GOG 



PHYSIOLOGY OF GONADS 



to those seeking further information about 

 this important new fiekl, the recent review 

 by Jacobsohn (19581. 



III. Endocrine Influences in Milk 

 Secretion 



A. ANTERIOR PITUITARY HORMONES 



1. Initiation of Secretion iLactogenesis) 



The early experiments leading to the 

 view that the anterior pituitary was not 

 only necessary for the initiation of milk 

 secretion, but in fact i)rovided a positive 

 lactogenic stimulus, are now well known 

 and the reader is referred to the reviews by 

 Folley (1952a, 1956) and Lyons (1958) for 

 further particulars. That pituitary prolactin 

 can evoke milk secretion in the suitably 

 de\-eloped mammary gland of the rabbit 

 with intact pituitary has been amply con- 

 firmed, and the original experiments of 

 Lyons (1942) involving the intraduct in- 

 jection of prolactin have been successfully 

 repeated by Meites and Turner (1947) and 



Fk;. 10.8. Liictation.'il lespon.scs in pseudoincg- 

 nant rabbit to different doses of prolactin injeclcd 

 intraductallv. (Fiom T. R. Bradley and P. M. 

 Clarke, J. Endo.ninol., 14, 28-36, 1956.) 



Bradley and Clarke (1956) (Fig. 10.8). 

 However, endogenous pituitary hormones 

 may have participated in the response in 

 such experiments and in the last 20 years 

 there has been considerable discussion as to 

 whether prolactin should be regarded as 

 the lactogenic hormone or as a component 

 of a lactogenic complex. This whole question 

 has been fully discussed in recent years (see 

 Folley, 1952a, 1956) and it now seems 

 reasonably certain that lactogenesis is a 

 response to the co-operative action of more 

 than one anterior pituitary hormone, that 

 is, to a lactogenic hormone complex of which 

 prolactin is an important component, as 

 first suggested by Folley and Young (1941 ) . 

 The recent reports by Nandi (1958a, b) 

 that STH -I- Cortisol can induce milk secre- 

 tion in triply operated mice with suitably 

 developed glands is further strong evidence 

 against regarding prolactin as the lactogenic 

 hormone. 



Secretory activity is evident in the mam- 

 mary gland during the second half of preg- 

 nancy, but abundant milk secretion does 

 not set in until parturition or shortly there- 

 after. The nature of the mechanism control- 

 ling the initiation of abundant secretion has 

 been the subject of speculation for many 

 years. The earlier theories w^ere discussed 

 l)y Turner ( 1939 ) in the second edition of 

 this book, and included the theory put 

 forward by Nelson with reference to the 

 guinea pig, that the high levels of blood 

 estrogen in late pregnancy suppressed the 

 secretion or release of prolactin from the 

 pituitary and had also a direct inhibitory 

 cttcct on the mammary parenchyma, the 

 fall in the levels of estrogen occurring at 

 parturition then allowing the anterior pitui- 

 tary to exert its full lactogenic effect. This 

 concept proved inadequate to exjilain ob- 

 servations in other species and it was later 

 extended by Folley and Malpress (1948b) 

 to embrace the concept of two thresholds 

 for oi:)posing influences of estrogen upon 

 jiituitary lactogenic function, a lower 

 threshold for stimulation and a higher one 

 for inhibition. Subsequent observations on 

 the inhibitory role of progesterone, in the 

 pix'sence of estrogen, on milk secretion, how- 

 ever, necessitated further modification of 

 the theorv. Before discussing these modifica- 



