and one female; mean age at death, 81.7 [range, 73–92] years) were donated to our department in accordance with the Japanese law entitled The Act on Body Donation for Medical and Dental Education (Act No. 56 of 1983). All donors voluntarily agreed before their deaths that their remains be used for educational and research purposes. Written informed consent was re-confirmed with the bereaved families, and no objections were raised. All cadavers were fixed with 8% formalin via arterial perfusion and preserved in 30% alcohol to maintain tissue integrity and prevent fungal growth. Cadavers with a history of major abdominal or pelvic surgery were excluded. Tissue blocks, including the ascending and descending mesocolons, retroperitoneum, and PeRF, were dissected en bloc for analysis. Histological Analysis Histological analysis was performed on six of the cadavers. In five cadavers, the entire abdomen, including the ascending and descending colons, retroperitoneum, and PeRF, was harvested en bloc, and a single transverse slice was obtained at the level crossing both colonic mesenteries using a diamond band saw (EXAKT 312; EXAKT Technologies, Inc., Germany). Representative tissue blocks containing the interface between the colonic mesentery and PeRF were excised for histological analysis. In the one remaining cadaver, the entire abdominal region was frozen at − 80°C and serially sectioned into 10-mm-thick transverse slices using a band saw (Nakajima Seisakusho, Y.K., Osaka, Japan), based on previous anatomical studies. 9 From these frozen slices, multiple smaller blocks were obtained to represent the craniocaudal extent of the adipose compartments of interest, particularly focusing on the boundary between the colonic mesentery and retroperitoneum as well as the lateral and posterior aspects of the PeRF. All tissue blocks were fixed in 10% neutral-buffered formalin for 24 hours, dehydrated using a graded ethanol series (70%, 80%, 90%, and 100%), cleared in xylene, and embedded in paraffin under negative pressure. The paraffin was replaced three times over 5 days to ensure thorough infiltration. Serial paraffin sections (5 µm thick) were cut using a rotary microtome (RM2235; Leica Biosystems Nussloch GmbH, Wetzlar, Germany) and stained with Elastica van Gieson and Masson’s trichrome staining. Microscopic examination focused on the zonal organization of the adipose compartments and the distribution of small blood vessels, particularly in relation to the mesenteric and perirenal regions. CoMBI One cadaver was used for three-dimensional morphological analysis using the CoMBI method. The right and left perirenal regions were dissected along with parts of the ascending and descending colons, kidneys, and abdominal walls. The specimens were fixed and embedded in opaque paraffin containing 6.25% w/w white crayon. 7 The specimen blocks were sectioned with 5 µm thickness using a rotary-type microtome (RX-860; Yamato Kohki Industrial Co. Ltd., Saitama, Japan), and the block-faces were captured using a digital camera (Nikon D5100; Nikon Corporation, Tokyo, Japan) with a macro lens (Nikon AF-S DX Micro Nikkor 40 mm f/2.8G; Nikon Corporation). The block-face was captured for every 20 sections cut; 207 and 390 serial block-face images of the right and left perirenal regions, respectively, were obtained with an interval of 100 µm. A few sections were obtained during block-face imaging and stained with Masson’s trichrome. Three-Dimensional Morphological Analysis Segmentation and three-dimensional reconstruction were performed using serial block-face images obtained using the CoMBI method. The original three-dimensional dataset of the left perirenal region consisted of 390 serial block-face images with 100-µm intervals. We created the smaller-sized dataset containing 187 block-face images with 200-µm intervals by selecting odd-numbered image files and excluding images of areas outside the region of interest. The images were aligned manually and used for three-dimensional analysis. The adipose compartments and organs within the block-face images were annotated by referring to the correlated sections stained with Masson’s trichrome. Segmentation of these structures was performed using an artificial intelligence-assisted protocol (Seg & Ref; https://github.com/SatoruMuro/SAM2GUIfor3Drecon) . 10 The segments were reconstructed into a three-dimensional surface model using 3D Slicer (version 5.2.2; https://www.slicer.org/) . 11 Adipose tissue is soft and extends over a wide area, hindering analysis of the entire morphology using sections and conventional microscopy. We were interested in the adipose tissue compartments and their relationship with the colon and abdominal wall. Thus, we performed block-face imaging for three-dimensional morphological analysis of perirenal adipose tissues and examined the correlation between block-face images and partial sections stained with Masson’s trichrome to annotate adipose tissues and organs. Ethical Approval This study was approved by the Human Subjects Research Ethics Review Committee of the Institute of Science, Tokyo, Japan (approval number M2019-075). All methods were performed in accordance with relevant guidelines and regulations. Usage of Generative AI and AI-Assisted Technologies During the preparation for this study, ChatGPT was used to improve the clarity and grammatical usage of English. After using these services, the authors reviewed and edited the content as required and take full responsibility for the content of the published article. RESULTS We identified the thin compartment of adipose tissues between the ascending and descending colonic mesenteries and PeRF and named it “thin adipose compartment (TAC).” The colon was clearly identified in the transverse section of the abdomen (Fig. 1A). The anterior surfaces of these colonic segments were covered by the peritoneum, which folded laterally at the colonic margins and continued toward the abdominal wall (Fig. 1B, C). Histological examination of the interface between the mesenteric and retroperitoneal adipose tissue revealed a TAC approximately 0.3–1.5 mm thick situated between them (Fig. 1D, E). This compartment was enclosed anteriorly and posteriorly by dense connective tissue, forming a discrete unit distinguishable from both the mesenteric and retroperitoneal fat. Small blood vessels were observed within the thin adipose compartment (Fig. 1F). The presence of the TAC was confirmed at different transverse levels, including the kidney. Tissue samples were collected from the lateral, anterior, and posterior areas of the perirenal adipose tissue from a transverse section of the abdomen at the kidney level (Fig. 2A). Histological analysis was performed in the lateral area near the colon and peritoneal reflection (Fig. 2B), anterior area including the interface between the mesenteric and retroperitoneal adipose tissue (Fig. 2C), and posterior area between the kidney and body wall (Fig. 2D). In the lateral area, a thin adipose compartment approximately 0.3–2 mm thick was observed between the PeRF and pararenal fat (PaRF). Perinephric veins were present within the PeRF, and small vessels were noted within the thin adipose compartment (Fig. 2B). The thin adipose compartment extended from the point of peritoneal reflection in three directions: (1) anteriorly beneath the peritoneum along the body wall, (2) anteriorly between the peritoneum and PeRF, and (3) posteriorly between the PaRF and PeRF. The compartment thickness was measured to be approximately 0.3–1 mm, lesser than the thickness of the TACs in the lateral and anterior areas. We observed the TAC at two different slice levels and aimed to capture its overall structure in three dimensions. However, conventional methods using torso slices and histological sections under a microscope were insufficient. In the block-face images obtained using the CoMBI method from a large tissue specimen, including the colon, kidney, and abdominal wall, distinct adipose compartments were identified: mesenteric fat, PeRF, renal hilar fat, and PaRF (Fig. 3A, B). Cross-sectional profiles of the blood vessels were