Spermatogenesis is defined as the sequence of events which leads to spermatatozoa (sperms) from primitive germ cells (spermatogonia).

Spermatogenesis begins at puberty (roughly 13 to 16 years) and continues well into old age.

The primitive germ cells( spermatogonia) remain dormant in the seminiferous tubules until the onset of puberty. With the onset of puberty the spermatogonia undergo several mitotic divisions which transforms them into primary spermatocytes, which are the largest germ cells in the seminiferous tubules.

Each primary spermatocyte undergoes a reduction division (meiotic division).  The first meiotic division gives rise to two haploid secondary spermatocytes (are about half the size of primary spermatocytes. The secondary spermatocytes then undergo a second mitotic division to form four haploid spermatids. The spermatids are about half the size of secondary spermatocytes.

The spermatids are then transformed into four mature sperms (by a differentiation process called spermiogenesis).

The entire process of spermatogenesis (including spermatogenesis) takes about two months.

The mature sperms (after spermiogenesis) enter the lumina of the seminiferous tubules.

Special cells (sertoli cells) lining the seminiferous tubules provide nutrition and support for the germ cells . They are also probably involved in the regulation of spermatogenesis. The sertoli cells secrete androgen binding protein (ABP), inhibin, and MIS. However they do not synthesize androgens. They contain the enzyme aromatase (CYP19) for converting androgens to estrogens. Inhibin inhibits follicle stimulating hormone secretion. MIS is responsible for the regression of the mullerian ducts in males during fetal life.

The sperms are then passively transported from the seminiferous tubules to the epididymis and stored until they become functionally mature.The epididymis is the elongated coil duct at the posterior border of the testis. It is continuous with the vas deferens (ductus deferens), which transports the sperms to the urethra.

Sperm Structure

The mature sperm consists of a head  and tail. It is free swimming.

The neck is the junction between the head and tail of the sperm.

The head of the sperm which contains the haploid nucleus is the largest part of the sperm.

The anterior(front end) of the nucleus is covered by a saccular organelle (sac like organelle) which contains several enzymes. The most important enzyme contained in the acrosome is acrosin.

The enzymes released from the acrosome facilitate the penetration of the zona pellucida and the corona radiata, while trying to fertilize the egg.

The tail of the sperm is made up of three pieces (three segments). The three segments are the middle piece (mid-piece), principal piece and end piece. The tail is responsible for the motility of the sperm. The middle piece is packed with large concentrations of mitochondria, which provide adequate the adenosine triphosphate (ATP) necessary for activity.

The mitochondrial (fibrous ) sheath is thought to be responsible for the lashing movements of the tail.

The membrane of late spermatids and spermatozoa contain a special type of germinal angiotensin II converting enzyme. Although the function of this enzyme in humans is not known, in mice desruption of the production of this enzyme reduces fertility.

Temperature and Spermatogenesis

For spermatogenesis to occur an optimal temperature which is considerably lower than that of the interior of the body must be maintained. The testis are normally maintained at a temperature of about 32 degrees celsius. The testis are kept cool by air circulating around the scrotum. It is also possible that a countercurrent heat exchange exists between the spermatic arteries and veins.

Hot baths (43- 45 degrees celsius for 30 minutes per day) and insulated clothing (eg atheletic supporters) reduce the sperm count , to as low as 90%.

It is also well established that if the testis are held intra-abdominally (in experimental animals or undescended due to embryological defect) can result in the degeneration of the tubular walls and sterility.

Hormones and Spermatogenesis

FSH (Follicle stimulating hormone) and androgens maintain spermatogenesis in the testis. LH (Leutenizing hormone) produces a high concentration of androgens in the testis. Maturation of spermatozoa (from spermatids) is also androgen dependent.

It is important to know that there is a high concentration of fluid in the rete testis. This helps in the concentration and reabsorption of the fluids. If this fails to occur, the sperms entering the epidedymis are diluted and infertility results.

References

Pages 22 to 23 Chapter 2, The Beginning of Human Development .The Developing Human. Clinically Oriented Embryology 6th edition by Keith L. Moore, Phd, FIAC, FRSM and TVN Persaud MD, PhD, DSc, FRCPPath (Lond).

Pages 412 to 414 Chapter 23, The Gonads; Development and Function of the Reproductive System. Review of Medical Physiology 20th Edition by William F Ganong.

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