Tubal epithelial alterations: evolving concepts.

In recent years, fallopian tubes have received increasing attention from the perspective of gynecologic oncology. Originally, fallopian tubes were treated as a minor organ in this area. However, since the hypothesis that high-grade serous carcinoma (HGSC), the most common histological type of ovarian cancer, originates from the fimbria of the fallopian tube, many novel findings have been reported [ 1 2 3 4 ]. Several new concepts have been introduced for neoplastic and precancerous lesions. However, because their pathological significance and malignant potential are not fully understood, some are difficult to treat both pathologically and clinically. In this review, we focus on the morphological changes and molecular abnormalities of the fallopian tube epithelium and organize these lesions from a histopathological perspective. Ethical approval was obtained from the Institutional Review Board of Akita University (IRB No. 3339). All studies were conducted in compliance with the Helsinki Declaration.

To facilitate subsequent discussion, we present a brief overview of the histology of the fallopian tube. The wall of the fallopian tube consists of 3 layers: mucosa, muscularis, and serosa. The mucosa forms complex folds, the architecture of which varies by region. For instance, the isthmus exhibits relatively few folds, whereas the ampulla displays more elaborate and intricate structures. The mucosal epithelium is primarily composed of ciliated and nonciliated cells. Most non-ciliated cells are secretory cells, and the mucosa is mainly composed of a mixture of ciliated and secretory cells. In addition, a small number of peg cells (also referred to as intercalated cells) and basal cells were also present. Peg cells are similar to secretory cells, and although their function and role have long remained unclear, recent studies have revealed that they are epithelial stem cells of the fallopian tube residing within the mucosa. These stem cells are predominantly located in the fimbrial end of the fallopian tube [ 5 6 ]. Basal cells, once thought to be epithelial in nature, have been shown to be immune cells, such as lymphocytes or histiocytes, upon further investigation ( Fig. 1 ) [ 7 8 9 ]. H&E, hematoxylin and eosin. Ciliated and secretory cells are the primary cellular components of fallopian tube mucosa and undergo cyclical morphological and functional changes in response to hormonal fluctuations throughout the menstrual cycle. During the follicular phase (which corresponds to the proliferative phase of the endometrium), secretory cells actively form secretory granules and the epithelial lining becomes taller. In addition, the proportion of ciliated cells increases during this phase, which is likely to facilitate ovum transport following ovulation. In contrast, during the luteal phase (which corresponds to the secretory phase of the endometrium), the height of the epithelium decreases owing to the release of secretory granules, and the proportion of ciliated cells also decreases [ 5 10 11 12 13 ]. These cyclic alterations reflect functional adaptation of the fallopian tube for gamete transport, fertilization, and early embryo movement. A notable histological feature of aging is the gradual increase in the number of secretory cells, whereas ciliated cells tend to decrease in number [ 13 14 15 ]. These changes may influence the physiological function of the fallopian tubes and have potential implications for fertility. Advances in genomic analysis technology have also led to new discoveries regarding the fallopian tubes. Ulrich et al. [ 16 ] performed single-cell RNA sequencing (scRNA-seq) on approximately 60,000 cells from healthy human fallopian tubes and classified epithelial cells into 4 ciliated subtypes defined by the expression of FOXJ1 and CAPS and 6 non-ciliated secretory subtypes characterized by PAX8 and KRT7 expression. Notably, this study identified previously unrecognized progenitor-like cell populations with stem cell properties. One secretory subtype expressing canonical stem cell markers, such as LGR5 , CD44 , and TNFRSF19 , was predicted to differentiate into mature secretory epithelial cells. Another subtype expressing genes associated with epithelial-mesenchymal transition, including ACTA2 and ZEB1 , demonstrated the potential to differentiate into not only ciliated epithelial cells but also vascular endothelial cells, suggesting a bipotent progenitor capacity that bridges epithelial and stromal lineages. There are also studies that have investigated the cellular origin of tumors based on scRNA-seq. Although the overall cellular composition is largely conserved across the 3 anatomical regions of the fallopian tube (fimbria, ampulla, and isthmus), subtle segment-specific transcriptomic differences have been observed. For instance, PGR expression is higher in ciliated cells of the isthmus, aligning with its known role in regulating ciliary motility for embryo transport to the uterus. Conversely, cells in the fimbria exhibited elevated expression of oxidative stress-related genes (e.g. TXNIP ), ovarian cancer-associated markers (e.g. BIRC3 ), and pro-inflammatory cytokines, such as CXCL1 , CXCL8, and CCL20 , reflecting the inflammatory microenvironment induced by ovulation. Hu et al. [ 17 ] performed an scRNA-seq analysis of 10 fallopian tube samples. Based on the expression of known marker genes, epithelial cells were broadly classified into secretory and ciliated cells, and secretory cells were further subdivided into 4 distinct clusters. Using the transcriptional signatures derived from these clusters, the authors constructed a reference matrix, which they then applied to bulk RNA sequencing data from The Cancer Genome Atlas (TCGA) and Australian Ovarian Cancer Study (AOCS) cohorts. TCGA samples were classified into one of the secretory cell subclusters, all of which were HGSC. In contrast, AOCS samples were also classified into secretory cell clusters and a subset was classified into the ciliated cell category. Most cases classified as ciliated cells have been reported as low-grade serous carcinoma (LGSC). These findings suggest that HGSC and LGSC arise from secretory and ciliated cells of the fallopian tube epithelium, respectively.

