Milk ejection in a marsupial, Macropus agilis.

Abstract

Marsupials give birth to young which are almost embryonic in form, and for the first few months of postnatal life they remain continuously attached to the nipples of the lactating mammary glands. The immature appearance of the neonate and the presence of an elaborate musculature in the wall of the pouch led observers to suggest that the contraction of the pouch functioned to compress the lactating mammary glands and thus force milk into the mouth of the young1. This view was dismissed when Enders2 demonstrated that milk was not expressed in the opossum, Didelphis virginiana, following electrical stimulation of the motor nerves supplying the muscles of the pouch, and he implied that milk was obtained solely by the sucking of the young. In most eutherian mammals very little milk can be extracted unless facilitated by an oxytocin-induced contraction of the myoepithelial cells surrounding the alveoli of the mammary gland. It is not known whether such a process of milk ejection exists in a marsupial, although oxytocin has been identified in the posterior pituitary gland3,4 and has been used as an aid in the collection of milk5. We recorded intramammary pressure from the lactating mammary gland of the agile wallaby, Macropus agilis, and examined the pressure responses evoked by injection of oxytocin and electrical stimulation of the presumptive oxytocin neurones. We report here three striking differences from those reported for eutherian mammals: the mammary gland of the wallaby, particularly during the early part of lactation ( 2,500 g).