observed within each adipose compartment. Histological sections prepared from the same specimen confirmed the presence of TACs between the mesentery and PeRF, and between the PeRF and PaRF (Fig. 3C–E). The TACs were enclosed on both sides by dense connective tissue and contained small blood vessels, consistent with previous observations (Fig. 1, 2). A clear boundary composed of dense connective tissues was observed between the PeRF and renal hilar fat, indicating that these are separate compartments (Fig. 3F). Wide-field block-face imaging obtained using CoMBI revealed that thin adipose compartments were consistently present (1) between the colonic mesentery and PeRF, (2) between the PeRF and PaRF, and (3) beneath the peritoneum along the abdominal wall. These compartments converged laterally with the PeRF, forming a triad-like junction (Fig. 3A–H). Serial block-face images demonstrated that these compartments extended vertically, suggesting a continuous distribution along the craniocaudal axis. Consistent morphological features were observed in the specimens from the right perirenal region (data not shown). This approach enabled wide-area visualization of the morphology of the adipose compartment, particularly the spatial continuity and anatomical relationship between the descending mesocolon and the adjacent perirenal and abdominal wall fat. To reveal the three-dimensional distribution of the adipose compartments, we created segments of the adipose compartments and organs in serial block-face images and reconstructed them into three-dimensional images (Fig. 4). The descending colon was enveloped by the mesenteric fat, which contained the vascular structures that supply the intestine. The anterior surface of the mesenteric fat was covered by the peritoneum, which folded laterally and continued toward the abdominal wall (Fig. 4A). Thin adipose compartments were identified posterior to the mesenteric fat and beneath the peritoneum along the abdominal wall, which converged laterally at the outer aspect of the PeRF and extended posteriorly toward the compartment located between the PeRF and PaRF (Fig. 4B). The PeRF, which enveloped the kidney, contained the perinephric vein (Fig. 4C). The renal hilar fat enclosed the renal artery, vein, and ureter, and covered the renal hilum (Fig. 4D). DISCUSSION In this study, a TAC was identified between the colonic mesentery and PeRF. It extended beyond the mesentery–retroperitoneum interface to connect with similar structures located subperitoneally and along the lateral and posterior margins of the PeRF (Fig. 5A). Using three-dimensional block-face imaging and histology, the compartment was shown to exhibit craniocaudal continuity, indicating a vertically organized adipose morphology. These findings suggest that this TAC should be reconsidered not as an amorphous filler, but as a spatially organized unit with structural, functional, and surgical significance. This may provide new anatomical insights for identifying safe dissection planes and understanding potential routes of tumor progression. Anatomical Significance of the Thin Adipose Compartment The conventional anatomical understanding of the relationship between the colonic mesentery and PeRF has centered on the concept of fusion fasciae, such as Toldt’s and Fredet’s fasciae, which are thought to arise from embryological peritoneal reflections. 12–15 However, consistent histological identification of such fasciae remains elusive. In the present study, we identified a thin adipose compartment between the colonic mesentery and PeRF, clearly delineated by dense connective tissue containing small vessels on both sides. Rather than representing a transitional zone, this compartment formed a distinct anatomical unit. Although similar structures have been previously described, they were often regarded as amorphous fat or poorly defined transition layers. 1,3,16–18 Anatomical interpretations of intra-abdominal connective tissue and fat can be divided into two frameworks: one highlighting fascia-like membranes as dissection planes, 4 and the other treating organs, vessels, and surrounding adipose tissue as integrated compartments. 8,19 Based on the latter view, our findings challenge the fascia-centered model and support adipose compartments as structured, functionally relevant units. The boundary structures between the colonic mesentery and the retroperitoneum, particularly Toldt’s and Gerota’s fasciae, have long attracted the attention of surgeons. 12,20 Toldt’s fascia is widely accepted to represent the interface between mesocolic fat and the retroperitoneal space. The precise anatomical and histological correlates of Toldt’s fascia remain a matter of ongoing debate. 12 Some studies have interpreted the TAC between the colonic mesentery and PeRF as corresponding to Toldt’s fascia. 3,16–18 Notably, these studies examined only limited anatomical regions, allowing for varying interpretations. Gerota classically proposed what was then a novel concept—the renal fascia, consisting of the anterior and posterior renal fasciae that enclose the kidney together with the PeRF. 21 However, multiple perspectives on the fascia thought to envelop the kidneys and great vessels exist. 20,22 Although the histological structure of Gerota’s fascia has been examined, its relationship with Toldt’s fascia remains unclear. 23 Figure 5B illustrates the compartmental structure of fat at the colonic mesentery–PeRF interface based on our findings. If we define the dense connective tissue located between the mesenteric fat and the TAC as Toldt’s fascia, and the dense connective tissue between the TAC and the PeRF as Gerota’s fascia, this would be largely consistent with previous anatomical descriptions. A clearer understanding of the TAC may contribute to elucidating the layered anatomical architecture of this interface. Developmental Implications As demonstrated in the present study, the fat layer between the peritoneum and abdominal musculature in the abdominal wall is divided into two compartments (Fig. 5): a TAC directly beneath the peritoneum, and a thicker compartment adjacent to the abdominal muscles. Based on their configuration and spatial relationship to the PeRF, we suggest that the urogenital structures—including the mesonephros, metanephros, and adrenal glands—originate within the TAC. The development of the PeRF and renal hilar fat likely partitions this thin layer into anterior and posterior portions, forming the lateral fat triradiate zone. The known formation of mesonephric ridges in the retroperitoneum and the fact that the adrenal cortex differentiates from cells derived from the coelomic epithelium 24,25 are consistent with the interpretation that these structures arise within the TAC beneath the peritoneum, rather than in the thicker adipose compartment adjacent to the abdominal musculature. The finding that the fat in the abdominal wall and retroperitoneum is not homogeneous but organized into discrete compartments likely reflects the spatial patterning of developmental processes, and may have important implications for surgical anatomy. Surgical Implications Traditionally, fasciae have been regarded as critical surgical landmarks that define dissection planes. By contrast, the significance of adipose tissue and its surrounding connective structures as anatomical planes of separation during surgery has been highlighted. 3,4 When considering these adipose-based planes, accounting for inter-individual variability—particularly in adipose tissue volume—is important. Given how thin the adipose compartment described in the present study is, its visibility may vary with individual adiposity. Adipose tissue is both a passive energy store and active endocrine organ with considerable inter-individual variability; its distribution differs by anatomical region. 