SCOUT refers to the regionally localized proliferation of secretory cells in the fallopian tube. It was proposed to be a phenomenon based on morphological analysis, specifically characterized by an increase in secretory cells. In relation to the p53 signature, which will be discussed later and shows no formation of ciliated cells, it has been investigated whether p53 abnormalities interrupt the normal process of ciliated cell differentiation or whether such abnormalities occur exclusively in specialized cells that do not belong to the ciliated cell differentiation pathway [ 18 ]. Specifically, SCOUTs are linear arrays of more than 30 non-ciliated cells. This broad concept originally included p53 signatures based on morphological definitions. It has been observed in the general population and in patients with BRCA mutations but is reported to be significantly more common in cases of ovarian HGSC [ 19 ]. A decrease in PAX2 expression is observed in approximately 90% of SCOUTs; among these, p53 abnormalities are classified as p53 signatures [ 18 ]. Ning et al. [ 20 ] further classified SCOUTs into 2 categories based on their morphological characteristics. Type I SCOUTs have a typical single- or double-layered tubal epithelium, consisting of either a layer of non-ciliated tubal cells or a mixture of non-ciliated and ciliated cells. In contrast, Type II SCOUTs are characterized by proliferation with mildly pseudostratified, closely arranged, elongated fusiform nuclei, resembling the endometrial epithelium. Molecular abnormalities of each lesion are also shown, with Type I SCOUT reporting the negative expression of ALDH1; weak or negative expression of LEF1, RCN1, RUNX2, and EZH2; and membrane expression of β-catenin. Type II SCOUT is characterized by the positive expression of ALDH1, LEF1, RCN1, EZH2, RUNX2, stathmin, and β-catenin in the nucleus and cytoplasm. A model for classifying PAX2-negative fallopian tube lesions based on a molecular marker analysis was presented, concluding that Type I SCOUTs are closest to the normal differentiation pathway, similar to Walthard cell nests. Conversely, it was proposed that Type II SCOUTs fall into the same category as precancerous lesions of HGSC, such as serous tubal intraepithelial lesions (STILs) and serous tubal intraepithelial carcinomas (STICs). It is important to note that, while SCOUTs are classified based on detailed molecular analyses, Ning et al. [ 20 ] did not explicitly address where p53 signatures—initially considered a subset of SCOUTs—would fall within this classification ( Fig. 2 ). SCOUT, secretory cell outgrowth. The idea of dividing SCOUTs into 2 categories based on the detailed analysis of molecular expression is innovative, but Type I SCOUTs are allowed to contain ciliated cells and SCOUTs with positive PAX2 expression (more than 30 non-lineage cells in a continuum as previously defined), which has undeniably made the concept of an abnormal fallopian tube cell population more difficult. An interesting report suggests that Type II SCOUTs, based on their morphological features and molecular abnormalities (aberrant β-catenin expression and CTNNB1 mutations), may represent precancerous lesions of tubal endometrioid carcinoma. Brouwer et al. [ 21 ] also demonstrated an association between these lesions and endometrioid carcinomas arising in gynecological organs. Currently, it is generally accepted that only lesions with decreased PAX2 expression are classified as SCOUTs, and some studies refer to them as “stem cell outgrowths” based on their stem cell-like properties [ 22 23 ]. While a groundbreaking concept, the definition remains ambiguous at present, and its clinical significance is not yet fully understood. Further investigation is warranted in the future. Type I SCOUTs also encompass papillary tubal hyperplasia (PTH), which is discussed later.

