Placental, or eutherian mammals, like ourselves, provide nutrients and oxygen to developing young via the placenta, connecting the foetus to the mother. Marsupials are traditionally thought to lack this organ, but it may be more accurate to say that they rely on it less. In tammar wallabies (pictured), mothers do develop a reduced placenta-like structure during their short pregnancies. Based on genetic analyses, it appears to behave like a typical placenta in early stages of pregnancy. Moreover, when the young wallaby continues to develop in its mother’s pouch, similarities in gene expression between lactating wallabies and both mouse and human placentas suggest that marsupial milk fulfils the same nutritional roles as a eutherian placenta later in pregnancy. As it is essential to the developing foetus, studying the placenta is difficult, so work on other mammals represents a valuable opportunity to learn more about the evolution and functions of this organ.
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