26 ,27 Accordingly, in individuals with minimal adipose volume, the two dense connective tissue layers of Toldt’s and Gerota’s fasciae may lie in close proximity or even fuse, causing the colonic mesenteric fat and PeRF to appear directly faced via a single connective tissue sheet. In such cases, the dissection plane may become indistinct and the layer recognized as Toldt’s fascia may shift depending on the operator’s interpretation. Boekestijn et al. suggested that retroperitoneal fascial structures may function as potential spaces, and their recognition can be influenced by surgical manipulation and the volume of surrounding fat. 28 Such considerations likely apply to not only the anterior region of the kidney, involving Toldt’s and Gerota’s fasciae, but also to the posterior region, involving Gerota’s fascia and the lateral conal fascia. 8 Limitations This study had several limitations. First, older male cadavers fixed in formalin were used; therefore, aging and preservation may have affected tissue features. Second, the all-male sample limits assessment of sex- and body-type–related variability. Third, although histological and three-dimensional analyses revealed adipose compartment structures, their clinical relevance was not directly examined. Future studies should compare different ages, sexes, and body types, and incorporate fresh tissue for dynamic analysis. Conclusions In conclusion, the presence of a TAC identified in this study suggests that retroperitoneal adipose tissue should be regarded as a spatially organized structural compartment. Accordingly, our findings offer a novel framework based on the compartmentalization and spatial distribution of adipose tissues. This approach has potential implications for improving the safety and precision of surgical dissection. Future studies should explore the relationship between adipose compartments and surrounding connective tissue layers in various organ systems, ultimately contributing to the development of comprehensive anatomical models for surgical navigation, imaging interpretation, and pathological assessment. Declarations Acknowledgements The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. The results of this research can potentially increase the overall knowledge of humankind and improve patient care. Therefore, the donors and their families deserve great gratitude. Conflict of Interest and Source of Funding: This study was supported by a grant (23K06299) from MEXT KAKENHI; however, the authors have no relevant financial relationships to disclose. References Wedel T, Heimke M, Fletcher J, et al. The retrocolic fascial system revisited for right hemicolectomy with complete mesocolic excision based on anatomical terminology: do we need the eponyms Toldt, Gerota, Fredet and Treitz? Colorectal Dis . 2023;25:764–774. Sugiyama Y, Muro S, Ban D, et al. Retroperitoneal fasciae as barriers for nerve and arterial passages connecting the retroperitoneal region to the peritoneal organs. J Anat . 2024;245:1–11. Culligan K, Walsh S, Dunne C, et al. 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Philadelphia: Churchill Livingstone Elsevier; 2009. Matsubara A, Murakami G, Niikura H, et al. Development of the human retroperitoneal fasciae. Cells Tissues Organs . 2009;190:286–296. Wozniak SE, Gee LL, Wachtel MS, et al. Adipose tissue: the new endocrine organ? A review article. Dig Dis Sci . 2009;54:1847–1856. Kershaw EE, Flier JS. Adipose tissue as an endocrine organ. J Clin Endocrinol Metab . 2004;89:2548–2556. Boekestijn B, Wasser MNJM, Mieog JSD, et al. Retroperitoneum revisited: a review of radiological literature and updated concept of retroperitoneal fascial anatomy with imaging features and correlating anatomy. Surg Radiol Anat . 2024;46:1165–1175. Additional Declarations The authors declare no competing interests. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7369510","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":500269260,"identity":"c5fcbd30-defb-4436-92b2-8640f3067bdd","order_by":0,"name":"Satoru Muro","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzUlEQVRIiWNgGAWjYNACGyBmb2Bg4AHzDhCjJQ2IeQ6TrEUiGaaFANCdkfvwAUOCXR7/zPdHN7xhsJNnYDyL3xqzG+nGBgwJycUSt5PZbs5hSDZsYDiXQEBLGpsE4w/mxAaglts8DMxA5WcMCGthSKhPnH/zMEhLPdFaDiduuMEM0nKYCC1nnjEbJCQcT9x4Jtns5hyD44ZtBP1yPI3xwYeE6sR5xw8+u/GmolqeX4JAiIEBwlSgk9gkzhDWgQb4e0jWMgpGwSgYBcMbAABod0WBISj6QgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0002-4709-6359","institution":"Institute of Science Tokyo","correspondingAuthor":true,"prefix":"","firstName":"Satoru","middleName":"","lastName":"Muro","suffix":""},{"id":500269327,"identity":"f573963f-429b-41f7-b6af-7423f26a4e4b","order_by":1,"name":"Atsuhiko Ochi","email":"","orcid":"https://orcid.org/0000-0003-4981-8818","institution":"Kameda Medical Center","correspondingAuthor":false,"prefix":"","firstName":"Atsuhiko","middleName":"","lastName":"Ochi","suffix":""},{"id":500269437,"identity":"3ab6f336-cede-438f-8a0b-3c1b32d9b039","order_by":2,"name":"Sho Mitsumaru","email":"","orcid":"","institution":"Institute of Science Tokyo","correspondingAuthor":false,"prefix":"","firstName":"Sho","middleName":"","lastName":"Mitsumaru","suffix":""},{"id":500269438,"identity":"c5f2a1af-2dd2-4630-b8a6-88ca739e836a","order_by":3,"name":"Yuki Tajika","email":"","orcid":"","institution":"Gunma Prefectural College of Health Sciences","correspondingAuthor":false,"prefix":"","firstName":"Yuki","middleName":"","lastName":"Tajika","suffix":""},{"id":500269507,"identity":"9c517f90-09cb-4c98-bd19-e648efeb1adb","order_by":4,"name":"Akimoto Nimura","email":"","orcid":"","institution":"Institute of Science Tokyo","correspondingAuthor":false,"prefix":"","firstName":"Akimoto","middleName":"","lastName":"Nimura","suffix":""},{"id":500269508,"identity":"d51e4153-d691-4184-99f9-393e07c992a5","order_by":5,"name":"Keiichi Akita","email":"","orcid":"","institution":"Institute of Science Tokyo","correspondingAuthor":false,"prefix":"","firstName":"Keiichi","middleName":"","lastName":"Akita","suffix":""}],"badges":[],"createdAt":"2025-08-14 03:59:53","currentVersionCode":1,"declarations":{"humanSubjects":true,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":true,"humanSubjectConsent":true,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-7369510/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7369510/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":89274009,"identity":"81093307-9030-4711-8dd9-bb28aa3debdb","added_by":"auto","created_at":"2025-08-18 09:12:32","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1527528,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe TAC between the colonic mesentery and perirenal fat\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(A) Transverse section of the abdomen showing the anatomical position of the ascending and descending colon.\u003c/p\u003e\n\u003cp\u003e(B, C) Magnified view of the red rectangular frames in A, showing the peritoneal reflections at the colonic margins and their continuation to the abdominal wall.\u003c/p\u003e\n\u003cp\u003e(D, E) Histological images (Elastica van Gieson staining) of the rectangular areas in B and E, showing a TAC (approximately 0.3–1.5 mm thick) between the MF and PeRF.\u003c/p\u003e\n\u003cp\u003e(F) Magnified view of the rectangular area in F. Small blood vessels can be seen within the compartment. The TAC is enclosed anteriorly and posteriorly by dense connective tissues, forming a distinguishable unit.\u003c/p\u003e\n\u003cp\u003eAC, ascending colon; AWM, abdominal wall muscle; DC, descending colon; MA, mesenteric artery; MF, mesenteric fat; MV, mesenteric vein; PeRF, perirenal fat; PM, psoas muscle; Pn, peritoneum; QL, quadratus lumborum muscle; TAC, thin adipose compartment\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7369510/v1/73b00ce5db5d977dfe6a5c31.png"},{"id":89272289,"identity":"bbde38cd-dd2f-4906-95b2-7dfe7a83ae9d","added_by":"auto","created_at":"2025-08-18 09:04:32","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":2353625,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDistribution of the TAC\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(A) Tissue sampling areas in the lateral, anterior, and posterior perirenal regions in a transverse abdominal section.\u003c/p\u003e\n\u003cp\u003e(B) Histological section (Masson’s trichrome staining) of the lateral region showing a TAC (~0.3–2 mm) between the PeRF and PaRF; small vessels can be seen in the compartment. The TAC extends from the peritoneal reflection in multiple directions, with variable thickness.