p53 signature is defined as “a lesion with at least 12 linear tubal secretory cells that show no morphological atypia but express p53 in a mutant pattern by immunohistochemistry” ( Fig. 3A-D ). This concept was proposed by Lee et al. [ 24 ] in 2007 in a detailed study on the pathogenesis of ovarian HGSC, a highly aggressive and lethal form of ovarian cancer. This landmark report identified the p53 signature to be closely associated with STIC, considered to be the immediate precursor lesion to HGSC, and showed that both shared common TP53 mutations [ 1 2 25 ]. In addition, these lesions were observed at a significantly higher frequency in risk-reducing bilateral salpingo-oophorectomy (RRSO) specimens from women with germline mutations in BRCA1/2 , which confer a high risk of ovarian and fallopian tube cancer [ 26 27 28 ]. These compelling studies have led to the widely accepted theory that the p53 signature represents an early step in the pathway leading to HGSCs, with progression occurring through the development of STIC. This theory is biologically plausible and straightforward because TP53 mutations are indeed the most characteristic and common genomic abnormality in HGSC tumorigenesis, often occurring early in the disease process and driving aggressive behavior [ 29 30 ]. H&E, hematoxylin and eosin; STIC, serous tubal intraepithelial carcinoma; STIL, serous tubal intraepithelial lesion. Conversely, there is a tendency to question the universality of this precursor-product relationship. This skepticism arises primarily from the observation that p53 signatures are relatively common, found in approximately 10%–30% of the general population, including women without any increased risk of ovarian cancer [ 24 27 31 ]. Furthermore, the vast majority of cases with the p53 signatures do not develop HGSC. This discrepancy suggests that the p53 signature may not inevitably progress to malignancy, and that additional factors are involved in the transformation process. At least some p53 signatures are considered to represent benign or adaptive responses of the tubal epithelium to hormonal, age-related, or environmental changes, rather than definitive steps towards cancer. Against this complex and often contradictory background, the precise clinical significance and role of the p53 signature in the pathogenesis of HGSC are still under investigation and are the subject of considerable debate among researchers and clinicians. To address this problem, novel studies have focused on variants of TP53 mutations in the p53 signature. Akahane et al. [ 32 ] suggested that by analyzing TP53 sequences, it is possible to identify those at a high risk of progression to malignant lesions. It is highly desirable to elucidate the significance of the relatively common phenomenon of p53 signature and the mechanism underlying progression to HGSC.

