REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1089 



growth, as may be deduced from his state- 

 ment: "iSo also the testicles of birds are 

 either small or entirely invisible when not 

 excited, but when urged by desire they be- 

 come very large; this is so remarkable in 

 pigeons and partridges that some persons 

 have supposed that they had no testicles 

 during winter." 



Unhke most female birds, the avian male 

 has two well developed gonads, of which the 

 left is usually the larger one. This fact was 

 observed in 1789 by Tannenberg and later 

 confirmed for the sparrow. Passer domesti- 

 cus (Etzold, 1891), and many other species 

 (Domm, 1939). Exceptions are the pigeon, 

 Columba livia (Riddle, 1918), and the tur- 

 key, Meleagris gallopavo (Law and Kosin, 

 1958) , in which the right testis is larger. The 

 difference in testicular size is not a reflec- 

 tion of lesser activity, for Macartney (1942) 

 established that the number of mitotic di- 

 visions was larger in the right than in the 

 left testis of the fowl. 



The testes are sus})ended in the body 

 cavity from the body wall by the short 

 mesorchium. The ex})osure to the high 

 internal temperature (40.5 to 41.6°C. ac- 

 cording to Williams, 1958a) does not affect 

 spermatogenesis adversely, although in 

 mammals such temperatures would cause de- 

 generation of the seminiferous tubules 

 (Moore, 1939). In order to account for this 

 phenomenon Cowles and Nordstrom (1946) 

 proposed, without experimental evidence, 

 that the abdominal air sacs might act as a 

 cooling mechanism for the testes. However, 

 Williams (1958a) found that the surface 

 temperature of the testes (41.30°C.) was 

 the same as the mean body temperature, 

 and, what was new, that the exposure of 

 the testis to lower temperatures (38.60° to 

 40.26°C.), by transplantation under the 

 skin, caused an acceleration in spermato- 

 genesis in the testes of young cockerels. On 

 the other hand, if pieces of testes were 

 transferred to saline with temperatures of 

 44.4° and 43.3°C., before transplantation, 

 destruction of the germinal epithelium oc- 

 curred (Williams, 1958b), the destruction 

 being roughly proportional to the degree 

 of heat used. Exposure to saline of body 

 temperature liad no effect. These results 

 seem to indicate that lower-than-body 

 temperatures are beneficial for si)ermato- 



genesis, as in the mammal, but that the 

 threshold at which higher temperatures in- 

 terfere with spermatogenesis is higher in 

 birds. Indirect evidence for the beneficial 

 effect on spermatogenesis of lower tempera- 

 ture is also found in Riley's (1937) investi- 

 gation of the diurnal rhythm of mitotic 

 activity in the testes of the house sparrow. 

 Maximal mitotic activity coincided with 

 the lowest body temperature. Macartney 

 (1942) could, however, not confirm these 

 observations for the domestic fowl. The 

 more direct experimental evidence of Wil- 

 liams (1958a, b) seems to confirm Riley's 

 hypothesis. 



The testis is surrounded by the tunica 

 albuginea. In seasonally breeding birds this 

 is replaced at the end of each breeding 

 season by a new tunica which forms under 

 the old one. Enclosed by the tunica albu- 

 ginea are the seminiferous tubules and in 

 the spaces between them the interstitial 

 cells. 



2. The Interstitiuin and Its Secretions 



The cells of Leydig originate in the sex- 

 ual cords of the embryonic gonad and mi- 

 grate to the intertubular spaces (Benoit, 

 1950a). Several lines of evidence show that 

 the Leydig cells secrete the androgenic hor- 

 mone. In brief the evidence is that: (1) In 

 hypophysectomized cocks the comb can be 

 maintained only when the Leydig cells are 

 histologically active (Nalbandov, Meyer 

 and McShan, 1946, 1951). (2) Selective 

 destruction of the germinal epithelium by 

 x-rays does not affect comb size (Benoit, 

 1950a). (3) When parts of testes regenerate 

 after ca.stration, one sometimes finds tissue 

 without Leydig cells but w^ith Sertoli cells; 

 in other cases Leydig cells are encountered 

 but there are no tubules. On the basis of 

 such dissociation, Benoit (1950a) concluded 

 that Sertoli cells do not secrete androgens, 

 whereas Leydig cells do. (4) Kumaran and 

 Turner (1949a) observed the presence of 

 birefringent crystals in the Leydig cells, 

 suggesting the production of the hormone 

 or its precursor in these cells. The concen- 

 tration of the birefringent material in- 

 creased with increasing age of young 

 cockerels. Such observations are consistent 

 with the increased rate of secretion of the 