\u003c/p\u003e\n\u003cp\u003e(C) Histological section (Masson’s trichrome staining) of the anterior region showing the TAC between the MF and PeRF.\u003c/p\u003e\n\u003cp\u003e(D) Posterior region between the kidney and body wall (Masson’s trichrome staining). The TAC was observed posterior to the PeRF.\u003c/p\u003e\n\u003cp\u003eTAC, thin adipose compartment; AC, ascending colon; AWM, abdominal wall muscle; MF, mesenteric fat; PaRF, pararenal fat; PeRF, perirenal fat; PeV, perinephric vein; PM, psoas muscle; Pn, peritoneum; QL, quadratus lumborum muscle\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7369510/v1/961416657e7b66b68ad2143e.png"},{"id":89272292,"identity":"1f09d55b-6085-4dd7-afea-a229549f9143","added_by":"auto","created_at":"2025-08-18 09:04:32","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1110668,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eWide-area visualization of adipose compartments using the correlative microscopy and block-face imaging method\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(A, B) Block-face images from large tissue specimens containing the colon, kidney, and abdominal wall, identifying the MF, PeRF, RHF, and PaRF.\u003c/p\u003e\n\u003cp\u003e(C–E) Histological sections (Masson’s trichrome staining) of the square frames in A and B, confirming the presence of TACs between the (C, E) mesentery and PeRF, as well as (D) PeRF and PaRF.\u003c/p\u003e\n\u003cp\u003e(F) Dense connective tissue boundary between the PeRF and RHF. (Masson’s trichrome staining)\u003c/p\u003e\n\u003cp\u003e(G, H) Traced and segmented images of A and B showing lateral convergence of the TACs and vertical craniocaudal continuity, forming a triad-like anatomical junction.\u003c/p\u003e\n\u003cp\u003eAWM, abdominal wall muscle; DC, descending colon; MA, mesenteric artery; MF, mesenteric fat; MV, mesenteric vein; PaRF, pararenal fat; PeRF, perirenal fat; PeV, perinephric vein; Pn, peritoneum; RA, renal artery; RHF, renal hilar fat; RV, renal vein; TAC, thin adipose compartment\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7369510/v1/22b92eaa0532493bd6c4304c.png"},{"id":89272295,"identity":"756c3202-1ea2-4dee-8752-7d0af4a5fee2","added_by":"auto","created_at":"2025-08-18 09:04:33","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":558871,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThree-dimensional reconstruction of the adipose compartments and associated organs\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(A) The descending colon surrounded by the MF and peritoneal fold continuing toward the abdominal wall.\u003c/p\u003e\n\u003cp\u003e(B) TACs located posterior to the mesentery and subperitoneally along the lateral abdominal wall, extending toward the compartment between the PeRF and PaRF.\u003c/p\u003e\n\u003cp\u003e(C) PeRF surrounding the kidney and containing PeVs.\u003c/p\u003e\n\u003cp\u003e(D) RHF enclosing the renal artery, vein, and ureter.\u003c/p\u003e\n\u003cp\u003eAWM, abdominal wall muscle; DC, descending colon; K, Kidney; MA, mesenteric artery; MF, mesenteric fat; MV, mesenteric vein; PaRF, pararenal fat; PeRF, perirenal fat; PeV, perinephric vein; Pn, peritoneum; RA, renal artery; RHF, renal hilar fat; RV, renal vein; TAC, thin adipose compartment; Ut, ureter\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7369510/v1/a6da883fe10e7b22b68d698b.png"},{"id":89272294,"identity":"b3885da6-6cb0-4524-99a8-74df8ba31e51","added_by":"auto","created_at":"2025-08-18 09:04:33","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":176486,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpatial distribution and continuity of thin adipose compartments revealed using histological and three-dimensional imaging.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(A) Schematic summary illustrating the anatomical continuity of the thin adipose compartments (TACs) identified in this study. These compartments are located at the interface between the colonic mesentery and PeRF, extending both to subperitoneal fat of the abdominal wall and posteriorly between the PeRF and PaRF (asterisks). The TACs converge to form a triad-like junction (fat triradiate zone) lateral to the PeRF. This spatially organized structure, enclosed by dense connective tissue and containing vascular elements, suggests a functionally distinct anatomical unit relevant to surgical dissection.\u003c/p\u003e\n\u003cp\u003e(B) Boundary structures between the colonic mesentery and the retroperitoneum. The TAC exists between the mesenteric and perirenal fat. Dense connective tissue is interposed at the boundaries between these different fat compartments. The dense connective tissue between the mesenteric fat and the TAC is considered to correspond to Toldt’s fascia, whereas the tissue between the TAC and the perirenal fat likely corresponds to Gerota’s fascia.\u003c/p\u003e\n\u003cp\u003eAWM, abdominal wall muscle; C, colon; K, Kidney; MF, mesenteric fat; PaRF, pararenal fat; PeRF, perirenal fat; PeV, perinephric vein; Pn, peritoneum; RHF, renal hilar fat; TAC, thin adipose compartment\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7369510/v1/c7dc45952cb810a7fe42db80.png"},{"id":89274396,"identity":"7e9e574e-9768-44d9-a3ec-c61adeb378ac","added_by":"auto","created_at":"2025-08-18 09:20:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6858337,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7369510/v1/4bf1192c-62d3-4721-ab4c-b3385badc35a.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eA Novel Thin Adipose Compartment at the Colonic Mesentery–Perirenal Fat Interface: Histological and Three-Dimensional Morphological Studies\u003c/p\u003e","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eThe anatomical relationship between the colonic mesentery and retroperitoneal structures in the abdomen has been reevaluated in parallel with advances in surgical techniques. Particularly, the ascending and descending mesocolons are thought to fuse with the parietal peritoneum during fetal development, forming a fusion (Toldt\u0026rsquo;s) fascia, which has traditionally been considered the boundary between the mesocolon and retroperitoneum.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e This fascial interface is reportedly not directly penetrated by vessels or nerves. Instead, nerves traverse from the retroperitoneal side into the mesocolon, avoiding this layer.\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e Such connective tissue planes are typically avascular and used as natural dissection layers during surgery.\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e Some reports have challenged the concept of Toldt\u0026rsquo;s fascia as a true fusion fascia, proposing that it represents a retracted extraperitoneal fascia formed during peritoneal remodeling.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e Regardless of the interpretation, these connective tissue layers serve as critical anatomical interfaces between the colonic mesentery and retroperitoneum, and a precise understanding of their structure is essential for evaluating tumor invasion and ensuring safe surgical dissection.\u003c/p\u003e\u003cp\u003ePrevious studies examining the interface between the ascending and descending mesocolons and retroperitoneal space primarily relied on histological observations from limited anatomical regions, hindering comprehensive understanding of the spatial continuity and three-dimensional architecture of these structures. Conventional anatomical approaches are insufficient to integrate macroscopic observations across extensive areas with microscopic analysis of fine structural features. To address this limitation, the present study employed correlative microscopy and block-face imaging (CoMBI), a method that enables integration of high-resolution histological and three-dimensional morphological analyses.