598 



PHYSIOLOGY OF GONADS 



5. Mam /nary Involution 



The involutionary changes which occur in 

 the mammary gland after weaning in vari- 

 ous species were described in the previous 

 edition of this book (Turner, 1939) and in a 

 later review by Folley (1952a). Since that 

 time, a few further studies have appeared. 



There is evidence that the course of the 

 histologic changes in the regressing mam- 

 mary gland may differ according to whether 

 the young are weaned after lactation has 

 reached its peak and is declining, or whether 

 they are removed soon after parturition, 

 when the effects of engorgement with milk 

 seem to be more marked (see, for example, 

 Williams, 1942, for the mouse). In rats 

 whose young were weaned soon after par- 

 turition Silver (1956) was able to re-estab- 

 lish lactation provided suckling was resumed 

 within 4 or 5 days; after that time irre- 

 versible changes in the capillary blood sup- 

 ply to the alveoli had set in. A further point 

 arises from a study on the cow by Mosimann 

 (1949) which indicates that the course of 

 the regressive changes in a gland which has 

 undergone one lactation only may differ 

 from those seen in glands from muciparous 

 animals. Oshima and Goto (1955) have used 

 quantitative histometric methods in a study 

 of the involuting rat mammary gland ; the 

 values which they obtained for the per- 

 centage parenchyma 7 to 10 days after re- 

 moval of the young agree quite well with 

 tiiose reported by Benson and Folley 

 ( 1957b) for rats weaned at the 4th day and 

 killed 9 days later. 



The biochemical changes occurring in 

 mammary tissue during involution arc of 

 some interest and have been studied in our 

 laboratory by McNaught (1956, 1957). She 

 studied mammary slices taken from rats 

 whose young were removed at the 10th day 

 and also slices from suckled glands, the es- 

 cajie of milk from which was prevented by 

 ligation of the galactophores, the other 

 glands in the same animals remaining intact 

 and serving as controls. Her results, some of 

 whicli are summarized in Figure 10.4, sug- 

 gest that functional changes which may be 

 taken as indicative of involution (decrease 

 in oxygen up-take, respiratory quotient 

 (R.Q.), and glucose up-take; increase in 

 lactic acid prcxUiction ) are seen as early as 



8 to 12 hours after weaning. Continued 

 suckling without removal of milk retards 

 tlie onset of these changes, but only for some 

 hours. Injections of oxytocin into the rats 

 after weaning (see page 607) did not retard 

 these biochemical changes. Essentially simi- 

 hii' results were independently reported by 

 Ota and Yokoyama (1958) and Mizuno and 

 Chikamune (i958). 



C. EXPERIMENTAL ANALYSIS OF HORMONAL 

 INFLUENCES 



1. Ovarian Hortnones in the Animal with 

 Intact Pituitary 



We shall see later (page 602) that the 

 mammogenic effects of the ovarian hor- 

 mones are largely dependent on the integrity 

 of the a'nterior pituitary and thus to ana- 

 lyze accurately the role of hormones in mam- 

 mary development it is necessary to use hy- 

 pophysectomized animals. Information of 

 considerable academic and practical impor- 

 tance has been obtained, however, from 

 studies in the animal with intact pituitary 

 and these we shall now consider. 



Early studies involving hormone adminis- 

 tration pointed to the conclusion that estro- 

 gens were in general resi)onsible for the 

 growth of the mammary (hicts, whereas pro- 

 gesterone was necessary for complete lobule- 

 alveolar growth (see reviews, l)y Turner, 

 1939; Folley and Malpress, 1948a; Folley, 

 1952a). The foundation for i^liis general 

 statement is now more sure, for as a result 

 of experimental studies over the last 10 

 years, what seemed to be exceptions to this 

 generalization have been shown to be other- 

 wise. In some species (mouse, rat, guinea 

 \)ig, and monkey) it is true that progester- 

 one alone, if given in sufficiently large doses, 

 will evoke duct and alveolar development in 

 the ovariectomized animal, but this is prob- 

 ably a pharmacologic rather than a physio- 

 logic effect. There are great differences in 

 the response of the mammary ducts to estro- 

 gen and on this basis it has become usual to 

 divide species into three broad categories 

 (see FoUey, 1956). It is, however, necessary 

 to add the warning that in the estrogen- 

 tre.'ited spayed animal progesterone from the 

 a(h'eiial eoiiex may synergize with the ex- 

 ogenous estrogen (see Folley, 1940; Trentin 

 and 1'ui'iier, 1947; Hohn, 1957) and it mav 



