From July to March, the testis of the spring-spawning freshwater goby Padogobius martensi is characterized by spermatogonial proliferation. A close correlation exists among type of proliferating spermatogonia, gonado-somatic (IG) profiles and morphological and functional variations of the Leydig cells. The IG reach their minimal levels by the end of summer and increase progressively but modestly during autumn and winter. Declining IG levels are associated with proliferation of primary spermatogonia only, whereas increasing IG levels are associated with predominant proliferation of secondary spermatogonia. Minimal IG levels are reached when the germinal epithelium is formed by a continuum of primary spermatogonia and associated Sertoli cells. The proliferation of secondary spermatogonia begins only at this time. Spermatogenesis in autumn occurs when spermatogonial cysts contain at the most 16 cells and it rarely results in the maturation of several cysts so that the amount of sperm cells produced is either negligible or scarce. A number of degenerating cells are usually present within the spermatogonial and meiotic cysts. Leydig cells are the unique cells that display features of steroidogenic cells: mitochondria with tubular cristae, extensive smooth endoplasmic reticulum (SER), 3ß-hydroxysteroid dehydrogenase (3ß-HSD) and glucose-6-phosphate dehydrogenase (G6PD) activity and sudanophilia. Light and dark Leydig cell varieties are always present. During regression, Leydig cells undergo a marked decrease in SER amount, mitochondrial sizes and number of mitochondrial cristae. In parallel, the 3ß-HSD and G6PD activities and sudanophilia decrease progressively until they become undetectable by the end of regression. In autumn, mitochondria increase in size, reaching sizes similar to those observed at the end of the spawning season in the light cells, but not in the dark cells. The SER, on the contrary, undergoes a modest and irregular increase only in a part of the Leydig cells, mostly of the light type. In parallel, the 3ß-HSD and G6PD activities increase until they become moderately intense by the end of autumn. At the end of winter, the SER is extensive and regularly dilated in both Leydig cell types, whereas mitochondria still have sizes similar to those observed in December. The 3ß-HSD and G6PD activities are strong and sudanophilia is again detectable. Sertoli cells undergo changes in shape and position in relation to the proliferation of primary spermatogonia and the development of cysts. A junction modulation occurs in association with these changes. Sertoli cells also undergo changes indicative of a decrease in activity immediately after spawning (loss of mitochondrial cristae and clarification of the mitochondrial matrix) and of an increase in activity by the end of the regressing phase (darkening of the mitochondrial matrix and increase in mitochondrial cristae, rough endoplasmic reticulum (RER) and free ribosomes). In addition, they are involved in the phagocytosis of degenerating germ cells at all stages of their development. Macrophages are found in the testis interstitium only, where they are usually adjacent to Leydig cells, myoid cells and blood capillaries and do not participate in the phagocytosis of degenerating germ cells. Myoid cells do not undergo ultrastructural changes except for an increase in the amount of heterochromatin by the end of spawning. The meaning of the autumnal spermatogenic wave and the relationships between the development of the germinal epithelium and the changes of the Leydig and Sertoli cells are discussed.
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