\u003csup\u003e\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e Using this method, we aimed to elucidate the structural continuity and spatial characteristics of the adipose compartment located between the colonic mesentery and perirenal fat (PeRF), along with both the ascending and descending colons, in a three-dimensional and systematic manner.\u003c/p\u003e\u003cp\u003eIn an earlier study, we demonstrated a previously unrecognized thin adipose compartment (TAC) in the region posterior to the kidney and reported its anatomical relationships with the posterior renal fascia and the lateral conal fascia.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e In the present study, we identified a TAC located anterior to the kidney, between the colonic mesentery and PeRF, and demonstrated its anatomical relationships with the fusion fascia. We aimed to define the anatomical characteristics and spatial extent of this adipose compartment, and to evaluate its continuity with adjacent structures, including the subperitoneal fat and posterior abdominal wall.\u003c/p\u003e"},{"header":"METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003ePreparation of Cadaveric Specimens\u003c/h2\u003e\u003cp\u003eSeven adult cadavers (six male and one female; mean age at death, 81.7 [range, 73\u0026ndash;92] years) were donated to our department in accordance with the Japanese law entitled The Act on Body Donation for Medical and Dental Education (Act No. 56 of 1983). All donors voluntarily agreed before their deaths that their remains be used for educational and research purposes. Written informed consent was re-confirmed with the bereaved families, and no objections were raised. All cadavers were fixed with 8% formalin via arterial perfusion and preserved in 30% alcohol to maintain tissue integrity and prevent fungal growth. Cadavers with a history of major abdominal or pelvic surgery were excluded. Tissue blocks, including the ascending and descending mesocolons, retroperitoneum, and PeRF, were dissected en bloc for analysis.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eHistological Analysis\u003c/h3\u003e\n\u003cp\u003eHistological analysis was performed on six of the cadavers. In five cadavers, the entire abdomen, including the ascending and descending colons, retroperitoneum, and PeRF, was harvested en bloc, and a single transverse slice was obtained at the level crossing both colonic mesenteries using a diamond band saw (EXAKT 312; EXAKT Technologies, Inc., Germany). Representative tissue blocks containing the interface between the colonic mesentery and PeRF were excised for histological analysis. In the one remaining cadaver, the entire abdominal region was frozen at \u0026minus;\u0026thinsp;80\u0026deg;C and serially sectioned into 10-mm-thick transverse slices using a band saw (Nakajima Seisakusho, Y.K., Osaka, Japan), based on previous anatomical studies.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e From these frozen slices, multiple smaller blocks were obtained to represent the craniocaudal extent of the adipose compartments of interest, particularly focusing on the boundary between the colonic mesentery and retroperitoneum as well as the lateral and posterior aspects of the PeRF.\u003c/p\u003e\u003cp\u003eAll tissue blocks were fixed in 10% neutral-buffered formalin for 24 hours, dehydrated using a graded ethanol series (70%, 80%, 90%, and 100%), cleared in xylene, and embedded in paraffin under negative pressure. The paraffin was replaced three times over 5 days to ensure thorough infiltration. Serial paraffin sections (5 \u0026micro;m thick) were cut using a rotary microtome (RM2235; Leica Biosystems Nussloch GmbH, Wetzlar, Germany) and stained with Elastica van Gieson and Masson\u0026rsquo;s trichrome staining. Microscopic examination focused on the zonal organization of the adipose compartments and the distribution of small blood vessels, particularly in relation to the mesenteric and perirenal regions.\u003c/p\u003e\n\u003ch3\u003eCoMBI\u003c/h3\u003e\n\u003cp\u003eOne cadaver was used for three-dimensional morphological analysis using the CoMBI method. The right and left perirenal regions were dissected along with parts of the ascending and descending colons, kidneys, and abdominal walls. The specimens were fixed and embedded in opaque paraffin containing 6.25% w/w white crayon.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e The specimen blocks were sectioned with 5 \u0026micro;m thickness using a rotary-type microtome (RX-860; Yamato Kohki Industrial Co. Ltd., Saitama, Japan), and the block-faces were captured using a digital camera (Nikon D5100; Nikon Corporation, Tokyo, Japan) with a macro lens (Nikon AF-S DX Micro Nikkor 40 mm f/2.8G; Nikon Corporation). The block-face was captured for every 20 sections cut; 207 and 390 serial block-face images of the right and left perirenal regions, respectively, were obtained with an interval of 100 \u0026micro;m. A few sections were obtained during block-face imaging and stained with Masson\u0026rsquo;s trichrome.\u003c/p\u003e\n\u003ch3\u003eThree-Dimensional Morphological Analysis\u003c/h3\u003e\n\u003cp\u003eSegmentation and three-dimensional reconstruction were performed using serial block-face images obtained using the CoMBI method. The original three-dimensional dataset of the left perirenal region consisted of 390 serial block-face images with 100-\u0026micro;m intervals. We created the smaller-sized dataset containing 187 block-face images with 200-\u0026micro;m intervals by selecting odd-numbered image files and excluding images of areas outside the region of interest. The images were aligned manually and used for three-dimensional analysis. The adipose compartments and organs within the block-face images were annotated by referring to the correlated sections stained with Masson\u0026rsquo;s trichrome. Segmentation of these structures was performed using an artificial intelligence-assisted protocol (Seg \u0026amp; Ref; \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://github.com/SatoruMuro/SAM2GUIfor3Drecon)\u003c/span\u003e\u003cspan address=\"https://github.com/SatoruMuro/SAM2GUIfor3Drecon)\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003csup\u003e10\u003c/sup\u003e The segments were reconstructed into a three-dimensional surface model using 3D Slicer (version 5.2.2; \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.slicer.org/)\u003c/span\u003e\u003cspan address=\"https://www.slicer.org/)\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003csup\u003e11\u003c/sup\u003e Adipose tissue is soft and extends over a wide area, hindering analysis of the entire morphology using sections and conventional microscopy. We were interested in the adipose tissue compartments and their relationship with the colon and abdominal wall. Thus, we performed block-face imaging for three-dimensional morphological analysis of perirenal adipose tissues and examined the correlation between block-face images and partial sections stained with Masson\u0026rsquo;s trichrome to annotate adipose tissues and organs.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was approved by the Human Subjects Research Ethics Review Committee of the Institute of Science, Tokyo, Japan (approval number M2019-075). All methods were performed in accordance with relevant guidelines and regulations.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eUsage of Generative AI and AI-Assisted Technologies\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDuring the preparation for this study, ChatGPT was used to improve the clarity and grammatical usage of English. After using these services, the authors reviewed and edited the content as required and take full responsibility for the content of the published article.\u003c/p\u003e"},{"header":"RESULTS","content":"\u003cp\u003eWe identified the thin compartment of adipose tissues between the ascending and descending colonic mesenteries and PeRF and named it \u0026ldquo;thin adipose compartment (TAC).