Expression of p57kip2 in germ cells and Leydig cells in human testis.

p57kip2, a KIP family cyclin-dependent kinase (Cdk) inhibitor, blocks the cell cycle by acting on multiple cyclin-Cdk complexes. To investigate the role of p57kip2 in human fertility, the expression of p57kip2 was investigated in testes from normal and obstructive azoospermic male patients who were positive for p57kip2 mRNA. In the seminiferous tubule, strong immunoreactivity of p57kip2 was found in nuclei of early spermatocytes, but not in the spermatogonia. The p57kip2 immunoreactivity in spermatocytes was markedly heterogeneous. Preleptotene spermatocytes showed strong p57kip2 immunoreactivity, but no visible signal was found in late pachytene spermatocytes. Nuclei of the elongating spermatids was also positive for p57kip2 immunoreactivity. Taken together, this suggests that p57kip2 may play a role in the regulation of meiotic progression of early spermatocytes and cell cycle arrest and differentiation of spermatids. p57kip2 immunoreactivity was found in the perinuclear region of the peritubular cells, but not in the Sertoli cells. In Leydig cells, moderate immunoreactivity of p57kip2 was largely found in the cytoplasm, suggesting the noble function of p57kip2 in the differentiation of adult Leydig cells.