Spermatogonia of mammals are always found adjacent to the basement membrane of the seminiferous tubule and are classified as either stem cells or differentiated sper-matogonia (19-26). The first category, stem cells, are called type A spermatogonia, whereas differentiated spermatogonia are designated as type B spermatogonia. Type A spermatogonia in primates are further divided into dark type A, or Ad, spermatogonia, and pale type A, or Ap, spermatogonia (17,19-26). The Ap spermatogonia divide to produce either more Ap spermatogonia,3 or the first generation of differentiated type B spermatogonia. Four generations of type B spermatogonia, designated B1, B2, B3, and B4, are present in the seminiferous epithelium of nonhuman primates of the Catarrhini. Humans are an exception in this regard, because only one generation of type B spermatogonia has been described in the seminiferous epithelia of men (16-18,23,24,26).
3 The important topic of spermatogonial stem cells and the process of stem cell renewal is discussed in the section entitled Stem Cell Renewal.
The differentiated spermatogonia are committed to produce more type B spermatogonia. These germ cells are diploid and are destined to either proliferate or, if they fail to proliferate, die. The mitoses of the differentiated spermatogonia are symmetrical; the two daughter cells are identical to one another, but the nuclei have smaller diameters and contain more heterochromatin than those of the mother cell.
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