The morphology and kinetics of spermatogonial degeneration in normal adult rats: An analysis using a simplified classification of the germinal epithelium

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Abstract
The phenomena of spermatogonial degeneration have been studied in normal adult rat testes using a simplified classification of the germinal epi-thelium based upon the six types of differentiating spermatogonia. The following features distinguished this from schemes based on acrosome development. Rather than 14 stages of unequal duration, there are only six stages, five of which are of the same length. The classification starts at the beginning of spermatogenesis with A1 spermatogonia rather than at the onset of spermiogenesis. The classification is derived from actual biological events in spermatogenesis, namely generation times of spermatogonia, rather than upon arbitrary events in acrosome development. Most importantly, this new classification can be used with most types of preparations and in most experimental conditions. Examination of tubular whole mounts reveals that degeneration preferentially occurs in types A2 and A3 and to a lesser extent A4 spermatogonia, and is rarely seen in generations of A1 In or B cells. Deterioration is first manifested in clusters of cells joined by the intercellular bridges as they complete DNA synthesis and enter the G2 phase of cell cycle. It is characterized by a denser staining of the nuclear membrane, coalescence of chromatin into several pyknotic bodies, and eventual extrusion of the nuclear mass, leaving a cytoplasmic ghost. The sequential steps in degeneration may often be traced from one end of a synctial chain to the other, suggesting that the process may start with just one cell and then spread via intercellular bridges to involve all spermatogonia within the clone. Quantitatively, degeneration is a relatively constant feature of spermatogonial development. Only 25% of the theoretically possible number of pre-leptotene spermatocytes are produced from the original population of A1 spermatogonia; most of this loss is incurred during the maturation of A2 and A3 generations. While the reason for spermatogonial degeneration in the normal germinal epithe-lium remain obscure, it is proposed that the numerical ratio of A spermatogonia to Sertoli cells may be a significant limiting factor.