The control of milk ejection in the mammal

Abstract

Extract

Occasionally the need arises for a review of a subject aimed at putting the available evidence into perspective. Over the past decade a considerable amount of work has been done on the milk-ejection mechanism of the mammal. Too frequently this reflex has been regarded as isolated from other aspects of mammalian reproduction. In this review, an attempt is made to outline our basic knowledge of the nature of the milk-ejection process and the mechanisms involved in it against the background of the mammalian reproductive system. For along time it has been known that the suckling stimulus has an effect on the ability of the mammal to secrete milk. At the same time it is known that there are no neural pathways to the anterior pituitary which releases the hormones controlling mammary gland activity. Thus the mechanism by which the stimuli of suckling could affect the release of anterior lobe hormones has not been at all clear. Evidence is rapidly accumulating for the idea that the posterior pituitary hormones, which must pass through the anterior pituitary gland on their way to the systemic circulation, may have an effect on the release of anterior lobe substances. This effect is made possible by the peculiar system of circulation which exists between the neurohypophyseal tract and the anterior pituitary. Evidence for this view is summarized in what follows, and its significance as a factor in the maintenance of milk secretion is considered. It has been thought worth while to present a generalized picture of the milk-ejection reflex and of certain other reproductive reflexes as recent developments within the mammals superimposed upon the “water balance” regulating mechanisms which are to be found throughout the entire vertebrate phylum. In doing this, a certain amount of over-simplification has been inevitable, but it is hoped that the review will place some of the problems of lactation in perspective and, in so doing, show the many gaps in our knowledge which must be filled before we have an adequate understanding of the process controlling the secretion and ejection of milk in the mammal.