GIO 



PHYSIOLOGY OF GONADS 



It is abo now obvious that oxytocin may 

 have to be injected to ensure milk ejection; 

 under certain circumstances, however, the 

 neurohypophyseal tissue remaining after 

 the removal of the posterior lol^e may be 

 capable of releasing oxytocin and permit- 

 ting milk ejection (see Benson and Cowie, 

 1956; Bintarningsih, Lyons, Johnson and Li. 

 1957, 1958). 



The earliest report on the maintenance 

 of lactation after hypophysectomy is that of 

 Gomez (1939, 1940), who found that hy- 

 pophysectomized lactating rats could rear 

 their litters if given anterior-pituitary ex- 

 tract, adrenal cortical extracts, glucose, and 

 posterior pituitary extract. These experi- 

 ments are difficult to assess because they are 

 reported only in abstract, but the use of pos- 

 terior pituitary extract at a time when the 

 role of oxytocin in milk ejection was not 

 generally recognized is worthy of note. Re- 

 cently, slight maintenance of milk secretion 

 in hypophysectomized rats has been ob- 

 tained with prolactin alone, and greater 

 maintenance when adrenocorticotrophic 

 hormone ( ACTH I or STH was administered 

 with prolactin (Cowie, 1957). Similar 

 studies were reported by Bintarningsih, 

 Lyons, Johnson and Li (1957, 1958) (see 

 also Lvons, Li and Johnson, 1958) in which 



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Fig. 10.10. Effect on the luilk yield of the cow 

 of injected hormones of the anterior pituitary. 

 (From the results of P. M. Cotes, J. A. Crichton, 

 S. J. Folley and F. G. Young, Nature, London. 

 164, 992-993, 1919.) 



considerable maintenance of milk secretion 

 was obtained in hypophysectomized rats 

 with prolactin and certain corticoids. Of 

 related interest is the observation by Elias 

 (1957) that Cortisol and prolactin can in- 

 duce secretory activity in explants of mouse 

 mammary gland growing on a synthetic 

 medium. (Tissue culture techniques have 

 been little exploited in mammary studies 

 and further developments in this field may 

 be expected.) 



The evidence to date suggests that, in the 

 rat, prolactin is an essential component of 

 the hormone complex involved in the main- 

 tenance of lactation with ACTH and STH 

 also participating, but further studies are 

 recjuired to determine the most favorable 

 balance of these factors. 



Preliminary studies on the maintenance of 

 lactation in the goat after hypophysectomy 

 suggest that both prolactin and STH are im- 

 portant in the initiation and maintenance of 

 milk secretioii (Cowie and Tindal, 1960). 

 Our knowledge of the process in other spe- 

 cies is derived from studies on the effect 

 of exogenous anterior pituitary hormones 

 on established lactation in intact animals— 

 galactopoietic effects (for reference see 

 Folley, 1952a, 1956). In the cow, consider- 

 able increase in milk yield can be obtained 

 by injecting STH (Cotes, Crichton, Folley 

 and Young, 1949), whereas prolactin has 

 a negligible galactopoietic effect (Fig. 10.10; 

 for discussion see also Folley, 1955). Re- 

 cently the precise relationship between the 

 dose of STH (ox) and the lactational re- 

 sponse in the cow was established in our lab- 

 oratory by Hutton (1957) who observed a 

 highly significant linear relationship be- 

 tween log doses of STH (single injection) 

 and the increase in milk yield obtained (Fig. 

 10.11 ) ; increases in fat yield relative to the 

 yield of nonfatty solids also occurred. In the 

 lactating rat, on the other hand, STH has 

 no galactopoietic effect (Meites, 1957b; 

 Cowie, Cox and Naito, 1957), whereas pro- 

 lactin has (Johnson and Meites, 1958). Such 

 studies must be interpreted with caution as 

 endogenous pituitary hormones were pres- 

 ent ; nevertheless, it seems reasonable to 

 conclude that STH is likely to be an im- 

 poi'tant factor in the maintenance of lacta- 

 tion in the row. 