\u0026rdquo;\u0026nbsp;The colon was clearly identified in the transverse section of the abdomen (Fig. 1A). The anterior surfaces of these colonic segments were covered by the peritoneum, which folded laterally at the colonic margins and continued toward the abdominal wall (Fig. 1B, C). Histological examination of the interface between the mesenteric and retroperitoneal adipose tissue revealed a TAC approximately 0.3\u0026ndash;1.5 mm thick situated between them (Fig. 1D, E). This compartment was enclosed anteriorly and posteriorly by dense connective tissue, forming a discrete unit distinguishable from both the mesenteric and retroperitoneal fat. Small blood vessels were observed within the thin adipose compartment (Fig. 1F).\u003c/p\u003e\n\u003cp\u003eThe presence of the TAC was confirmed at different transverse levels, including the kidney. Tissue samples were collected from the lateral, anterior, and posterior areas of the perirenal adipose tissue from a transverse section of the abdomen at the kidney level (Fig. 2A). Histological analysis was performed in the lateral area near the colon and peritoneal reflection (Fig. 2B), anterior area including the interface between the mesenteric and retroperitoneal adipose tissue (Fig. 2C), and posterior area between the kidney and body wall (Fig. 2D). In the lateral area, a thin adipose compartment approximately 0.3\u0026ndash;2 mm thick was observed between the PeRF and pararenal fat (PaRF). Perinephric veins were present within the PeRF, and small vessels were noted within the thin adipose compartment (Fig. 2B). The thin adipose compartment extended from the point of peritoneal reflection in three directions: (1) anteriorly beneath the peritoneum along the body wall, (2) anteriorly between the peritoneum and PeRF, and (3) posteriorly between the PaRF and PeRF. The compartment thickness was measured to be approximately 0.3\u0026ndash;1 mm, lesser than the thickness of the TACs in the lateral and anterior areas.\u003c/p\u003e\n\u003cp\u003eWe observed the TAC at two different slice levels and aimed to capture its overall structure in three dimensions. However, conventional methods using torso slices and histological sections under a microscope were insufficient. In the block-face images obtained using the CoMBI method from a large tissue specimen, including the colon, kidney, and abdominal wall, distinct adipose compartments were identified: mesenteric fat, PeRF, renal hilar fat, and PaRF (Fig. 3A, B). Cross-sectional profiles of the blood vessels were observed within each adipose compartment. Histological sections prepared from the same specimen confirmed the presence of TACs between the mesentery and PeRF, and between the PeRF and PaRF (Fig. 3C\u0026ndash;E). The TACs were enclosed on both sides by dense connective tissue and contained small blood vessels, consistent with previous observations (Fig. 1, 2). A clear boundary composed of dense connective tissues was observed between the PeRF and renal hilar fat, indicating that these are separate compartments (Fig. 3F). Wide-field block-face imaging obtained using CoMBI revealed that thin adipose compartments were consistently present (1) between the colonic mesentery and PeRF, (2) between the PeRF and PaRF, and (3) beneath the peritoneum along the abdominal wall. These compartments converged laterally with the PeRF, forming a triad-like junction (Fig. 3A\u0026ndash;H). Serial block-face images demonstrated that these compartments extended vertically, suggesting a continuous distribution along the craniocaudal axis. Consistent morphological features were observed in the specimens from the right perirenal region (data not shown).\u0026nbsp;This approach enabled wide-area visualization of the morphology of the adipose compartment, particularly the spatial continuity and anatomical relationship between the descending mesocolon and the adjacent perirenal and abdominal wall fat.\u003c/p\u003e\n\u003cp\u003eTo reveal the three-dimensional distribution of the adipose compartments, we created segments of the adipose compartments and organs in serial block-face images and reconstructed them into three-dimensional images (Fig. 4). The descending colon was enveloped by the mesenteric fat, which contained the vascular structures that supply the intestine. The anterior surface of the mesenteric fat was covered by the peritoneum, which folded laterally and continued toward the abdominal wall (Fig. 4A). Thin adipose compartments were identified posterior to the mesenteric fat and beneath the peritoneum along the abdominal wall, which converged laterally at the outer aspect of the PeRF and extended posteriorly toward the compartment located between the PeRF and PaRF (Fig. 4B). The PeRF, which enveloped the kidney, contained the perinephric vein (Fig. 4C). The renal hilar fat enclosed the renal artery, vein, and ureter, and covered the renal hilum (Fig. 4D).\u003c/p\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eIn this study, a TAC was identified between the colonic mesentery and PeRF. It extended beyond the mesentery\u0026ndash;retroperitoneum interface to connect with similar structures located subperitoneally and along the lateral and posterior margins of the PeRF (Fig. 5A). Using three-dimensional block-face imaging and histology, the compartment was shown to exhibit craniocaudal continuity, indicating a vertically organized adipose morphology. These findings suggest that this TAC should be reconsidered not as an amorphous filler, but as a spatially organized unit with structural, functional, and surgical significance. This may provide new anatomical insights for identifying safe dissection planes and understanding potential routes of tumor progression.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAnatomical Significance of the Thin Adipose Compartment\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe conventional anatomical understanding of the relationship between the colonic mesentery and PeRF has centered on the concept of fusion fasciae, such as Toldt\u0026rsquo;s and Fredet\u0026rsquo;s fasciae, which are thought to arise from embryological peritoneal reflections.\u003csup\u003e12\u0026ndash;15\u003c/sup\u003e However, consistent histological identification of such fasciae remains elusive. In the present study, we identified a thin adipose compartment between the colonic mesentery and PeRF, clearly delineated by dense connective tissue containing small vessels on both sides. Rather than representing a transitional zone, this compartment formed a distinct anatomical unit. Although similar structures have been previously described, they were often regarded as amorphous fat or poorly defined transition layers.\u003csup\u003e1,3,16\u0026ndash;18\u003c/sup\u003e Anatomical interpretations of intra-abdominal connective tissue and fat can be divided into two frameworks: one highlighting fascia-like membranes as dissection planes,\u003csup\u003e4\u003c/sup\u003e and the other treating organs, vessels, and surrounding adipose tissue as integrated compartments.\u003csup\u003e8,19\u003c/sup\u003e Based on the latter view, our findings challenge the fascia-centered model and support adipose compartments as structured, functionally relevant units.\u003c/p\u003e\n\u003cp\u003eThe boundary structures between the colonic mesentery and the retroperitoneum, particularly Toldt\u0026rsquo;s and Gerota\u0026rsquo;s fasciae, have long attracted the attention of surgeons.\u003csup\u003e12,20\u003c/sup\u003e Toldt\u0026rsquo;s fascia is widely accepted to represent the interface between mesocolic fat and the retroperitoneal space. The precise anatomical and histological correlates of Toldt\u0026rsquo;s fascia remain a matter of ongoing debate.\u003csup\u003e12\u003c/sup\u003e Some studies have interpreted the TAC between the colonic mesentery and PeRF as corresponding to Toldt\u0026rsquo;s fascia.\u003csup\u003e3,16\u0026ndash;18\u003c/sup\u003e Notably, these studies examined only limited anatomical regions, allowing for varying interpretations. Gerota classically proposed what was then a novel concept\u0026mdash;the renal fascia, consisting of the anterior and posterior renal fasciae that enclose the kidney together with the PeRF.\u003csup\u003e21\u003c/sup\u003e However, multiple perspectives on the fascia thought to envelop the kidneys and great vessels exist.