THE HORMONES IN PREGNANCY 



subsequent work by various investigators, well summarized by 

 C. W. Turner of Missouri, himself one of the leaders in this 

 work, in Sex and Internal Secretions} It now appears that 

 the second of the three more or less distinct stages of growth 

 and function of the mammary gland, already referred to and 

 illustrated (Fig. 29), involves somewhat different responses 

 to the hormones in different species. The first stage, that of 

 preliminary growth of the duct system to the adult virginal 

 state, occurs under the influence of estrogenic hormone as 

 already described. In the second stage, the ends of the ducts 

 proliferate and branch into numerous terminal alveoli. This 

 stage in many animals requires the action of progesterone ; in 

 a few species (of which the guinea pig is an example) pro- 

 gesterone is not needed at all and the proliferation can be 

 completely induced by the estrogens ; in some other species 

 growth of the alveoli is at least facilitated or speeded up by 

 progesterone, though not actually dependent upon that hor- 

 mone. Judging from various observations which have been 

 reported on monkeys, the primate mammary gland is among 

 this intermediate group. Whether this is also true of the 

 human we do not know at present. The third stage, that of 

 secretion of milk, is brought about by the lactogenic hor- 

 mone of the pituitary. It is a striking evidence of the potency 

 of these hormones to induce lactation that the rudimentary 

 mammary glands of male animals can readily be made to 

 lactate by a suitable course of treatment with the estrogen- 

 prolactin or estrin-progesterone-prolactin sequence. 



We have yet to discover how the pituitary gland is stimu- 

 lated to exert its lactogenic effect during pregnancy. The 

 reason the flow of milk does not begin until just after parturi- 

 tion, and then begins suddenly, is that lactation is inhibited 

 by the estrogenic hormone of the placenta. Once the placenta 

 is out of the way, the flow of milk is released. 



8 Sex and Internal Secretions, edited by Edgar Allen, Baltimore, 2d 

 ed., 1939; Chapter XI, The Mammary Glands, by C. W. Turner. 



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