Renewal of spermatogonia in man

The spermatogonia of normal adult human testis were investigated in view of clarifying their mode of proliferation and renewal. Three main types of spermatogonia were identified: the dark type A spermatogonia ( Ad ) tentatively considered as the stem cells, the pale type A spermatogonia ( Ap ) and the type B spermatogonia (B), these being the more and more differentiated elements giving rise to preleptotene spermatocytes. The dark and pale type A spermatogonia were present in all stages of the cycle of the seminiferous epithelium, the type B spermatogonia were found in stages VI, I and II of the cycle and the preleptotene spermatocytes in stages III and IV of the cycle. The type A spermatogonia divided preferentially in stage V of the cycle and the type B spermatogonia in stage II of the cycle. Quantitative data on spermatogonia and preleptotene spermatocytes revealed that the cell ratio Ad: Ap: B: Pl was equal to 1:1:2:4. This indicated that the spermatogonial stem cells divided to produce equal numbers of new stem cells ( Ad ) and of the more differentiated pale type A spermatogonia ( Ap ). Each one of the latter gave rise to two type B spermatogonia which in turn produced four spermatocytes. The arrangement in pairs of the dark and pale type A spermatogonia throughout the duration of the cycle indicated that the mitoses of spermatogonial stem cells are “equivalent” in nature; therefore, the possibility of having “differential” mitoses to explain the renewal of spermatogonial stem cells should be abandoned. Lastly, the frequent arrangement of the two classes of type A spermatogonia in homogeneous clusters indicated that the impetus which facilitates the differentiation of stem cells into the more differentiated elements ( Ap ) may affect homogeneous and compact groups of stem cells.