The interstitial cells of the trout testis (Oncorhynchus mykiss): ultrastructural characterization and changes throughout the reproductive cycle.

The major non-vascular cell types present in the interstitial compartment in trout testes have been ultrastructurally characterized and cell changes in the course of the two first reproductive cycles have been studied. Three major cell types are always present fibroblasts, myoid cells and Leydig cells. Their structure varies with the maturational stage of the gonad. Fibroblasts are centrally located in the interstitial areas. Numerous typical myoid cells are always present near the basal lamina. However, at the end of a cycle some of them display degenerative changes, then disappear. At the beginning of the next cycle, fibroblasts appear to differentiate to new myoid cells. Leydig cells present before the start of the first spermatogenesis are replaced by new ones which probably also arise from fibroblasts. These cells progressively differentiate during spermatogenesis, so that most of the Leydig cells present during the spermiation phase are fully-differentiated steroidogenic cells. At the end of a cycle, a certain number of the Leydig cells disappear and are replaced at the beginning of the next cycle by new ones most likely derived from fibroblasts. The remaining Leydig cells dedifferentiate to cells which may redifferentiate during gonadal activity. Macrophages are present mostly at the end of a cycle and participate at this time in the removal of dead interstitial cells. In conclusion, at least some fibroblast-like cells are precursor cells, able to replace myoid cells and Leydig cells which disappear between two consecutive reproductive cycles.