MAMMALIAN TESTIS 



313 



thcrniy, or heating of the testes by other 

 methods produces temporary depression of 

 the sperm count (see Hotchkiss, 1944b, for 

 review) . These experiments support the con- 

 cept of the thermoregulatory function of the 

 scrotum. These studies also indicate that 

 spermatogenesis in mammals can proceed 

 only at an optimal temperature. 



Cryptorchism in man has received con- 

 siderable attention because it is a common 

 clinical problem. The incidence of cryptor- 

 chism in man is given as 10 per cent at 

 birth, 2 per cent at puberty, and 0.2 per 

 cent at maturity (Nelson, 1953). The tes- 

 tis of man develops between the adrenal 

 glands medially and the body wall laterally, 

 ventral to the metanephros, and in the 20- 

 mm. fetus is not far from the groin. The 

 gubernaculum develops from the plica in- 

 guinalis and attaches to a pocket of the 

 abdominal wall which forms the vaginal 

 process (Wyndham, 1943). At the 2nd 

 month of fetal life, the testis is elongated, 

 extending from the diaphragm to the site of 

 the future abdominal inguinal ring. By the 

 3rd month, the cranial end of the testis 

 undergoes involution. By the 4th to the 7th 

 month, the testis is in the iliac fossa near 

 the internal ring. Descent occurs in the 

 seventh month, and the testis becomes scro- 

 tal during the 8th fetal month. The guber- 

 naculum shortens as descent takes place 

 and finally becomes a vestige in the adult. 



Prenatal descent of the testis into the 

 scrotum also occurs in mammals, such as 

 the monkey, horse, bull, pig, sheep, and 

 goat. Descent occurs postnatally in the 

 opossum, whereas it takes place in the pu- 

 bertal period in rats, mice, rabbits, and 

 guinea pigs. The monkey occupies a some- 

 what intermediate position. The testes de- 

 scend before birth but then migrate back 

 into the abdominal cavity until puberty 

 when descent occurs for the second time 



TABLE 5.1 



Comparison of average volume of structures in the 

 testis of man and the rat 

 All figures are percentages of total testicular 

 volume. (From E. C. Roosen-Runge, Fertil. & 

 Steril., 7, 251, 1956.) 



Interstitial tissue 



Leydig cells 



Basement membrane 



Total interstitial space 



Spermatogonia 



Spermatocytes 



Spermatids and spermatozoa 



Abnormal germ cells 



Residual bodies 



Total germ cells 



Sertoli cells 



Lumen 



Space 



Man 



22.0 

 3.1 

 9.1 



34.2 



7.8 

 14.4 

 9.1 

 0.7 



32.0 

 17.4 

 10.6 



5.8 



8.0 

 1.7 

 2.4 



12.1 

 1,7 



14.7 



41.1 

 0.1 

 1.2 



58.8 

 8.4 



19.5 

 1.1 



(Wells, 1944) . In seasonal breeders like the 

 ground squirrel, the testes alternate between 

 scrotum and abdomen with the breeding and 

 nonbreeding seasons. 



The mechanism of testicular descent is 

 not entirely understood. The gubernaculum 

 seems to act as a guide for the descending 

 testis, but it is not essential for descent. 

 Excision of the gubernaculum does not pre- 

 vent descent (Wells, 1943a, b). Growth of 

 the testis is not a determining factor in 

 its downward migration. Martins (1943) 

 showed that substitute testes in the form of 

 paraffin pellets can be made to descend in 

 castrated rats by the administration of tes- 

 tosterone. Therefore, it appears that descent 

 is determined by androgens affecting the 

 testis and accessory structures. The ques- 

 tions as to what androgens are responsi- 

 ble for testicular descent, where they are 

 formed, and how they are controlled remain 

 unanswered. Because Leydig cells appear in 

 the fetal testis of man during the fourth 

 month and because Ferner and Runge 



24. Right testis after another month, at 3 years and 1 day. Tubular diameter is 120 to 

 150 fi. Tubules are tightly packed and it is difficult to find Leydig cells. Cellular debris 

 has been cleared from most tubule.s, and a new generation of spermatids with orderly ar- 

 rangement is present. 



25. Right testis 5 months later, at 3 years, 5 months, 1 day. Tubular diameter is 150 ^l. 

 Leydig cells are extremely rare. There are many presperm cells and a few mature sperm cells 

 free in the lumen. 



26 to 2S. Adult testis, age 11 years, 6 months, 20 days. 26. Seminiferous tubule lumen 

 lined by j^oung spermatids. 27. Tubule lined by spermatids with spermlike heads. 2S. Tubule 

 containing mature sperm cells about ready to be shed. (From G. van Wagenen and M. L. 

 Simpson, Anat. Rec, 118, 231, 1954.) 



