The median eminence in normal, ovariectomized, and ovariectomized‐estradiol‐treated hamsters: An ultrastructural study

Abstract

In an effort to define more clearly the effect various plasma concentrations of estrogen have on the morphology and function of tanycytes, the present investigation examined the median eminence (ME) of normally cycling, ovariectomized, and ovariectomized-estradiol-treated hamsters. In normally-cycling animals, when endogenous estrogen was at its highest level (day 4 or proestrus), numerous microappendages arose from the luminal surfaces of tanycytes located in the ventrolateral region of the ME. Large blebs (1.0-5.0-micrometers diameter), miniblebs (1.0-micrometer diameter), and microvilli dominated the surfaces of these cells. Large blebs appeared to have been formed by the coalescence of several miniblebs and were composed of cytoplasmic ground substance devoid of organelles. The peduncular shape of many of these blebs suggested their involvement in an apocrinelike secretion by the tanycyte. When endogenous estrogen levels were low (day 1 of the estrous cycle), the tanycytes of normally cycling hamsters possessed slightly fewer microappendages. Following ovariectomy, large blebs were nearly absent from the luminal surfaces of tanycytes, and the number of miniblebs and microvilli were also greatly reduced. Subcutaneous injections of 17-beta estradiol benzoate restored the large blebs to the tanycyte surface. The number and variety of tanycytic microappendages in these animals resembled those in normally cycling hamsters on day 4 of the estrous cycle. The present study demonstrates that tanycytes of the hamster ME are sensitive to estrogen and vary in their morphology in relation to the animal's reproductive status. These changes in tanycyte morphology can be correlated directly to functions of absorption (microvilli) and secretion (blebs). The sensitivity of tanycytes to estrogen suggests that these cells may also play a role in the hypophyseal-ovarian feedback mechanism.

Ultrastructural localization of vimentin immunoreactivity and gene expression in tanycytes and their alterations in hamsters kept under different photoperiods

“…In hamsters, especially, surface morphology of tanycytes has been reported to change in relation to the animal's reproductive status (Merchant and Dollar 1981). Nerve terminals of LHRH neurons are concentrated in the external zone of the median eminence around the tuberoinfundibular sulcus, in which the terminals of tanycytic processes are also concentrated (Kozlowski and Coates 1985).…”

Dendritic growth of arcuate neuroendocrine neurons following orchidectomy in adult rats

“…Previous studies have shown that castration results in ultrastructural changes in tanycytes and nerve terminals in the rat median eminence (Stoeckart et al, 1974(Stoeckart et al, , 1975Merchant and Dollar, 1981;Coates and Davis, 1982;King and Letourneau, 1994). Because Fluoro-Gold uptake by arcuate neuroendocrine neurons is critically dependent on the relationship between fenestrated capillaries and nerve terminals in the median eminence, such alterations could lead to differential sampling of neuroendocrine neurons between experimental and control groups.…”