The neurons of the arcuate nucleus were studied ultrastructurally in female hamsters that were given saline or 1 mg mercuric chloride/day during one estrous cycle. Membranous whorls, composed of 4–26 concentric layers of smooth endoplasmic reticulum, were found in 7–8 of the neurons of control animals in diestrus (D3) and in proestrus (D4). Whorls were found in 5–16 of the neurons in mercury-treated animals on all days of the experimental period. The whorls appeared to be associated with RER and Golgi bodies. The levels of FSH and LH were measured in the plasma and pituitaries of both groups of animals. There was a significantly higher concentration of FSH in the pituitaries of experimental animals on D3 and of LH on D1C2 of the subsequent cycle; plasma levels of FSH and LH were similar in both groups throughout the experimental period. The relationship between the levels of gonadotropins and the presence of membranous whorls is disc
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