REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1131 



ovulation cycle of the fowl. In the meantime 

 the indirect evidence suggesting that estro- 

 gen has its depressing effect on gonado- 

 trophin release by way of a neural 

 mechanism will be cited: (1) estrogen ad- 

 ministration to chickens causes changes in 

 the neurosecretory cells of the paraventric- 

 ular nucleus of the hyopthalamus (Legait, 

 1959) ; (2) estrogen causes gonadotrophin 

 inhibition by way of the hypothalamus in 

 mammals (Flerko, 1957) ; this is discussed 

 in detail in the chapter by Everett; (3) 

 progesterone, another steroid hormone, 

 causes release of gonadotrophins from the 

 anterior pituitary by way of a neural mech- 

 anism. This possibility is discussed in the 

 present chapter and, for mammals, in the 

 chapter by Everett. 



3. Androgen 



Androgens, apparently, have an effect on 

 the ovaries of hypophysectomized pigeons 

 which corresponds to that on the testes. Chu 

 and You (1946) obtained 4 to 6 mm. fol- 

 licles filled with yolk in androgen-treated 

 hypophysectomized pigeons. A stimulating 

 effect on the ovary was found also in intact 

 immature pigeons, although the testes of 

 immature males failed to respond (Chu and 

 You, 1946). A similar situation exists in 

 sparrows in which injections of testosterone 

 (0.5 to 1.0 mg. per day) stimulated the 

 ovarian follicles to such a degree that their 

 diameters were approximately 75 per cent 

 larger than the average in the controls 

 (Ringoen, 1943). Androgen administration 

 also resulted in a modification of the follicu- 

 lar epithelium from simple to stratified 

 (Ringoen, 1943). 



Breneman (1955, 1956) studied the effect 

 of androgen on the ovaries and follicular de- 

 velopment in intact, 30-day-old chickens. 

 Five fjLg. TP given daily increased ovarian 

 weight, but administration of either 1.0 or 

 25 ^g. did not have this effect (Breneman, 

 1956). Doses of 0.1, 1.0, 10.0, 50, and 100 

 fig. TP increased follicle area of the ovary 

 significantly, but after a maximum w^as 

 reached with the 0.1- and 10-/>ig. levels, the 

 response tended to decrease. After the ad- 

 ministration of 100 fjig., the follicular area 

 was less than when the smaller amounts 

 were given, but it was still larger than in 



the controls. The increase in follicle area 

 may well be the result of inhibition of the 

 interstitium combined with stimulation of 

 the follicles. Breneman (1955) emphasized 

 that androgen administration increases the 

 height of the follicular epithelium. Analysis 

 of variance of Breneman's data (by me) 

 showed that only the comparison of control 

 versus all androgen levels combined was 

 significant (p < 0.05) . The lack of a dose- 

 response relationship over such a wide 

 range of doses suggests that the difference 

 between controls and androgen-treated pul- 

 lets was the result of a sampling error, un- 

 less the assumption of an all-or-none re- 

 sponse is made with respect to increases of 

 follicular epithelial height. Androgen, ap- 

 parently, did not affect pituitary gonadotro- 

 phin assays. Breneman's (1955) statement 

 that low doses of estrachol and testosterone 

 facilitate the ovarian response to PMS is 

 not supported by evidence, because no data 

 are given on the effect of PMS alone. Nelson 

 and Stabler (1940» injected large doses of 

 TP (140 mg. in 30 days) into young female 

 sparrow hawks (Falco s. sparverius) and 

 found no effect on either left or right ovary. 

 The limited data available on different spe- 

 cies suggest the following provisional gen- 

 eralization: if androgens stimulate the testes 

 of a species (sparrows, pigeons), then an- 

 drogens will, under similar conditions, also 

 stimulate the ovaries of the females. 



Testosterone can cause the anterior pitui- 

 tary to release LH, which results in ovula- 

 tion. The incidence of premature ovulations 

 after testosterone injection is about 41 per 

 cent compared with 95 per cent after jiro- 

 gesterone injection. Testosterone-induced 

 LH release is mediated by a neural mecha- 

 nism which will be discussed more fully 

 under progesterone effects in the regulation 

 of ovarian activity. 



Testosterone, thus, seems to be capable of 

 affecting ovarian activity in two ways. One 

 is by a direct effect on the ovary, as in hy- 

 pophysectomized pigeons, and the other is 

 by an effect on the neural components which 

 regulate anterior pituitary activity. 



4- Progesterone 



Progesterone probably plays a very im- 

 l)ortant role in the regulation of ovarian ac- 