STILs are regarded as precancerous lesions of HGSC. They are considered a precursor to STIC and histologically defined as an intraepithelial lesion of the fallopian tube that exhibits cytologic atypia that is insufficient for the diagnosis of STIC [ 1 2 25 ]. Specifically, an increased nuclear-to-cytoplasmic ratio and chromatin abnormalities may be permitted; however, a significant loss of nuclear polarity is not acceptable for STIL. Immunohistochemically, STIL typically demonstrates aberrant p53 expression and a mildly elevated Ki-67 labeling index ( Fig. 3E-H ). In contrast, STIC is characterized by marked nuclear atypia, comparable to that observed in HGSC. Recognition of such pronounced atypia is essential for a pathological evaluation. Some studies have proposed diagnostic algorithms and flowcharts to clarify the classification of p53-aberrant lesions [ 33 34 ]. STIL is considered an intermediate lesion between the p53 signature and STIC, and it has been proposed that the p53 signature and STIL together comprise a spectrum of early serous proliferations (ESPs) [ 35 ]. While it is well established that STIC constitutes the most frequent precursor lesion to HGSC, several issues remain unresolved. The foremost concern is the diagnostic reproducibility. Given that STIL and HGSC may represent a morphological continuum, consistent diagnostic criteria, especially for atypical nuclear features, are essential, but challenging to implement uniformly across observers. STICs exhibiting conspicuous architectural features such as marked polarity loss or papillary protrusion into the lumen are more readily identified; however, borderline cases between STIL and STIC can be diagnostically ambiguous [ 36 ]. Another unresolved issue is the malignant potential of isolated STIC. Although a progression risk of 7.1% at 3 years and up to 27.5% with a 10-year follow-up has been reported, further longitudinal studies are necessary to validate this figure [ 37 38 ]. In addition, the concept of precursor escape has emerged, suggesting that the p53 signature or STILs may exfoliate into the peritoneal cavity and directly give rise to HGSC or primary peritoneal carcinoma, even in the absence of an identifiable STIC [ 39 40 ]. Finally, the clinical management following the STIC diagnosis remains undefined. Currently, there are no standardized treatment or surveillance protocols. BRCA1/2 genetic testing and semiannual follow-up, including gynecologic examinations, blood tumor marker measurement, and expert transvaginal ultrasonography, have been proposed; however, these strategies require further validation [ 41 42 ]. Several studies utilizing spatial transcriptomic technologies have revealed detailed genomic abnormalities and heterogeneity within STIC [ 22 43 ]. The expression of IGFBP2 is reportedly associated with the risk of progression from STIC to HGSC. Furthermore, based on transcriptional profiles, STICs have been classified into 4 subtypes, illustrating a continuum from dormant STICs, more closely resembling normal fallopian tube epithelium, to proliferative STICs with a higher risk of progression to HGSC. Interestingly, copy number alterations were also identified, with upregulation of the 8q24 region, including the MYC gene, which is commonly observed in both HGSC and high-risk STICs. Morphologically, dormant STICs exhibit mild atypia, whereas proliferative STICs display marked nuclear atypia. These findings suggest that STICs can be further stratified based on malignancy potential and that such stratification may serve as a biomarker for assessing the risk of progression to HGSC. In conclusion, we have provided an overview of fallopian tube lesions as precursor lesions of HGSC. Accumulating evidence suggests that the classical linear sequence from the p53 signature through STIL to STIC and ultimately to HGSC may not fully capture the complexity of HGSC pathogenesis. The current WHO classification emphasizes the importance of thorough examination of fallopian tubes when evaluating HGSC, particularly in the context of RRSO [ 44 ]. As fallopian tube sampling becomes increasingly routine in gynecologic pathology, it is anticipated that further studies will clarify the roles and interrelationships of these precursor lesions in HGSC development. Although diagnostic algorithms integrating morphologic and immunohistochemical features exist, interobserver reproducibility remains limited in distinguishing STIL from STIC, particularly with regard to the assessment of nuclear atypia comparable to that seen in HGSC, underscoring the need for more objective and reproducible diagnostic criteria and evidence-based management strategies.

β-Catenin signature is a unique concept discovered by our group [ 45 ]. This refers to a phenomenon in the fallopian tube epithelium characterized by an immunohistochemically abnormal expression of β-catenin (nuclear and/or cytoplasmic expression) in a linear array of 12 or more consecutive cells ( Fig. 4A-D ). We identified β-catenin signatures in approximately 20% of the fallopian tubes in the general population. Among these, hotspot mutations in CTNNB1 have been identified in a subset. At first glance, this phenomenon resembles the p53 signature; however, its implications are distinct. H&E, hematoxylin and eosin. First, tumors driven by CTNNB1 mutations specifically arising in the fallopian tube are exceedingly rare, and such mutations are almost never observed in HGSC [ 46 ]. Second, the fact that β-catenin signatures were significantly more frequent in younger individuals is particularly intriguing. This suggests that the phenomenon is distinct from age-related genomic alterations and may indicate the presence of a mechanism for eliminating mutated cells. β-catenin abnormalities are associated with Type II SCOUTs. Although a portion of β-catenin signatures morphologically corresponded to Type II SCOUTs, many exhibited different histological characteristics. One important distinction is that, whereas Brouwer et al. [ 21 ] initiated their study from the identification of Type II SCOUTs, our group began their investigation by focusing on β-catenin-aberrant cells. Although the clinical significance of β-catenin signatures remains unclear, these findings underscore the diversity of genomic alterations that occur in the fallopian tube. The presence of genetically abnormal cell clusters that are likely not precancerous raises the fundamental question of what truly defines “cancer.” Although β-catenin signatures share certain morphologic and molecular features with Type II SCOUTs and endometrioid tubal intraepithelial neoplasia (E-TIN), as discussed below, their relatively frequent occurrence, together with the extreme rarity of corresponding malignant counterparts, suggests that they may constitute a biologically distinct category of genetically altered epithelial cell clusters rather than obligate precursors of endometrioid-type malignancies.

