Mammalian spermatogenesis is a complex biological process that occurs within a highly organized tissue, the seminiferous epithelium. The coordinated maturation of spermatogonia, spermatocytes, and spermatids suggests the existence of precise programs of gene expression in these cells and in their neighboring somatic Sertoli cells. The objective of this study was to identify the genes that execute these programs. Rat seminiferous tubules at stages I, II-III, IV-V, VI, VIIa,b, VIIc,d, VIII, IX-XI, XII, and XIII-XIV of the cycle were isolated by microdissection, whereas Sertoli cells, spermatogonia plus early spermatocytes, pachytene spermatocytes, and round spermatids were purified from enzymatically dispersed testes. Microarray analysis by using Rat Genome 230 2.0 arrays identified 16,971 probe sets that recognized testicular transcripts, and 398 of these were identified as testis-specific. Expression of 1,286 probe sets were found to differ at least 4-fold between two cell types and also across the stages of the cycle. Pathway and annotated cluster analyses of those probe sets predicted that entire biological pathways and processes are regulated cyclically in specific cells. Important among these are the cell cycle, DNA repair, and embryonic neuron development. Taken together, these data indicate that stage-regulated gene expression is a widespread and fundamental characteristic of spermatogenic cells and Sertoli cells.array analysis ͉ spermatogenesis ͉ seminiferous tubules ͉ stages of the cycle of the seminiferous epithellum ͉ contraception I n mammals, spermatogenesis encompasses a series of precisely timed cellular events that take place in a highly organized tissue, the seminiferous epithelium. Within each cross-section of this tissue, spermatogonia, spermatocytes, and spermatids are in intimate physical association with somatic Sertoli cells, and, together, these cells progress synchronously through the stages of the cycle of the seminiferous epithelium (1, 2). Four cycles are required for spermatogonia, and their progeny to complete spermatogenesis. The synchrony of germ cell development has a significant effect on the structures and functions of both the germ cells and their associated somatic Sertoli cells, suggesting the existence of precise and coordinated cyclic programs of gene expression. However, because investigators have evaluated the stage-dependent changes in expression of only a few genes, whether or not cyclic gene expression is a rare or prevalent characteristic of these cells is unknown (3-7). It is also not known whether genes encoding components of important biological pathways and processes are coordinately regulated in a cyclic manner. However, such genes may represent new targets for the development of male contraceptives.Given the coordinated nature of spermatogenesis, we hypothesized that the expression of large numbers of genes differ substantially, both between specific cell types within the seminiferous epithelium and within a given cell type as it progresses through the stages of...