798 VII. LIPID DISTRIBUTION IN SPECIFIC TISSUES 



view on the chemistry and physiology of prolactin was compiled by White, 

 in 1949. 512 



In addition to luteotropin, other hormones are necessary for the produc- 

 tion of milk. Thus, although luteotropin is effective in developing the 

 mammary tissue in preparation for lactation, thyroxine is also required if 

 milk secretion is actually to occur. Apparently the ovaries secrete an 

 inhibitory substance; in ovariectomized rats prolactin caused secretion of 

 milk, irrespective of whether or not thyroxine was present. 514 However, 

 Desclin 514 was unable to demonstrate that prolactin alone or with thyroxine 

 exerted an effect upon the mammary gland of the hypophysectomized rat. 

 After parturition, this pituitary hormone (prolactin) is released, resulting 

 in a stimulation of milk production. 515 Folley and Young 516 reported that 

 an increase in milk yield occurred in cows following the injection of pro- 

 lactin; this was accompanied by a substantial increase in the milk fat con- 

 tent. The average daily production of milk fat rose nearly 50% during the 

 period of five successive injections of prolactin. According to Smith and 

 Dastur, 517 thyroxine also increased the yield of milk and of milk fat in 

 lactating cows. However, there was no important alteration in the nature 

 of the milk fat during the period when its yield was being enhanced through 

 the administration of thyroxine. 



(S) The Origin of the Milk Lipids 



Comparatively large amounts of fats are secreted by the mammary 

 gland during periods of high milk production. According to Folley, 518 

 a cow yielding 50 to 60 pounds of milk daily will secrete about 1 kg. of 

 fat per day; this is about one-fourth of the total dry weight of the udder. 

 Several possible sources of this fat have been suggested, namely that it is 

 derived from dietary fat, from body fat transported to the mammary 

 gland in the blood, and also from fat synthesized by the mammary gland 

 in situ. The first suggestion as to the source of milk fat is not adequate 

 to explain the marked differences in fatty acid composition and in glyceride 

 structure between milk fats and food fats, as well as the several body fats. 



a. Dietary Fat as a Source of Milk Fat. Smith and associates 519 have 



614 L. Desclin, Compt. rend. soc. biol, 142, 1172-1174 (1948). 



515 W. O. Nelson, Physiol. Revs., 16, 488-526 (1936). 



516 S. J. Folley and F. G. Young, Biochem. J., 33, 192-197 (1939). 



617 J. A. B. Smith and N. N. Dastur, Biochem. J., 34, 1093-1107 (1940). 



618 S. J. Folley, "Aspects of Fat Metabolism in the Ruminant," in R. T. Williams, 

 Lipid Metabolism, Biochem. Soc. Symposia, No. 9, Cambridge Univ. Press, 52-65 

 (1952). 



819 F. H. Smith, C. A. Wells, and P. V. Ewing, Georgia Expt. Sta., Bull. No. 122, 95- 

 111 (June, 1916). 



