The development of male germ cells in the seminiferous tubules of the testis involves three main phases: spermatogonial multiplication, meiosis, and spermiogenesis. In the seminiferous epithelium, cells in these developmental phases are arranged in defined associations or stages. Along the seminiferous tubules, these stages follow each other in regular fashion, giving rise to the wave of the seminiferous epithelium in most mammals. The time interval between the appearance of the same cell association at a given point of the tubule is called the cycle of the seminiferous epithelium. The dependence of spermatogenesis on pituitary FSH and on androgens secreted by Leydig cells in the testicular interstitial tissue is well documented. The somatic components of the seminiferous epithelium, the Sertoli cells, are the primary targets of these hormones. However, the local mechanisms, i.e. how Sertoli cells interact with the germ cells at various stages of the seminiferous epithelial cycle, have remained poorly understood. New possibilities for biochemical studies of this interaction have become available with the transillumination-assisted microdissection technique. It permits a collection of seminiferous tubular segments at defined stages of the cycle in unstained living conditions.