MILK EJECTION IN A MARSUPIAL, MACROPUS-AGILIS

Marsupials give birth to young which are almost embryonic in form, and for the 1st mo. 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 to the suggestion that the contraction of the pouch functioned to compress the lactating mammary glands and thus force milk into the mouth of the young. This view was dismissed when it was demonstrated that milk was not expressed in the opossum, Didelphis virginiana, following electrical stimulation of the motor nerves supplying the muscles of the pouch. Possibly, 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 was identified in the posterior pituitary gland and was used as an aid in the collection of milk. Intramammary pressure from the lactating mammary gland of the agile wallaby, M. agilis was recorded and the pressure responses evoked by injection of oxytocin and electrical stimulation of the presumptive oxytocin neurons were examined. Three striking differences from those reported for eutherian mammals were reported: the mammary gland of the wallaby, particularly during the early part of lactation (< 100 days), was more sensitive to oxytocin, produced higher intraductal pressures and contracted at lower frequencies of brain stimulation (4 pulses/s). The wallaby, far from discarding during its evolution the milk-ejection response to oxytocin, apparently refined the response to permit it to feed simultaneously both newly born pouch young (< 1 g) and young at foot (> 2500 g).