Spermiogenesis
The mammalian haploid spermatid is a round, unflagellated cell that looks nothing like the mature vertebrate sperm. The next step in sperm maturation, then, is spermiogenesis (or spermateliosis), the differentiation of the sperm cell. For fertilization to occur, the sperm has to meet and bind with the egg, and spermiogenesis prepares the sperm for these functions of motility and interaction. The first steps involve the construction of the acrosomal vesicle from the Golgi apparatus. The acrosome forms a cap that covers the sperm nucleus. As the acrosomal cap is formed, the nucleus rotates so that the cap will be facing the basal membrane of the seminiferous tubule. This rotation is necessary because the flagellum is beginning to form from the centriole on the other side of the nucleus, and this flagellum will extend into the lumen. During the last stage of spermiogenesis, the nucleus flattens and condenses, the remaining cytoplasm (the “cytoplasmic droplet”) is jettisoned, and the mitochondria form a ring around the base of the flagellum.
One of the major changes in the nucleus is the replacement of the histones by protamines. Transcription of the gene for protamine is seen in the early haploid cells (spermatids), although translation is delayed for several days (Peschon et al. 1987). Protamines are relatively small proteins that are over 60% arginine. During spermiogenesis, the nucleosomes dissociate, and the histones of the haploid nucleus are eventually replaced by protamines. This causes the complete shutdown of transcription in the nucleus and facilitates its assuming an almost crystalline structure. The resulting sperm then enter the lumen of the tubule.
In the mouse, the entire development process from stem cell to spermatozoon takes 34.5 days. The spermatogonial stages last 8 days, meiosis lasts 13 days, and spermiogenesis takes up another 13.5 days. In humans, spermatic development takes nearly twice as long to complete. Because the type A1 spermatogonia are stem cells, spermatogenesis can occur continuously. Each day, some 100 million sperm are made in each human testicle, and each ejaculation releases 200 million sperm. Unused sperm are either resorbed or passed out of the body in urine. During his lifetime, a human male can produce 10[sup]12[/sup] to 10[sup]13[/sup] sperm (Reijo et al. 1995).
Developmental Biology, 6th Edition
Scott F. Gilbert
Sinauer Associates, Inc., Publishers,
