Review: Marsupials: Placental Mammals with a Difference

A full term tammar fetus, still enclosed in the fetal membranes and with the placenta still attached to the endometrium of the uterus.

Diagram of the basic difference in structure between the eutherian blastocyst, with its inner cell mass, and that of the marsupial, which totally lacks an inner cell mass. The genes that regulate this differentiation are Cdx2, Nanog and Oct4 (Pou5f1). Cdx2 is expressed in the trophoblast, while Nanog and Oct4 are expressed in the inner cell mass and downregulate Cdx2.

Composition of the fetal fluids and hormones and growth factors expressed by the placenta of the tammar. The yolk sac fluid is rich in nutrients, with specific secretion of proteins and amino acids. It maintains a high concentration of glucose compared to that of the maternal circulation. So far the mRNA of several hormones has been isolated from the placenta. It also synthesizes significant quantities of prostaglandin F2a at parturition. (Data from and BR Menzies, AJ Pask, G Shaw and MB Renfree, unpublished results).

Maternal recognition of pregnancy occurs in marsupials as in eutherians. The endometrium becomes highly proliferative on the gravid uterus. When blastocyst transfers were done, the endometrium on the gravid side responded by proliferation regardless of whether it was the uterus ipsilateral or contralateral to the corpus luteum and its secretion of progesterone. These data provided the first evidence that there is a maternal recognition of pregnancy in marsupials, and that the effect was due to the presence or absence of the developing fetal membranes (placenta) that we now know are capable of hormonal secretion (Data from []). F, follicle; CL, corpus luteum; V, vesicle with embryonic disc.

Model to show the some of the functions of the tammar chorio-vitelline placenta during pregnancy and parturition. (Data from ).