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As Sertoli cells normally stop dividing by Day 21, we did not count the number of Sertoli cells in adult FORKO mice in this study; there was no evidence of mitosis of Sertoli cell nuclei at any ages examined in the present investigation. The finding that some tubules in a cross-section appear normal while others show varying degrees of altered epithelium is apparently not unique to the FORKO testis and has also been reported for other knockout mouse models. While the underlying reasons for such a phenomenon are still not fully understood, several speculations can be advanced. It is possible that the protein(s) regulated by the FSH-R cascade and that are affected in FORKO mice may reside in only some Sertoli cells and not in others. It is also possible that not all Sertoli cells encompassing a given tubule express the FSH receptor, or equal amounts of it, at the same time. Alternately, there may be different subpopulations among Sertoli cells that are functionally different from the others. Thus, one population may be involved more in fluid transport, while another in elevated protein synthesis, and yet another in endocytosis.