spermatogenesis n : development of spermatozoa

English

Noun

spermatogenesis

1. The process of sperm production in the testes.

Spermatogenesis is the process by which male spermatogonia develop into mature spermatozoa. Spermatozoa are the mature male gametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version of gametogenesis. In mammals it occurs in the male testes and epididymis in a stepwise fashion, and for humans takes approximately 64 days. Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for sexual reproduction. It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age. The entire process can be broken up into several distinct stages, each corresponding to a particular type of cell:

Purpose

Spermatogenesis produces mature male gametes, commonly called sperm but specifically known as spermatozoa, which are able to fertilize the counterpart female gamete, the oocyte, during conception to produce a single-celled individual known as a zygote. This is the cornerstone of sexual reproduction and involves the two gametes both contributing half the normal set of chromosomes (haploid) to result in a chromosomally normal (diploid) zygote.

To preserve the number of chromosomes in the offspring, which differs between species, each gamete must have half the usual number of chromosomes present in other body cells. Otherwise, the offspring will have twice the normal number of chromosomes, and serious abnormalities may result. In humans, chromosomal abnormalities arising from incorrect spermatogenesis can result in Down Syndrome, Klinefelter's Syndrome, and spontaneous abortion. Most chromosomally abnormal zygotes will not survive for long after conception; however, plant reproduction is a little more robust, and viable new species may arise from cases of polyploidy.

Location

Spermatogenesis takes place within several structures of the male reproductive system. The initial stages occur within the testes and progress to the epididymis where the developing gametes mature and are stored until ejaculation. The seminiferous tubules of the testes are the starting point for the process, where stem cells adjacent to the inner tubule wall divide in a centripetal direction—beginning at the walls and proceeding into the innermost part, or lumen—to produce immature sperm. Maturation occurs in the epididymis and involves the acquisition of a tail and hence motility.

Stages

Spermatocytogenesis

At all stages of differentiation, the spermatogenic cells are in close contact with Sertoli cells which are thought to provide structural and metabolic support to the developing sperm cells. A single Sertoli cell extends from the basement membrane to the lumen of the seminiferous tubule, although the cytoplasmic processes are difficult to distinguish at the light microscopic level.

Sertoli cells serve a number of functions during spermatogenesis, they support the developing gametes in the following ways:

• Maintain the environment necessary for development and maturation via the blood-testis barrier
• Secrete substances initiating meiosis
• Secrete supporting testicular fluid
• Secrete androgen-binding protein, which concentrates testosterone in close proximity to the developing gametes
  • Testosterone is needed in very high quantities for maintenance of the reproductive tract, and ABP allows a much higher level of fertility
• Secrete hormones effecting pituitary gland control of spermatogenesis, particularly the polypeptide hormone, inhibin
• Phagocytose residual cytoplasm left over from spermiogenesis
• They release Antimullerian hormone which prevents formation of the Mullerian Duct / Oviduct.

Influencing factors

The process of spermatogenesis is highly sensitive to fluctuations in the environment, particularly hormones and temperature. Testosterone is required in large local concentrations to maintain the process, which is achieved via the binding of testosterone by androgen binding protein present in the seminiferous tubules. Testosterone is produced by interstitial cells, also known as Leydig cells, which preside adjacent to the seminiferous tubules.

Seminiferous epithelium is sensitive to elevated temperature in humans and some other species, and will be adversely affected by temperatures as high as normal body temperature. Consequently, the testes are located outside the body in a sack of skin called the scrotum. The optimal temperature is maintained at 2°C (man) - 8°C (mouse) below body temperature. This is achieved by regulation of blood flow and positioning towards and away from the heat of the body by the cremasteric muscle and the dartos smooth muscle in the scrotum.

Dietary deficiencies (such as vitamins B, E and A), anabolic steroids, metals (cadmium and lead), x-ray exposure, dioxin, alcohol, and infectious diseases will also adversely affect the rate of spermatogenesis.

Hormonal control

Hormonal control of spermatogenesis varies among species. In humans the mechanism are not completely understood, however it is known that initiation of spermatogenesis occurs at puberty due to the interaction of the hypothalamus, pituitary gland and Leydig cells. If the pituitary gland is removed, spermatogenesis can still be initiated by follicle stimulating hormone and testosterone.

Follicle stimulating hormone stimulates both the production of androgen binding protein by Sertoli cells, and the formation of the blood-testis barrier. Androgen binding protein is essential to concentrating testosterone in levels high enough to initiate and maintain spermatogenesis, which can be 20-50 times higher than the concentration found in blood. Follicle stimulating hormone may initiate the sequestering of testosterone in the testes, but once developed only testosterone is required to maintain spermatogenesis. However, increasing the levels of follicle stimulating hormone will increase the production of spermatozoa by preventing the apoptosis of type A spermatogonia. The hormone inhibin acts to decrease the levels of follicle stimulating hormone.

The Sertoli cells themselves mediate parts of spermatogenesis though hormone production. They are capable of producing the hormones estradiol and inhibin. The Leydig cells are also capable of producing estradiol in addition to their main product testosterone.

See also

• Folliculogenesis
• Germ cells
• Oogenesis
• Sertoli cells
• Sperm count

References

• ">http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TCM-3RM01S4-G&_coverDate=11%2F30%2F1997&_alid=496201858&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=5174&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=038f066d33248ee72545b48d21e1dd8c}}
• BARDIN CW: Pituitary-testicular axis. In: YEN SS , JAFFEE RB , eds: Reproductive Endocrinology, 3rd ed. Philadelphia: WB Saunders, 1991
• CHAMBERS CV , SHAFER MA , ADGER H , et al: Microflora of the urethra in adolescent boys: relationships to sexual activity and nongonococcal urethritis. J Ped 110:314-321, 1987
• CZYBA JC , GIROD C: Development of normal testis. In: HAFEZ ESE , ed: Descended and Cryptorchid Testis. The Hague, Martinus Nijhoff, 1980.
• Whitmore WF, Kars L, Gittes RF: The role of germinal epithelium and spermatogenesis in the privileged survival of intratesticular grafts. J Urol 1985;134:782.

External links

• Spermatogenesis - male reproductive physiology - Am Fam Physician 2000;62:1095.