Necturus Testis and Reproduction 71 



all urodele amphibians, is similar to but less complex than that of the 

 amniote testis. 



The biochemical parameters measured in N. lewisi testis, indicating 

 active steroidogenesis, essentially correspond with the topographical 

 location and morphological differentiation of the Leydig cells. Thus, it 

 would appear that these Leydig cells are responsible for the production 

 of steroids by the testis of N. lewisi. Their steroidogenic potential also 

 appears to be related to the cycle of the seminiferous epithelium, since 

 spermiation, followed by regression of the mature lobules, is the event 

 signaling hypertrophy of the surrounding interlobular tissue. It is possi- 

 ble that similar changes also occur in mammals but have never been 

 detected, since not only are different germ cell stages normally found 

 throughout the testis year-round in most common laboratory animals 

 but differentiation is not synchronized in all germ cells of any one tes- 

 ticular region. Recent studies have in fact shown that when Leydig cells 

 from adult rats are separated on density gradients, at least three func- 

 tionally distinct populations can be identified (O'Shaugnessy et al. 

 1981). Whether these Leydig cells are randomly distributed throughout 

 the testis of the rat or actually occur in specific relation to certain stages 

 of germ cell development, however, remains to be elucidated. An intrig- 

 uing question is the mechanism by which the spermiated, degenerating 

 lobules of Necturus testis initiate the hypertrophy and differentiation of 

 the Leydig cells associated with these regions. This seems to be a gener- 

 alized phenomenon, since there are scattered reports in the literature 

 demonstrating, in many mammalian species, that damage to the semi- 

 niferous epithelium, either by physical or chemical agents, results in the 

 hypertrophy of Leydig cells adjacent to these lesions (Neaves 1975; Aoki 

 and Fawcett 1978). Furthermore, inhibition of spermatogenesis due to 

 cryptorchidism is similarly associated with an increase in development 

 of Leydig cells as well as an enhancement of androgen secretion (deK- 

 retser et al. 1980). As yet, however, further investigations are required to 

 determine the molecular events controlling this interesting relationship 

 between the spermatogenic tissue and Leydig cells. 



It has been assumed that, in all species, at least some stages of 

 spermatogenesis are androgen-dependent (Callard, I. P. et al. 1978; 

 Rodriguez-Rigau et al. 1980). In Necturus testis, therefore, it seems 

 paradoxical that lobular regions which contain differentiating germ cells 

 are found to be associated with poorly developed Leydig cells possessing 

 low androgen synthesizing abilities. It is possible, however, that these 

 Leydig cells do synthesize low but adequate quantities of androgens that 

 suffice for local spermatogenetic requirements. Among non-mammals it 

 has been commonly demonstrated that spermatogenetic activity and 

 Leydig cell development are often temporally separated during the 

 annual cycle (Lofts and Bern 1972; Callard, I. P. et al. 1978). Where 



