Estrogen-dependent changes in the functional interrelationships among neurons, ependymal cells and glial cells of the arcuate nucleus. Cytometric studies in the female albino mouse.

The synchronizing effect of ethinylestradiol (4 micrograms/g b.w.) on neurons of the arcuate nucleus 700-950 micrometers caudal to the posterior edge of the optic chiasma was studied by karyometry in 6-week-old albino mice during proestrus. The caudal portion of the arcuate nucleus was identified as the most estrogen-sensitive subdivision; all neurons showed an increase in their nuclear area (mean transect, profile area of the nucleus) 1 h following administration of ethinylestradiol. This hypothalamic region was selected for the subsequent electron-microscopic cytometric study to analyze functional interrelationships among neurons, ependymal cells and glial cells. Six and 12 days after ovariectomy no significant change in the nuclear area of neurons and ependymal cells was found 850-950 micrometers behind the posterior slope of the optic chiasma, but the neurons exhibited a decrease in the number of polyribosomes, the volume fraction (Vvmi) and the surface density of the inner membrane of mitochondria (Svmi). A similar decrease in Vvmi and Svmi was measured in the apical part of ependymal cells and in the pericapillary profiles of ependymal and glial cells, which was accompanied by a reduction in the surface density of ependymal processes extending into the ventricular lumen. In addition, no change of Vvmi and Svmi was seen in the basal subnuclear part of ependymal cells. This bipolar functional reaction of ependymal cells after ovariectomy is discussed as an indicator of ependymal control of neuronal activity by sequestering biologically active agents, e.g., transmitters of neurohormones, in their apical and basal extensions facing the ventricular surface or the pericapillary space.