E-TIN refers to lesions among Type II SCOUTs that exhibit structural atypia resembling endometrioid hyperplasia. Hecht et al. [ 47 ] conducted a histological analysis of 5,190 pairs of fimbrial sections and identified 4 cases of E-TIN. Of these, 3 cases were associated with neoplastic lesions in the endometrium (endometrioid intraepithelial neoplasm and endometrioid carcinoma). Subsequent reports described additional cases, although no instances of recurrence following resection have been documented to date [ 48 49 ]. E-TIN is considered a precursor lesion that arises in continuity with Type II SCOUTs and may progress to tubal endometrioid carcinoma. Given that SCOUTs and β-catenin signatures are relatively common findings in the fallopian tubes of the general population, whereas E-TIN and tubal endometrioid carcinoma remain exceedingly rare entities (with Hecht et al. [ 47 ] reporting only one case of tubal carcinoma), it is conceivable that these lesions do not necessarily exist along a continuous neoplastic sequence. This situation is somewhat analogous to the relationship between p53 signatures, STIC, and HGSC, although E-TIN and tubal endometrioid carcinoma are far rarer. In recent years, the phenomenon of synchronous endometrial and ovarian carcinomas has been increasingly recognized and studied [ 50 51 ]. Given the anatomical and logical connections between the uterus and ovaries, the fallopian tube has garnered attention as a potential conduit that links these 2 sites. However, the role of the fallopian tube in endometrioid tumorigenesis remains an area of ongoing research. Morphologically, tubal endometriosis and tubal involvement by endometrioid carcinoma of the uterine corpus are important differential diagnoses for E-TIN ( Fig. 4E and F ). In addition, retrograde migration of shed endometrial tissue into the fallopian tube during menstruation can sometimes mimic endometrioid tumors, making the distinction challenging in certain cases. An accurate diagnosis requires careful consideration of clinical information, the presence or absence of endometrial stroma, and the degree of cytological atypia. It is important to recognize that E-TIN is extremely rare.

PTH is a histopathological entity characterized by the proliferation of tubal epithelial cells exhibiting mild to moderate nuclear atypia arranged in small papillary architectures ( Fig. 5 ) [ 52 ]. PTH, papillary tubal hyperplasia. Similar lesions have previously been referred to in various terms, including proliferative epithelial lesion, epithelial hyperplasia, and mucosal epithelial proliferation [ 53 54 55 ]. In 2011, Kurman et al. [ 52 ] introduced a more stringent definition and proposed the concept of PTH, defined as tubal proliferations exhibiting papillary tufting and detached clusters of bland epithelium. It is often associated with psammoma bodies. Traditionally, such changes have been regarded as reactive phenomena, attributed to inflammation or hormonal fluctuations, and are not considered to be associated with neoplastic processes. However, Robey and Silva [ 54 ] conducted a comprehensive analysis and proposed a potential association between these lesions and LGSC. Building on this hypothesis, Kurman et al. [ 52 ] examined a cohort of 1,000 LGSC cases and identified 22 cases of tubal implants. These cases, along with an additional 7 cases of PTH, were subjected to further analyses. Notably, PTH was identified in 20 of the 22 tubal implants cases. Furthermore, in cases of PTH not associated with carcinoma, a high prevalence of concurrent pelvic inflammatory changes and endosalpingiosis was observed. Given that KRAS mutations are frequently detected in LGSC, the study has examined the presence of KRAS mutations in PTH associated with LGSC. However, no KRAS mutations have been demonstrated in PTH [ 56 ]. In contrast, KRAS mutations have been reported in endosalpingiosis in the study of Chui and Shih [ 57 ]. Although there is little controversy regarding serous borderline tumors (SBTs) as precursors of LGSC, whether PTH or endosalpingiosis may represent earlier precursor lesions of SBT remains unclear. Further investigations are warranted to clarify the potential tumorigenic sequence.

We have provided an overview of the complex and poorly understood fallopian tube lesions ( Fig. 6 ). Although much remains unknown, we hope that this will serve to consolidate the current body of knowledge. The widely accepted theory that HGSC originates in the fallopian tube begins with meticulous and detailed histopathological analyses. By accurately recognizing and analyzing abnormalities, further insights into fallopian tube pathology will continue to accumulate. ESP, early serous proliferation; HGSC, high-grade serous carcinoma; PTH, papillary tubal hyperplasia; SCOUT, secretory cell outgrowth; STIC, serous tubal intraepithelial carcinoma; STIN, serous tubal intraepithelial neoplasm.