Spermatids, which are small round cells, undergo spermiogenesis, a remarkable metamorphosis resulting in the formation of streamlined spermatozoa. During this process, the nucleus of the spermatid decreases in size and a tail is formed. The tail contains microtubular structures that propel the sperm. The chromatin material in the sperm nucleus condenses and most of the cytoplasm is lost. The acrosome, a membrane-enclosed structure on the head of the sperm, acts as a lysosome and contains hydrolytic enzymes important for fertilization. The acrosomal reaction activates these enzymes. Spermiation, the release of sperm, is controlled by Sertoli cells. Spermatogenesis takes approximately 72 days, and a cohort of adjacent spermatogonia enter the process every 16 days, resulting in staggered production along the seminiferous tubule. Sertoli cells, the true epithelial cells of the seminiferous epithelium, provide structural support and form junctions with all stages of sperm cells. These cells guide sperm cells towards the lumen as they progress through spermatogenesis. The breakdown of Sertoli-sperm cell junctions is necessary for spermiation. Tight junctions are also formed between adjacent Sertoli cells.
Fig. 44.4,
S3).
Spermatids are small round cells that undergo a remarkable metamorphosis called spermiogenesis (
Fig. 44.5). The products of spermiogenesis are the streamlined spermatozoa see
Fig. 44.4 , S4). As the spermatid matures into a spermatozoon the size of the nucleus decreases and a prominent tail is formed. The tail contains microtubular structures that propel sperm, similar to a flagellum. The chromatin material in the sperm nucleus condenses, and most of the cytoplasm is lost. The acrosome is a membrane-enclosed structure on the head of the sperm that acts as a lysosome
CHAPTER 44 The Male and Female Reproductive Systems 789
Ligamentous remnant Vas deferens of processus vaginalis
Fibrous septum Head of epididymis
Body of epididymis
Tail of epididymis • Fig. 44.3 Histology of a testicular lobule. (From Young B etal. Wheater’s Functional Histology. A Text and Colour Atlas. 5th ed. London:ChurchillLivingstone;2006.) and contains hydrolytic enzymes that are important for fertilization. These enzymes remain inactive until the acrosomal reaction occurs (see
Fig. 44.4 , S4) are found at the luminal surface of the seminiferous tubule. Release of sperm, or spermiation, is controlled by Sertoli cells. The process of • Fig. 44.4 Histology of a seminiferous tubule. M, myoid cell just outside the basal lamina; S1, primary spermatocyte; S3, spermatid; S4, mature spermatid or spermatozoon; SB and SA, spermatogonia; St,Sertolicell.(FromYoungBetal.Wheater’s Functional Histology. A Text and Colour Atlas.5thed.London:ChurchillLivingstone;2006.) spermatogenesis takes about 72 days. A cohort of adjacent spermatogonia enter the process every 16 days so that the process is staggered at one point along a seminiferous tubule. In addition the process is staggered along the length of a seminiferous tubule (i.e., not all spermatogonia enter the process of spermatogenesis at the same time along the •Fig. 44.5 Structureofspermcellsduringtheprocessofspermatogenesisandspermiogenesis.
entire length of the tubule or in synchrony with every other tubule; there are about 500 seminiferous tubules per testis; see later). Because the seminiferous tubules within one testis are about 400 m in length, spermatozoa are continually being generated at many sites within the testis at any given time.
The Sertoli Cell
Sertoli cells are the true epithelial cells of the seminiferous epithelium and extend from the basal lamina to the lumen (see
Fig. 44.4 , St). Sertoli cells surround sperm cells and provide structural support within the epithelium, and they form adhering and gap junctions with all stages of sperm cells. Through formation and breakdown of these junctions, Sertoli cells guide sperm cells toward the lumen as they advance to later stages of spermatogenesis. Spermiation requires the final breakdown of Sertoli–sperm cell junctions.
Another important structural feature of Sertoli cells is the formation of tight junctions between adjacent Sertoli cells (