FIRST PART OF 

 PREGNANCY 



MILK 

 SECRETION 



Fig. 58. Mammary gland changes in relation to reproduction. (Figures are a modifica- 

 tion of a figure by Corner: Hormones in Human Reproduction, Princeton, Princeton 

 University Press. The figure in the latter work was based on a figure by C. D. Turner: 

 Chap. XI of Sex and Internal Secretions, by Allen, et al., Baltimore, Williams & Wilkins, 

 1939.) Factors involved in mammary gland development and secretion are somewhat as 

 follows: (A, A') Condition of the young, infantile gland. (B) Development from a 

 simple, branched, tubular gland of the immature animal (A') into a compound tubular 

 gland presumably under the direct stimulation of estrogen, according to one theory, or 

 by the action of estrogen upon the pituitary gland which then releases mammogen I, 

 producing these changes, according to Turner, et al.: Chap. XI, Sex and Internal Secre- 

 tions, by Allen, et al., Baltimore, Williams & Wilkins. (C) Transformation of the com- 

 pound tubular gland into a compound tubulo-alveolar gland under the influence of proges- 

 terone, during the first part of pregnancy, or, according to Turner, et a!., by the influence 

 of estrogen plus progesterone which causes the pituitary to release a second mammogen 

 which produces the alveolar transformation. (D) Effect of the latter part of pregnancy 

 is to bring about a development of the cells of the acini of the acinous or alveolar system. 

 The unit shown in (D) represents a simple, branched, acinous gland, in which there are 

 six alveoli or acini associated with the duct. (E) Affect of parturition is to release the 

 lactogenic hormone (prolactin; luteotrophin) from the pituitary gland which brings about 

 milk secretion. During pregnancy the high levels of estrogen presumably inhibit milk 

 secretion. However, following pregnancy the level of estrogen is lowered permitting 

 lactogenic-hormone action upon the alveoli of the gland. 



The removal of the placenta and embryo at any time during gestation permits milk 

 flow, provided the mammary glands are sufficiently developed. In the human, any remains 

 of the placenta after birth inhibit milk secretion, probably because the estrogenic hormone 

 is elaborated by the placental remnants. (See Selye, '48, p. 829.) 



In the rabbit, estrogen and progesterone are necessary for the elaboration of the duct 

 and secretory acini; in the guinea pig and goat, and to some extent in the primates, 

 including the human female, estrogen alone is capable of producing the development of 

 the entire duct and acinous system. (See Turner, '48, p. 430.) 



105 



