Variation in the Stock of Testicular Stem Cells and in the Yield of Spermatogonial Divisions in Ram and Bull Testes*

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Abstract
Two types of stem spermatogonia- type A0 and A1 spermatogonia- have been observed in ram and bull testes. Type A0 spermatogonia are present in the impuberal testis. Their total number/testis does not vary greatly during life of bull. Type A0 spermatogonia give rise to type A1 spermatogonia which appear at the onset of spermatogenesis; their total number/testis increases during and after puberty. Total numbers of type A0 and A1 spermatogonia/testis are not correlated in the ram while total numbers of type A1 spermatogonia and Sertoli cells are correlated. Hemicastration performed in impuberal calves induces an increase in numbers of Sertoli cell/type A1 spermatogonia/testis. In adult ram, the total number of A0 and A1 spermatogonia decreases during the non sexual season and increase again in the sexual season. After hypophysectomy performed in adult rams, the number of A0 spermatogonia/testis is not modified while that of A1 spermatogonia decreases. In ram and bull, 6 spermatogonial generations were detected before the germ cells undergo meiosis. In the adult ram and bull, the type A1 spermatogonia originate mostly after divisions of A2 spermatogonia. Few of them arise from A0 spermatogonia. The yield of primary spermatocytes from spermatogonial divisions is very low, one third and one half of the expected number in the bull and ram respectively. In ram, thus yield positively correlated to LH plasma levels (0.5; P = 0.05). In hypophysectomized ram, this yield is nearly equal to zero.