A special cranial nucleus (CSF-contacting nucleus) in primates

Background: In higher-order animal brains, twelve pairs of cranial nuclei are known, each performing specific functions. Past 30 years of our studies on rat brain reveal that there is a unique nucleus (CSF-contacting nucleus) in the brain. The extraordinary feature of this unique nucleus is that it is not connected to any substantial organ but to the CSF. The identification of this special nucleus will provide the structural basis for information transmission and functional modulation between the neurons and the body fluids. Methods: The present study is based on the past 30 years of our relevant achievements. In this study, we used the primate rhesus monkeys as the model and injected the tracer CB-HRP into the CSF. After 48 h, the monkeys were perfused and the brain was dissected out, and sectioned for CB staining. The CB positive structures were observed under confocal and electron microscopy. The three-dimensional (3D) structure of the CB positive neurons cluster was reconstructed by computer software. Results: Our results show that (1) CB labelling is confined within the ventricle, but not leakage into the brain parenchyma. (2) From the midbrain inferior colliculus superior border plane ventral to the aqueduct to the upper part of the fourth ventricle floor, a large number of CB positive neurons are consistently located, form a cluster, and are symmetrically located on both sides of the midline. (3) 3D reconstruction shows that the CB positive neurons cluster in the monkey brain occupies certain space. The rostral part is large and caudal part is thin appearing a “rivet”-like shape. (4) Under electron microscopy, the CB positive neurons show different types of synaptic connections with the non-CSF-contacting structures in the brain. Some of the processes stretch directly into the ventricle cavity. Conclusions: The CSF-contacting nucleus is existed objectively in primates. It is also regarded as the XIII pair of cranial nucleus (CSF-contacting nucleus), which is specialised in the communications between the nerve and the CSF. Our results are crucial for completing the neural structures and understanding the coordination of the entire body functions.