REPRODUCTION OF A MARSUPIAL - FROM UTERUS TO POUCH

Parturition in marsupials is a remarkable process because the maternal endocrine system must be able to respond to signals from the tiny alt ricial neonate which may be as small as 5 mg (honey possum) or as 'lar ge' as 800 mg (red and grey kangaroos). From the relatively few studie s that have been conducted, it is clear that many features involved in the control of birth are held in common with those of eutherians. Bas ed on current data on parturition in one species, the tammar wallaby, it is now clear that prostaglandins F-2 alpha and PGE(2) (PGs) are the key regulators. The active oxytocic peptide in tammars is mesotocin ( MT), which appears to facilitate birth. In the second half of gestatio n the myometrium becomes sensitive to mesotocin. As the fetus nears te rm, PG production in both the endometrium and yolk-sac placenta increa ses. A surge of PG occurs at term which stimulates uterine contraction s, It induces stereotyped parturient behaviour a few minutes before em ergence of the fetus, so that it can safely climb from the urogenital sinus to the pouch. Treatment with prostaglandin synthetase inhibitors through late pregnancy prevents birth. The fetus determines the timin g of birth and premature birth can be induced with the glucocorticoid agonist dexamethasone. Thus, maturation of the fetal hypothalamo-pitui tary-adrenal axis may lead to release of cortisol. Cortisol, in rum, m ay increase PG production by the uterus or placenta eventually leading to the parturient surge of this hormone in the plasma. Rising PG leve ls, either alone or in concert with MT, presumably lead to increased u terine contractions to ensure completion of the birth process once it is started. The small size of the term fetus (under 400 mg) and the lo w mass of the placenta (under 100 mg) suggests that paracrine mechanis ms between the fetus, placenta and uterus may play a more important ro le in regulating PG production or uterine oxytocin receptor levels tha n occurs in most eutherians. However, the nature of the fete-placental signals and the interactions between PGs and MT remain enigmatic.