The neurohemal contact area of the median eminence was examined in adult, young, neonatal, and fetal rats in freeze-fracture preparations. While no specializations of the membranes of axonic terminals abutting on the perivascular space were observed, adjacent areas of the same membranes are rich in signs of hormone release. Signs of exocytosis are defined in the manner of Theodosis et al. (1978). Exocytotic pits with a single dense granule as a core are identified on P-faces along with mounds on corresponding E faces. These features appear near the time of birth, are especially numerous at 3 days, 3 weeks, and 5 weeks, and continue in lesser numbers into adulthood. Even more numerous and appearing even earlier, by the last day of fetal life, were P-face clusters of large particles usually in a pit, and pits without particles. These fall into 2 significantly different categories distinguished by the size of the pit. E face counterparts are large and small protuberances, respectively. Fenestrae of adult size and distribution are observed along the endothelium of portal vessels from the 20th day of fetal life on. Their frequency correlates well with other structural manifestations of a median eminence ready for the onset of functional activity at about the time of birth (Monroe and Paull 1974).
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