Spermatogenesis

A blood testis barrier prevents some compounds in blood from entering the seminiferous tubules where sperm are produced. This prevents compounds such as heavy metals and drugs from damaging sperm. Mitotic and meiotic cell divisions occur during spermatogenesis. Spermatogonia undergo several mitotic divisions to become primary spermatocytes, which undergo meiotic divisions to become secondary spermatocytes, then spermatids, and finally spermatozoa or sperm. Spermatogenesis results in a multiplication in the number of sperm produced and a reduction in the genetic material in sperm to half the amount of other cells in the body (haploid).

Spermatogenesis is a continuous process in males unless conditions are abnormal such as when heat stress increases testicular temperature. The interval required for the production of sperm from the first cell division of spermatogonia until a spermatozoon or sperm is released into the lumen of the seminiferous tubule varies with species. For instance, in farm animals it only requires 34 days for spermatogenesis in boars; however, 61 days are required in bulls. Compared with mammals, sper-matogenesis in birds is about four times faster and about four times the number of sperm are produced per gram of testis. The duct system from the testes (efferent ducts) transports sperm to the epididymis, a long coiled tube. Length of the epididymis is about 18 meters in boars and 75 meters in stallions. Sperm cells undergo a maturation process in the epididymis during 7 to 14 days, depending on the species. Sperm that enter the epididymis are not motile or capable of fertilization until maturational changes occur in the epididymis.

Table 1 Testicular characteristics of farm animals

Weight of both

Daily sperm

Age at

Species

testes (g)

production (x 109)

puberty (mo.)a

Boar

720

16.2

58

Ram

500

9.5

49

Bull, beef

650

5.9

9 12

Stallion

340

5.3

12 20

Rabbit

6

0.2

34

Dog

32 88

0.9

6 12

aSperm ejaculated. (Compiled from the literature.)

aSperm ejaculated. (Compiled from the literature.)

If males have increased body temperature due to fever or exposure to hot climatic conditions, testicular temperature can increase and result in a reduction in the number of sperm produced or the production of sperm that are less fertile or infertile.12 If bulls or boars are heat-stressed for an interval of only three days, this can cause temporary infertility. After heat stress, boars must be exposed to a cool environment for five weeks and bulls must be cool for seven weeks before normal sperm are produced and animals are fertile. Heat stress also causes infertility in birds and the effect appears to be greater in males than in females.

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