\u003csup\u003e20,22\u003c/sup\u003e Although the histological structure of Gerota\u0026rsquo;s fascia has been examined, its relationship with Toldt\u0026rsquo;s fascia remains unclear.\u003csup\u003e23\u003c/sup\u003e Figure 5B illustrates the compartmental structure of fat at the colonic mesentery\u0026ndash;PeRF interface based on our findings. If we define the dense connective tissue located between the mesenteric fat and the TAC as Toldt\u0026rsquo;s fascia, and the dense connective tissue between the TAC and the PeRF as Gerota\u0026rsquo;s fascia, this would be largely consistent with previous anatomical descriptions. A clearer understanding of the TAC may contribute to elucidating the layered anatomical architecture of this interface.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDevelopmental Implications\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAs demonstrated in the present study, the fat layer between the peritoneum and abdominal musculature in the abdominal wall is divided into two compartments (Fig. 5): a TAC directly beneath the peritoneum, and a thicker compartment adjacent to the abdominal muscles. Based on their configuration and spatial relationship to the PeRF, we suggest that the urogenital structures\u0026mdash;including the mesonephros, metanephros, and adrenal glands\u0026mdash;originate within the TAC. The development of the PeRF and renal hilar fat likely partitions this thin layer into anterior and posterior portions, forming the lateral fat triradiate zone. The known formation of mesonephric ridges in the retroperitoneum and the fact that the adrenal cortex differentiates from cells derived from the coelomic epithelium\u003csup\u003e24,25\u003c/sup\u003e are consistent with the interpretation that these structures arise within the TAC beneath the peritoneum, rather than in the thicker adipose compartment adjacent to the abdominal musculature. The finding that the fat in the abdominal wall and retroperitoneum is not homogeneous but organized into discrete compartments likely reflects the spatial patterning of developmental processes, and may have important implications for surgical anatomy.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSurgical Implications\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTraditionally, fasciae have been regarded as critical surgical landmarks that define dissection planes. By contrast, the significance of adipose tissue and its surrounding connective structures as anatomical planes of separation during surgery has been highlighted.\u003csup\u003e3,4\u003c/sup\u003e When considering these adipose-based planes, accounting for inter-individual variability\u0026mdash;particularly in adipose tissue volume\u0026mdash;is important. Given how thin the adipose compartment described in the present study is, its visibility may vary with individual adiposity. Adipose tissue is both a passive energy store and active endocrine organ with considerable inter-individual variability; its distribution differs by anatomical region.\u003csup\u003e26\u003c/sup\u003e\u003csup\u003e,27\u003c/sup\u003e Accordingly, in individuals with minimal adipose volume, the two dense connective tissue layers of Toldt\u0026rsquo;s and Gerota\u0026rsquo;s fasciae may lie in close proximity or even fuse, causing the colonic mesenteric fat and PeRF to appear directly faced via a single connective tissue sheet. In such cases, the dissection plane may become indistinct and the layer recognized as Toldt\u0026rsquo;s fascia may shift depending on the operator\u0026rsquo;s interpretation. Boekestijn et al. suggested that retroperitoneal fascial structures may function as potential spaces, and their recognition can be influenced by surgical manipulation and the volume of surrounding fat.\u003csup\u003e28\u0026nbsp;\u003c/sup\u003eSuch considerations likely apply to not only the anterior region of the kidney, involving Toldt\u0026rsquo;s and Gerota\u0026rsquo;s fasciae, but also to the posterior region, involving Gerota\u0026rsquo;s fascia and the lateral conal fascia.\u003csup\u003e8\u003c/sup\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eLimitations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study had several limitations. First, older male cadavers fixed in formalin were used; therefore, aging and preservation may have affected tissue features. Second, the all-male sample limits assessment of sex- and body-type\u0026ndash;related variability. Third, although histological and three-dimensional analyses revealed adipose compartment structures, their clinical relevance was not directly examined. Future studies should compare different ages, sexes, and body types, and incorporate fresh tissue for dynamic analysis.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn conclusion, the presence of a TAC identified in this study suggests that retroperitoneal adipose tissue should be regarded as a spatially organized structural compartment. Accordingly, our findings offer a novel framework based on the compartmentalization and spatial distribution of adipose tissues. This approach has potential implications for improving the safety and precision of surgical dissection. Future studies should explore the relationship between adipose compartments and surrounding connective tissue layers in various organ systems, ultimately contributing to the development of comprehensive anatomical models for surgical navigation, imaging interpretation, and pathological assessment.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eThe authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. The results of this research can potentially increase the overall knowledge of humankind and improve patient care. Therefore, the donors and their families deserve great gratitude.\u003c/p\u003e\n\u003ch2\u003eConflict of Interest and Source of Funding: \u003c/h2\u003e\n\u003cp\u003eThis study was supported by a grant (23K06299) from MEXT KAKENHI; however, the authors have no relevant financial relationships to disclose.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eWedel T, Heimke M, Fletcher J, et al. The retrocolic fascial system revisited for right hemicolectomy with complete mesocolic excision based on anatomical terminology: do we need the eponyms Toldt, Gerota, Fredet and Treitz? \u003cem\u003eColorectal Dis\u003c/em\u003e. 2023;25:764\u0026ndash;774.\u003c/li\u003e\n\u003cli\u003eSugiyama Y, Muro S, Ban D, et al. Retroperitoneal fasciae as barriers for nerve and arterial passages connecting the retroperitoneal region to the peritoneal organs. \u003cem\u003eJ Anat\u003c/em\u003e. 2024;245:1\u0026ndash;11.\u003c/li\u003e\n\u003cli\u003eCulligan K, Walsh S, Dunne C, et al. The mesocolon: a histological and electron microscopic characterization of the mesenteric attachment of the colon prior to and after surgical mobilization. \u003cem\u003eAnn Surg.\u003c/em\u003e 2014;260:1048\u0026ndash;1056.\u003c/li\u003e\n\u003cli\u003eChen S, Yang G, Li Q, et al. There is no fusion fascia in the abdomen and extraperitoneal fascia always surrounds the mesentery. \u003cem\u003eJ Anat.\u003c/em\u003e 2023;242:796\u0026ndash;805.\u003c/li\u003e\n\u003cli\u003eTajika Y, Murakami T, Iijima K, et al\u003cem\u003e.\u003c/em\u003e A novel imaging method for correlating 2D light microscopic data and 3D volume data based on block-face imaging. \u003cem\u003eSci Rep.\u003c/em\u003e 2017;7:3645.\u003c/li\u003e\n\u003cli\u003eTajika Y, Ishii N, Morimura Y, et al\u003cem\u003e.\u003c/em\u003e Correlative microscopy and block-face imaging (CoMBI): a 3D imaging method with wide applicability in the field of biological science. \u003cem\u003eAnat Sci Int. \u003c/em\u003e2023;98\u003cem\u003e:\u003c/em\u003e353\u0026ndash;359.\u003c/li\u003e\n\u003cli\u003eIshii N, Tajika Y, Murakami T, et al. Correlative microscopy and block-face imaging (CoMBI) method for both paraffin-embedded and frozen specimens. \u003cem\u003eSci Rep.\u003c/em\u003e 2021;11:13108.\u003c/li\u003e\n\u003cli\u003eOchi A, Muro S, Mitsumaru S, et al. Anatomy of adipose compartments and fascial structures in the posterolateral region of the kidney with special focus on the thin adipose compartment. 