MAMMARY GLAND AND LACTATION 



623 



of doiil)tful validity, because the milk-ejec- 

 tion effect observed may have been due to 

 5-hydroxytryptamine (see Linzell, 1955), 

 and more recent attempts to assay the level 

 of oxytocin in the blood have not been 

 entirely satisfactory or conclusive. There 

 seem to be other polypeptide substances in 

 blood which possess oxytocic activity, al- 

 though the thiogly collate inactivation test 

 indicates that these are different from oxy- 

 tocin (Robertson and Hawker, 1957), and 

 no marked changes in the blood oxytocic 

 activity associated with suckling or milking 

 have been detected (Hawker and Roberts, 

 1957; Hawker, 1958). However, it would 

 seem doubtful whether the present assay 

 techniques are sufficiently sensitive and spe- 

 cific to detect changes in blood oxytocin of 

 the magnitude likely to be associated with 

 milking or suckling. In the lactating cow 

 the intravenous injection of 0.05 to 2.0 I.U. 

 oxytocin will cause milk ejection (Bilek and 

 .Tanovsk>% 1956; Donker, 1958), in the goat 

 0.01 to 1 I.U. (Cowie, cited by Folley, 

 1952b; Denamur and Martinet, 1953), in 

 the sow 0.2 to 1.0 I.U. (Braude, 1954; Whit- 

 tlestone, 1954; Cross, Goodwin and Silver, 

 1958) in the rabbit 0.05 I.U. (Cross, 1955b) , 

 and in the lactating woman 0.01 I.U. (Bel- 

 ler, Krumholz and Zeininger, 1958) . If these 

 (loses give any indication of the quantity 

 of endogenous oxytocin released, then the 

 concentration in the peripheral blood is 

 likely to be very small ; indeed Cross, Good- 

 win and Silver (1958) calculated that a 

 threshold dose (10 mU.) of oxytocin in 

 the sow w^ould give a plasma concentration 

 of about 1 (U,U. per ml, and until it can be 

 shown that the assay techniques are suf- 

 ficiently sensitive to detect the changes 

 in oxytocin concentration produced by in- 

 travenous injections of "physiologic" doses 

 of oxytocin, no great reliance can be placed 

 on the results of assays. 



Attempts have been made to demonstrate 

 alterations in the hormone content of the 

 neural lobe following the suckling or milk- 

 ing stimulus. In the goat and cow no de- 

 tectable changes have been reported, but in 

 the smaller species (dog, cat, rat, guinea 

 pig) decreases have been described (see 

 Cowie and Folley, 1957). It is likely that in 

 many species the amount released is small 



relative to tlie total hormone content of the 

 gland and within the limits of error of the 



assay. 



D. EFFECTOR CONTRACTILE MECHANISM OF 

 THE MAMMARY GLAND 



In the last 10 years considerable research 

 has been devoted to a study of the effector 

 contractile tissue in the mammary gland; 

 this work has recently been reviewed in 

 some detail (see Folley, 1956) and only the 

 salient features need be mentioned here. 



Although earlier histologists had from 

 time to time figured myoepithelial or "bas- 

 ket" cells in close association with the mam- 

 mary alveoli, the morphology and distribu- 

 tion of the cells remained vague until 

 Richardson (1949) published a detailed and 

 illuminating description (Fig. 10.18). His 

 beautiful observations have since been con- 

 firmed and supplemented by Linzell (1952) 

 and Silver (1954). Richardson also disposed 

 of the oft repeated view that smooth-mus- 

 cle fibers around the alveoli played an im- 

 l)ortant role in milk ejection. From a study 

 of the general orientation of the myoepithe- 

 lial cells and the precise relationship between 

 these cells and the folds in the secretory epi- 

 thelium from contracted glands, Richardson 

 considered it reasonable to regard the myo- 

 epithelium as the contractile tissue in the 

 mammary gland which responds to oxytocin 

 causing contraction of the alveoli and wid- 

 ening of the ducts. The evidence adduced by 

 Richardson, although good, was neverthe- 

 less circumstantial, and it was desirable that 

 attempts be made to visualize the contrac- 

 tion of the myoepithelial cells in response to 

 oxj^tocin. In this connection it is of interest 

 to recall that Gaines (1915) reported that 

 when a drop of pituitrin was placed on the 

 cut surface of the mammary gland from a 

 lactating guinea pig, minute white dots ap- 

 peared within a few seconds beneath the 

 pituitrin and slowly swelled to tiny milky 

 rivulets streaming beautifully through the 

 clear liquid. Much later the local effects of 

 posterior pituitary extract on the mammary 

 gland were studied by Zaks (1951) in the 

 living mouse, when it was reported that it 

 caused contraction of the alveoli and ex- 

 pansion of the ducts. These observations 

 were considerablv extended bv Linzell 



