REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1113 



days of uge. Some of these right ovaries 

 resulting from sinistral ovariectomy have 

 follicles which can be ovulated (Nal))andov, 

 1959a». A number of observations indicate 

 that the apparent effect of the age at which 

 ovariectomy is performed on the type of de- 

 velopment of the rudiment has an endocri- 

 nologic basis. (1) The 20-day-old chick 

 secretes detectable amounts of estrogen as 

 determined by bioassay of the blood ( Korn- 

 feld and Nalbandov, 1954). (2) Small doses 

 of estrogen (2 ^g. per 100 gm.) inhibit the 

 develoi)ment of the rudimentary gonad of 

 ovariectomized pullets (Kornfeld and Nal- 

 bandov, 1957; Kornfeld, 1958). These doses 

 are too small to affect oviduct weight signif- 

 icantly and may, thus, be in the range of the 

 amounts secreted by the immature ovary. 

 (3) If the rudiment develops in spite of es- 

 trogen treatment, the incidence of ovaries 

 and ovotestes is greater than that of testes 

 (Taber and Salley, 1954; Kornfeld and Nal- 

 bandov, 1954). Histologic examinations 

 show also that estrogen inhibits medullary 

 tissue more than cortical tissue (Taber, 

 Clay tor. Knight, Flowers, Gambrell and 

 Ayers, 1958). 



The following explanation of the age ef- 

 fect is thus in agreement with the experi- 

 mental evidence. Estrogen secretion by the 

 left ovary inhibits the development of the 

 medullary tissue of the right gonad. If this 

 inhibition is removed before 30 days, the 

 medulla can still proliferate. By 30 days of 

 age the estrogen inhibition has apparently 

 destroyed the potential of the medulla to 

 develop and thus no proliferation occurs 

 even when the inhibition is removed. It is 

 understood, of course, that this critical 

 point, may vary between individuals within 

 a strain and even more so from strain to 

 strain. 



The possible stimulation of cortical tissue 

 development by estrogen is not as clear-cut. 

 Taber and Salley's (1954) and Kornfeld and 

 Nalbandov's (1954) experiments indicate 

 that estrogen favors the development of 

 cortical tissue. However, in a series of ex- 

 periments by Taber, Claytor, Knight, 

 Flowers, Gambrell and Ayers (1958) in- 

 volving large numbers of birds, no evidence 

 was obtained that would support such a 

 conclusion. 



The development of the rudimentary 

 gonad seems to be under the control of the 

 anterior pituitary. Hypophysectomy (Korn- 

 feld and Nalbandov, 1954) , prolactin in- 

 jections (Kornfeld and Nalbandov, 1954), 

 and estrogen administration (Taber and 

 Salley, 1954; Kornfeld and Nalbandov, 

 1954; Kornfeld, 1958; Taber, Claytor, 

 Knight, Flowers, Gambrell and Ayers, 

 1958) inhibit its development. However, 

 replacement therapy in the case of hypophy- 

 sectomized birds (Kornfeld and Nalbandov, 

 1954) or in the case of estrogen-inhibited 

 birds (Kornfeld and Nalbandov, 1954; 

 Kornfeld, 1958; Taber, Claytor, Knight, 

 Flowers, Gambrell and Ayers, 1958) does 

 not stimulate the rudiment, nor do mam- 

 malian or avian gonadotrophins in poulards 

 (sinistrally ovariectomized hens) (Korn- 

 feld and Nalbandov, 1954; Kornfeld, 1958; 

 Taber, Claytor, Knight, Flowers, Gambrell 

 and Ayers, 1958). The lack of development 

 in the rudiment cannot be explained by the 

 lack of the "third gonadotrophin," because, 

 in some of these investigations, chicken 

 pituitary preparations were used (Kornfeld, 

 1958; Taber, Claytor, Knight, Flowers, 

 Gambrell and Ayers, 1958) . It seems that 

 the estrogen inhibits the rudiment by a di- 

 rect action, as proposed by Kornfeld (1958) . 

 Recently, Kornfeld (1960) obtained ad- 

 ditional evidence that estrogen is the main 

 agent preventing development of the rudi- 

 ment. Injections of 17a-etliyl-19-nortestos- 

 terone, an anti-estrogen, resulted in its de- 

 velopment to four times the control size in 

 100-day-old chickens. The lack of develop- 

 ment after hypophysectomy in the poulard 

 may be the result of deficiency of thyroid 

 or adrenal hormones or of generalized meta- 

 bolic disturbances caused by the hypophy- 

 sectomy. It is known that adrenalectomy 

 (Hewitt, 1947) or inanition (Taber, Salley 

 and Knight, 1956) inhibits rudimentary 

 gonad development, and hypophysectomy 

 causes a sharp decrease in thyroid weight 

 (Baum and JMeyer, 1956). As has already 

 been shown in this chapter, reduced thyroid 

 activity severely restricts gonadal develop- 

 ment. The inhibitory effect of prolactin on 

 rudiment development may be explained by 

 a direct inhibitory effect of prolactin on the 

 rudiment or by possible side effects of pro- 



