The effect of sex hormones on lactation in the rat

Abstract

The mechanism which secures the onset of full lactation at about the time of parturition has long been under investigation. It was once believed that during the gestation period when the mammary gland is undergoing development, lactation was restrained by some inhibitor, the influence of which was removed at parturition, the stimulus of lactation being regarded as of a negative character, that is, merely the removal of an inhibition. The important discovery of Stricker and Grueter (1928) that the anterior lobe of the hypophysis is the source of a lactogenic hormone, prolactin, which provides a positive lactation stimulus has necessitated modification of the earlier view. It should be emphasized that the influence of the hypophyseal lactogenic hormone must be operative to some extent during pregnancy, since numerous histological and cytological studies (e. g. that of Weatherford (1929) on the rat) have shown that mammary secretion begins in the latter half of pregnancy. This is well illustrated by the fact that in late pregnancy considerable quantities of a secretion containing the typical milk constituents can be obtained, by regular milking, from the udders of cows pregnant for the first time (Asdell 1925). The results of Cutler and Lewis (1933) indicate from another angle that the secretory phase of the lactation cycle begins in mid-pregnancy, since by the complement-fixation reaction they could always detect caseinogen in extracts of the udders of pregnant heifers after the 24th week of gestation, but in no case before the 20th week. The great increase in the rate of milk secretion which follows parturition, indicates, however, either that during pregnancy some influence is operative which partially inhibits lactation or else that during this period the lactation stimulus is only partial. In agreement with the former alternative is the fact that in the cow pregnancy hastens the normal decline in lactation, the depressing effect of pregnancy coming into evidence at about the 5th month (Hammond and Sanders 1923; Ragsdale, Turner and Brody 1924). It is interesting to recall that in 1926, Gaines and Davidson were of the opinion that the accelerated decline in lactation due to pregnancy was due to the operation of a then hypothetical depressing hormone.