An electron microscopic study of the human infundibulum.

Abstract

An electron microscopic analysis was performed on biopsies of the infundibulum from humans undergoing hypophysectomy. Two types of nerve fibers can be distinguished by their dense core vesicles, both of which terminate in the perivascular space of small vessels within the infundibulum. Type A fibers contain dense core vesicles measuring 1,500–3,000 A in diameter; type B fibers contain dense core vesicles measuring 500–1,000 A in diameter. Smaller clear vesicles (200–500 A) are found within the nerve endings in an inverse proportion to dense core vesicles. Herring bodies contain either type A or type B dense core vesicles, but frequently are filled with non-neurosecretory elements (mitochondria, dense bodies) which can also be found in nerve endings. These observations support other evidence that two types of neural control are involved in anterior pituitary regulation, but a more precise correlation between structure and function is not possible. The vascular bed of the neural infundibulum is characterized by blood vessels whose structure ranks them with venules. Short amuscular capillary segments show a cuff formed by pituicytes reminiscent of that formed by astrocytes around cerebral capillaries.