10 August 2025, PREPRINT (Version 1). Available at Research Square [https://doi.org/10.21203/rs.3.rs-7324278/v1]\u003c/li\u003e\n\u003cli\u003eMuro S, Tsukada Y, Harada M, et al. Spatial distribution of smooth muscle tissue in the male pelvic floor with special reference to the lateral extent of the rectourethralis muscle: application to prostatectomy and proctectomy. \u003cem\u003eClin Anat\u003c/em\u003e. 2018;31:1167\u0026ndash;1176.\u003c/li\u003e\n\u003cli\u003eMuro S, Ibara T, Nimura A, et al. Seg and Ref: a newly developed toolset for artificial intelligence-powered segmentation and interactive refinement for labor-saving three-dimensional reconstruction. \u003cem\u003eMicroscopy (Oxf)\u003c/em\u003e. 3 March 2025; [Epub ahead of print]. Available at: https://doi.org/10.1093/jmicro/dfaf015.\u003c/li\u003e\n\u003cli\u003eFedorov A, Beichel R, Kalpathy-Cramer J, et al. 3D Slicer as an Image Computing Platform for the Quantitative Imaging Network. \u003cem\u003eMagn Reson Imaging\u003c/em\u003e. 2012;30:1323\u0026ndash;1341. \u003c/li\u003e\n\u003cli\u003eLiang JT, Huang J, Chen TC, et al. The Toldt fascia: a historic review and surgical implications in complete mesocolic excision for colon cancer. \u003cem\u003eAsian J Surg. \u003c/em\u003e2019;42:1\u0026ndash;5.\u003c/li\u003e\n\u003cli\u003eGarcia-Granero A, Pellino G, Frasson M, et al. The fusion fascia of Fredet: an important embryological landmark for complete mesocolic excision and D3-lymphadenectomy in right colon cancer. \u003cem\u003eSurg Endosc\u003c/em\u003e. 2019;33:3842\u0026ndash;3850.\u003c/li\u003e\n\u003cli\u003eBlears EE, Fortunato R. A view from the shoulders of giants: using history to optimize the modern minimally invasive right hemicolectomy. \u003cem\u003eWorld J Colorectal Surg\u003c/em\u003e. 2021;10:55\u0026ndash;61.\u003c/li\u003e\n\u003cli\u003eBrachini G, Cirillo B, Matteucci M, et al. A systematic review of varying definitions and the clinical significance of Fredet\u0026rsquo;s fascia in the era of complete mesocolic excision. \u003cem\u003eJ Clin Med\u003c/em\u003e. 2023;12:6233.\u003c/li\u003e\n\u003cli\u003eCulligan K, Sehgal R, Mulligan D, et al. A detailed appraisal of mesocolic lymphangiology - an immunohistochemical and stereological analysis. \u003cem\u003eJ Anat.\u003c/em\u003e 2014;225:463\u0026ndash;472.\u003c/li\u003e\n\u003cli\u003eCoffey JC, Culligan K, Walsh LG, et al. An appraisal of the computed axial tomographic appearance of the human mesentery based on mesenteric contiguity from the duodenojejunal flexure to the mesorectal level. \u003cem\u003eEur Radiol\u003c/em\u003e. 2016;26:714\u0026ndash;721.\u003c/li\u003e\n\u003cli\u003eCoffey JC, Byrnes KG, Walsh DJ, et al. Update on the mesentery: structure, function, and role in disease. \u003cem\u003eLancet Gastroenterol Hepatol.\u003c/em\u003e 2022;7:96\u0026ndash;106.\u003c/li\u003e\n\u003cli\u003eOchi A, Muro S, Adachi T, et al. Zoning inside the renal fascia: the anatomical relationship between the urinary system and perirenal fat. \u003cem\u003eInt J Urol. \u003c/em\u003e2020;27:625\u0026ndash;633.\u003c/li\u003e\n\u003cli\u003eAmin M, Blandford AT, Polk HC Jr. Renal fascia of Gerota. \u003cem\u003eUrology\u003c/em\u003e. 1976;7:1\u0026ndash;3.\u003c/li\u003e\n\u003cli\u003eGerota D. Arch Anat. Entwickl, Anat. Abteil, Jahrg. 1895;265\u0026ndash;285.\u003c/li\u003e\n\u003cli\u003eChesbrough RM, Burkhard TK, Martinez AJ, et al. Gerota versus Zuckerkandl: the renal fascia revisited. \u003cem\u003eRadiology\u003c/em\u003e. 1989;173:845\u0026ndash;846.\u003c/li\u003e\n\u003cli\u003eKobayashi Y, Edamura K, Sadahira T, et al. What is the identity of Gerota fascia? Histological study with cadavers. \u003cem\u003eInt J Urol\u003c/em\u003e. 2025;32:62\u0026ndash;68.\u003c/li\u003e\n\u003cli\u003eSchoenwolf GC, Larsen WJ. Larsen\u0026rsquo;s human embryology. Philadelphia: Churchill Livingstone Elsevier; 2009.\u003c/li\u003e\n\u003cli\u003eMatsubara A, Murakami G, Niikura H, et al. Development of the human retroperitoneal fasciae. \u003cem\u003eCells Tissues Organs\u003c/em\u003e. 2009;190:286\u0026ndash;296.\u003c/li\u003e\n\u003cli\u003eWozniak SE, Gee LL, Wachtel MS, et al. Adipose tissue: the new endocrine organ? A review article. \u003cem\u003eDig Dis Sci\u003c/em\u003e. 2009;54:1847\u0026ndash;1856.\u003c/li\u003e\n\u003cli\u003eKershaw EE, Flier JS. Adipose tissue as an endocrine organ. \u003cem\u003eJ Clin Endocrinol Metab\u003c/em\u003e. 2004;89:2548\u0026ndash;2556.\u003c/li\u003e\n\u003cli\u003eBoekestijn B, Wasser MNJM, Mieog JSD, et al. Retroperitoneum revisited: a review of radiological literature and updated concept of retroperitoneal fascial anatomy with imaging features and correlating anatomy. \u003cem\u003eSurg Radiol Anat\u003c/em\u003e. 2024;46:1165\u0026ndash;1175.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"Institute of Science Tokyo","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"colonic mesentery, retroperitoneum, Toldt’s fascia, histology, three-dimensional reconstruction, intra-abdominal anatomy, surgical navigation","lastPublishedDoi":"10.21203/rs.3.rs-7369510/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7369510/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eObjective: \u003c/strong\u003eTo elucidate the anatomical characteristics and three-dimensional continuity of a previously unrecognized thin adipose compartment between the colonic mesentery and retroperitoneum, using correlative microscopy and block-face imaging (CoMBI).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSummary Background Data: \u003c/strong\u003eThe interface between the colonic mesentery and retroperitoneum has traditionally been considered a fusion fascia (e.g., Toldt’s fascia). However, emerging evidence suggests more complex fascial remodeling, with previous studies lacking comprehensive spatial context.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMethods: \u003c/strong\u003eSeven adult cadavers were examined. Histological analysis was conducted on six specimens using paraffin sections stained with Elastica van Gieson and Masson’s trichrome. One cadaver underwent three-dimensional morphological analysis using CoMBI. Serial block-face images of the perirenal region were captured at 100 μm intervals, and three-dimensional reconstruction segmentation was performed.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults: \u003c/strong\u003eA distinct thin adipose compartment (0.3–2.0 mm thick) was consistently observed between the colonic mesentery and perirenal fat, enclosed by dense connective tissue and containing small vessels. Similar compartments were also found between the perirenal fat and pararenal fat, and beneath the peritoneum along the abdominal wall. These compartments extended in three directions from the peritoneal reflection and demonstrated craniocaudal continuity, laterally, these form a triad-like junction.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusions: \u003c/strong\u003eThe thin adipose compartment represents a structurally organized anatomical unit rather than amorphous filler. Its consistent continuity and integration with adjacent structures support a compartment-based framework of intra-abdominal anatomy, with implications for surgical navigation.\u003c/p\u003e","manuscriptTitle":"A Novel Thin Adipose Compartment at the Colonic Mesentery–Perirenal Fat Interface: Histological and Three-Dimensional Morphological Studies","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-18 09:04:27","doi":"10.21203/rs.3.rs-7369510/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a6b2f1ad-5d20-417a-8e8a-cf1a47eeb553","owner":[],"postedDate":"August 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":53141852,"name":"Surgery"}],"tags":[],"updatedAt":"2025-08-18T09:04:28+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-18 09:04:27","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7369510","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7369510","identity":"rs-7369510","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}