Innervatiuon of steroid-producing cells in the gonads of fish.

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The innervation of the gonads of Oreochromis niloticus examined immunohistochemically and ultrastructurally. In testis, many thick nerve bundles are distributed in the wall surrounding the efferent duct. The neurons appear to terminate at the surface of smooth muscle cells in the efferent duct wall. Thin nerve bundles penetrate the interstices among lobules containing cysts of spermatogenic germ cells. terminal ends of these nerve fibers also make contact with smooth muscle cells enclosing lobules and with the surface of steroid-producing cells. These observations suggest that innervation of the testis may not only control the contracting and relaxing of smooth muscle cells but also control steroid secretion. In ovary, thick nerve bundles were localized in the area near the ovarian artery and vein in the ovarian wall on the side facing the mesentery. Groups of a few axons ramified from the thick nerves and were terminated in the proximity of clusters of steroid-producing cells which are distributed in the interstitial area among yolky oocytes. The axon terminals were in intimate relation with the steroid-producing cells. The terminals contained many clear vesicles, a few dense-cored vesicles and some mitochondria. Moreover, a few terminals were observed on the surface of steroid-producing theca cells surrounding yolky oocytes. Our observations of direct innervation of steroid-producing cells bring to light a possible new avenue for regulation of steroid production in the tilapia ovary.