Figure 219

Schematic diagram illustrating the generations of spermatogenic cells. This diagram shows the clonal nature of the successive generations of spermatogenic cells. Cytoplasmic division is complete only in the primitive type A dark spermatogonia that serve as stem cells. All other spermatogenic cells remain connected by intercellular bridges as they undergo mitotic and meiotic division and differentiation of the spermatids. The cells separate into individual spermatozoa as they are released from the seminiferous epithelium. The residual bodies remain connected and are phagocytosed by the Sertoli cells. (From Dym M, Fawcett DW. Biol Reprocl 1971;4:195-215.)

residual bodies spermatozoa residual bodies spermatozoa type A dark spermatogonia type A pale spermatogonia

late spermatids type A pale spermatogonia secondary spermatocytes early spermatids type B dark spermatogonia primary spermatocytes late spermatids type A dark spermatogonia type A pale spermatogonia tvoe A pale spermatogonia type A pale spermatogonia type A pale spermatogonia secondary spermatocytes early spermatids type B dark spermatogonia primary spermatocytes the progeny of an initial pair of Ap spermatogonia are connected, much like a strand of pearls. These cytoplasmic connections remain intact to the last stages of spermatid maturation and are essential for the synchronous development of each clone from an original pair of Ap cells.

After several divisions, type A spermatogonia differentiate into type B spermatogonia. The appearance of type B spermatogonia represents the last event in the spermatogo-nial phase.

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