Spermatogonial stem cells in rodents and other nonprimate mammals

As in all renewing tissues, stem cells are at the basis of the spermatogenic process. Spermatogonial stem cells are single cells located on the basal membrane of the seminiferous tubules and are called A-single (As) spermatogo-

From: Adult Stem Cells Edited by: K. Turksen © Humana Press Inc., Totowa, NJ

Fig. 1. (A) Section of a mouse testis showing cross sections through seminiferous tubules in the wall where spermatogenesis takes place. In between the seminiferous tubules, there is the interstitial tissue. (B) Part of a seminiferous tubule and interstitial tissue showing the cell types in the testis: arrowheads, Sertoli cells; asterisks, Leydig cells in the interstitial tissue; arrows, spermatogonia; c, spermatocytes; r, round spermatids; e, elongated spermatids.

Fig. 1. (A) Section of a mouse testis showing cross sections through seminiferous tubules in the wall where spermatogenesis takes place. In between the seminiferous tubules, there is the interstitial tissue. (B) Part of a seminiferous tubule and interstitial tissue showing the cell types in the testis: arrowheads, Sertoli cells; asterisks, Leydig cells in the interstitial tissue; arrows, spermatogonia; c, spermatocytes; r, round spermatids; e, elongated spermatids.

nia (4-7). On division, spermatogonial stem cells give rise either to two new single cells or to a pair of daughter cells (Apr spermatogonia) that do not complete cytokinesis and stay connected by an intercellular bridge (8,9). In all further divisions starting with the pair, cytokinesis will also be incomplete, leading to the formation of increasingly large syncytia of germ cells. Hence, all differentiating progeny of a spermatogonial stem cell will stay connected by intercellular bridges, which is a unique characteristic compared to other renewing tissues. As Apr spermatogonia are morphologically similar to As spermatogonia, the intercellular bridge is the first visible expression of the entrance into the differentiation pathway.

It is not known yet whether the divisions of spermatogonial stem cells are symmetrical (7). If the stem cells produce either two new stem cells or a differentiating pair, the divisions can be called symmetrical. However, preceding such a division, there might be one in which one of the daughter cells remains in the stem line and the other may already be predestined to produce a pair at its next division.

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