Mammals demonstrate many unique aspects of development. Internal development within the maternal body requires many adaptations that are not seen in egg-laying animals. The embryo has to cope with many additional technical challenges. For example, it needs to take oxygen and nutrients from maternal circulation, while at the same time not provoking the maternal immune system to attack it. That said, there are also many advantages of internal (viviparous) development. The embryo is protected inside of its mother; it is well nourished and does not require external temperature regulation since the mother’s body provides a stable environment. These advantages are the reason that, even in taxonomic groups of egg laying animals, there are also some species that retain their eggs inside the body and give birth to already formed younglings. Mammals, however, brought this strategy to a qualitatively new and more advanced level. The yolk content of a mammal’s egg is drastically reduced compared to other terrestrial vertebrates, like reptiles and birds. Mammalian eggs basically underwent extreme miniaturization. Being 100 microns in diameter, the human egg is at the border of what is visible to the naked eye. Internal development and miniaturization of the egg cell resulted in great ignorance about mammalian development before the 20th century. To eliminate the need for yolk and to solve the problem of oxygen supply, some mammals use a unique, complex organ, called the placenta. The placenta is partially made of embryonic trophoblast, and partially made of maternal organ derived from the inner layer of the uterus, called endometrium. The uterus itself is an evolutionary novelty, an organ that provides a favorable environment for embryonic and fetal development in mammals. So far, human technology has been unable to make an artificial uterus. The uterus is extremely elastic, allowing for exponential growth of a mammal fetus. This muscular organ also provides power to expel the baby from the maternal body once it comes time to deliver. While the uterus is a permanent female organ, the placenta is a temporary organ. It develops along with the embryo, and genetically represents a mosaic of maternal and embryonic cells. The placenta is highly vascularized, but interestingly does not allow for the mixing of maternal and fetal blood. At the same time, it is a barrier between the two organisms, and the life-giving connection between them. Embryonic and fetal hemoglobin molecules have higher oxygen-binding affinity than the adult molecule. This is needed in order to “steal” the oxygen from maternal circulation, otherwise the mother and baby’s hemoglobin would equally attract oxygen, and it would not be transferred from the maternal to the fetal blood stream. There are three sets of hemoglobin genes: embryonic, fetal, and adult. The switch to the production of adult hemoglobin coincides with birth. In utero, the umbilical cord is attached to the placenta, bringing oxygen and nutrients to the fetus and carrying carbon dioxide and the harmful products of metabolism away. We can say that the baby uses its mother’s lungs, digestive system, and excretory system via the connection provided by the placenta. The placenta also suppresses the maternal immune system, so that the baby can develop safely inside its mother’s body without being attacked by maternal antibodies.
1. Identify the only non-placental mammal in North America
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a
raccoon (Procyon lotor)
b
opossum (Didelphis virginiana)
c
kangaroo (Macropus giganteus)
d
wombat (Vombatus ursinus)