Immunohistochemical study of human pineal vessels

BACKGROUND: The pineal gland is a neuroendocrine organ located in the epithalamic area of the brain. By using the melatonin, a pineal hormone, the pineal gland synchronizes the work of the internal physiological systems of the body with the circadian light-darkness cycle. Melatonin is synthesized in pinealocytes, the endocrine cells of the pineal gland, and secreted into the bloodstream. However, the structural features of the blood vessels in the pineal gland are still not well understood. AIM: The purpose of this study was to elucidate the intraorgan localization and immunohistochemical pattern of the blood vessels of the pineal gland of human, which had not been previously studied. MATERIALS AND METHODS: In the research, immunohistochemistry methods were applied using two selective markers of blood vessels, the antibodies to von Willebrand factor and type IV collagen. Von Willebrand factor is expressed selectively in endothelial cells that form blood vessels, including small capillaries, while type IV collagen is inherent to the basement membrane that separates the vascular endothelium from the underlying tissue. RESULTS: The immunohistochemical reaction to both markers clearly visualize the blood vessels of the human pineal gland, which in both cases were observed mainly in the connective tissue septa (trabeculae), or, in the absence of a regular lobular structure, in the connective tissue layers. In lobules surrounded by connective tissue trabeculae and containing a large number of densely packed pinealocytes, von Willebrand factor- and type IV collagen-immunoreactive structures were very rare, and in many cases were not observed. The found phenomenon of distribution of blood vessels in the human pineal gland is described for the first time. CONCLUSIONS: Since blood vessel markers with well-proven selectivity were used, the results obtained with their usage can be considered reliable; this gives grounds with a high degree of probability to assert that the majority of pinealocytes in the human pineal gland do not have direct contact with blood vessels and, accordingly, cannot secrete melatonin directly into the bloodstream. On the basis of the results obtained, a hypothesis is proposed that the hormone secretion from pinealocytes into blood vessels is mediated by astroglial cells.