308 



PHYSIOLOGY OF GONADS 



a lumen appears in the tubules and sper- 

 matogenesis begins. At 94 days, maturation 

 of the Lej^dig cells is noted, and spermio- 

 genesis occurs. The diameter of the tubules 

 at maturity is about 100 jx, but the tubule 

 continues to increase to about 160 /x when 

 the goat is 135 days of age. Formation of 

 sperm occurs earlier in goats than in rams, 

 bulls, and boars (Yao and Eaton, 1954). 



As in common laboratory animals, the 

 time sequence in the testicular maturation 

 of farm animals is determined by genie fac- 

 tors, which obviously is an important phe- 

 nomenon economically. In different strains 

 of the ram, for instance, there is a variation 

 of 5 weeks in the time of appearance of 

 primary spermatocytes, of 9 weeks in the 

 appearance of secondary spermatocytes, 

 and of 2 weeks for spermatids (Carmon and 

 Green, 1952). 



Among the primates, postnatal develop- 

 ment has been studied intensely in the mon- 

 key and man. In the rhesus monkey (Fig. 

 5.1), the tubules attain a diameter of 70 

 to 80 fx during fetal life (van Wagenen and 

 Simpson, 1954). Only spermatogonia and 

 Stertoli cells containing basal nuclei are 

 present. Mature Leydig cells are also identi- 

 fiable. Shortly after birth, regression occurs 

 in the tubules, which decrease to a diameter 

 of 50 to 60 )u, and in the Leydig cells, which 

 dedifferentiate into mesenchymal cells. The 

 presence of mature Leydig cells and of dif- 

 ferentiated Sertoli cells in the fetal testis 



and their involution shortly after birth 

 may be related to the secretion of a fetal 

 morphogenic substance (c/. chapter by 

 Burns). 



During the first year after birth, the few 

 spermatogonia increase in number and size. 

 During the second year, they become more 

 numerous and the tubules increase in length. 

 However, the germinal epithelium remains 

 quiescent until late in the third year. At 

 this time, the Sertoli cells increase in num- 

 ber and differentiate. The Leydig cells ma- 

 ture again. The tubular diameter now is 

 about 100 jx. A lumen appears when the 

 tubules are 100 to 150 /x in diameter. Sper- 

 matids appear, and orderly spermatogenesis 

 occurs. The prepubertal period in the mon- 

 key is about a fifth that of man; except 

 for the factor of time, the sequence of de- 

 velopment in the monkey testis resembles 

 that in man (Figs. 5.2 and 5.3). In the 

 pubertal monkey Leydig cells and tubules 

 are stimulated simultaneously. Some ob- 

 servers have reported that maturation of 

 Leydig cells occurs after tubular maturation 

 in humans (Sniffen, 1952; Albert, Under- 

 dahl, Greene and Lorenz, 1953a) and in 

 bulls (Hooker, 1948). However, this point 

 may depend on the choice of the criteria for 

 tubular stimulation (tubular wall versus 

 germinal epithelium) and for function of 

 Leydig cells (morphologic differentiation 

 versus secretory activity) . 



Some interesting details on the relative 



Grams 



8000 

 7000 

 6000 

 5000 

 4000 

 3000 

 2000 

 1000 



Cm 



4 

 3 

 2 



I 



Age in Years 



Fig. 5.1. Graphic representation of changes in testes of the rliesus monkey during de- 

 velopment. Coordinates are body weight and age of animals, and length of testes. (From 

 G. van Wagenen and M. E. Simpson, Anat. Rec, 118, 231, 1954.) 



