Spermatogenesis: how sperm is produced


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Sperm production, or, ‘spermatogenesis’ begins during puberty and continues throughout life. Every day a man makes millions of sperm. Since only one or two sperm fertilizes an egg, it seems mother nature has tried to ensure our species survival by overdoing it a little.

Sperm formation occurs in the seminiferous tubules of the testis. This process is started by primitive stem cells called spermatogonia found on the outer edge of each tubule. Spermatogonia cells go through rapid mitotic divisions to build up the stem cell line. From birth until puberty, all such divisions simply produce more stem cells. During puberty, however, FSH (follicle stimulating hormone) is secreted in increasing amounts by the anterior pituitary gland, and from this time on, each division of a spermatogonium produces one stem cell (type A daughter cell) and another cell called a type B daughter cell. The type A cell remains at the tubule edge to maintain the stem cell population. The type B cell gets pushed toward the tubule lumen, where it becomes a primary spermatocyte, destined to undergo meiosis and form four sperm. Meiosis is a special type of nuclear division that occurs for the most part only in the gonads (testes and ovaries). It differs from mitosis in two major ways. Meiosis consists of two successive divisions of the nucleus (called meiosis I & II) and results in four (instead of two) daughter cells, or more precisely, four gametes. In spermatogenesis, the gametes are called spermatids. Spermatids have only half as much genetic material as other body cells. In humans, this is 23 chromosomes rather than the usual 46. Then, when the sperm and egg (which also has only 23 chromosomes) meet, forming a fertilized egg (or zygote), the normal 46 chromosomes is again established and then is maintained in the body cells that form after via the process of mitosis.

As meiosis occurs, the dividing cells (primary then secondary spermatocytes) are pushed toward the lumen of the tubule. Thus, the progress of meiosis can be followed from the tubule edge to the lumen. The spermatids, which are the products of meiosis, are not functional sperm. They are nonmotile cells and have too much excess baggage to function well in reproduction. They must undergo further changes, in which their excess cytoplasm is stripped away and a tail is formed. In this last stage of sperm development, called spermiogenesis, all the excess cytoplasm is sloughed off, and what remains is compacted into the three regions of the mature sperm – the head, midpiece, and tail. The mature sperm is a greatly streamlined cell equipped with a high rate of metabolism and means of propelling itself, enabling it to move long distances in a short time to get the egg. It is a prime example of the fit between form and function.

The sperm head contains DNA, the genetic material. Essentially, it is the nucleus of the spermatid. Anterior to the nucleus is the helmet-like acrosome, which is similar to a large lysosome. When a sperm comes into close contact with an egg (oocyte), the acrosomal membrane breaks down and releases enzymes that help the sperm penetrate through the follicle cells that surround the egg. Filaments, which form the tail, arise from centrioles in the midpiece. Mitochondria wrapped tightly around these filaments provide the ATP needed for the whip-like movements of the tail that propel the sperm.

The entire process of spermatogenesis, from the formation of a primary spermatocyte to release of immature sperm in the tubule lumen, takes 64 – 72 days. Sperm in the lumen are unable to ‘swim’ and incapable of fertilizing an egg. They are moved by peristalsis through the tubules of the testes into the epididymis. There they undergo further maturation, which results in increased motility and fertilizing power.

Environmental threats can alter the normal process of sperm formation. For example, some common antibiotics such as penicillin and tetracycline may suppress sperm formation. Radiation, lead, certain pesticides, marijuana, tobacco, and excessive alcohol can cause production of abnormal sperm (two headed, multiple tailed etc).