Further Observations on the Numbers of Spermatogonia, Spermatocytes, and Spermatids Connected by Intercellular Bridges in the Mammalian Testis1

Abstract

The presence of true intercellular bridges connecting groups of spermatocytes and spermatids in the mammalian testis has been described repeatedly in electron microscopic studies, and recently Gondos and Zemjanis (1970) have reported pairs of interconnected spermatogonia in monkey testis. The present paper describes the fine structure of bridges between developing germ cells in a variety of mammalian species and considers the numbers of cells involved and the significance of this syncytial arrangement in the overall organization of the seminiferous epithelium. In addition to spermatocytes and spermatids, spermatogonia type A and type B were also found to be joined by bridges. In rodents the type A have oval nuclei containing a uniformly granular nucleoplasm and one or more nucleoli at or near the nuclear membrane. The type B possess spherical nuclei with several clumps of chromatin near the periphery and additional small clumps associated with a centrally placed nucleolus. If a pair of connected type A spermatogonia initiates spermatogenesis and the daughter cells of all subsequent divisions remain connected, then one would expect after the eight ensuing mitotic and meiotic divisions, hundreds of conjoined spermatids. The theoretical maximum numbers are probably never achieved since considerable cell death is known to occur normally at certain stages of the spermatogenic cycle. Degenerations are invariably associated with divisions but it appears that some members of a group of conjoined cells may degenerate without the degenerative process extending to the entire group. A possible explanation for this limitation of the process resides in transient specializations of the preexisting intercellular bridges during division that temporarily isolate cell territories within the syncytium. This consists of a series of 8–12 parallel cisternae oriented perpendicular to the long axis of the bridge. These cisternae are separated by intervening layers of relatively dense cytoplasmic material. Since these septa appear to interrupt cytoplasmic continuity, it is possible that they may prevent spread of degenerative changes throughout the syncytium and permit selective degeneration of certain cellular units. The total number of viable spermatids which remains interconnected is indeterminate, and probably variable, but may well be as many as a hundred.