Extraembryonic membranes are membranous structures that appear parallel to the embryo and which play important roles in embryonic development. They form from the embryo but do not become part of the individual organism after its birth.
The extraembryonic membranes that may be present in vertebrates are the yolk sac, the amnion, the chorion, the allantois and the placenta.
The presence of each extraembryonic membrane varies according to the vertebrate class.
In fish and amphibians, only the yolk sac is present. In reptiles and birds, in addition to the yolk sac, the amnion, the chorion and the allantois are also present. In placental mammals, in addition to all these membranes, the placenta is also present.
The yolk sac is formed from the covering of the vitellus by cells originating from the primitive gut.
The yolk sac stores vitellus, the main source of nutrition for non-placental embryos.
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The allantois is the extraembryonic membrane whose function is to store excretions of the embryo.
In placental mammals, the allantois is present but it does not exert that function, since embryonic wastes are collected by the mother’s body through the placenta.
The allantois is an adaptation to dry land because in embryos of oviparous terrestrial organisms, such as reptiles and birds, metabolic wastes cannot be immediately excreted to aquatic surroundings (like fish and amphibian larvae do). Therefore, the appearance of a structure capable of storing embryonic excretions until hatching was necessary.
The amnion is the membrane that covers the embryo. The chorion is the membrane that covers the amnion, the yolk sac and the allantois. The space delimited by the chorion and the amnion is called the amniotic cavity and it is filled with amniotic fluid. The amniotic cavity has the function of preventing the drying out of the embryo and protecting it against mechanical shocks.
The amnion is also an adaptation to dry land since one of its functions is to prevent the embryo from drying out.
The chorioallantois membrane is formed by juxtaposition of certain regions of the chorion and the allantois. Since it is porous, the chorioallantois membrane allows the passage of gases between the embryo and the exterior, thus making aerobic cellular respiration possible.
A true placenta is present in placental mammals.
The placenta is formed from the embryo's chorion and the mother’s endometrium. Its main function is to allow the exchange of substances between the fetus and the mother’s body.
From the mother to the fetus, the main substances transferred through the placenta are water, oxygen, nutrients and antibodies. From the fetus to the mother, the main substances transferred are metabolic wastes, including urea (nitrogen waste), and carbon dioxide.
Under normal conditions, cells do not cross the placenta during gestation. The placenta has smooth mucosa which separate the richly vascularized region in contact with the mother’s endometrium from the umbilical cord in contact with fetal blood. This barrier is known as the placental barrier. Although permeable to some substances (selective permeability), the placental barrier prevents the passage of cells.
The placenta has an endocrine function, since it secretes the hormones progesterone and estrogen, which maintain the endometrium (the internal covering of the uterus) and prevent menses during pregnancy. The placenta also secretes other important hormones for pregnancy regulation.
The umbilical cord is a set of blood vessels that connects the fetus with the placenta. In the fetus, one end of the cord is inserted into the center of the abdominal wall (the scar of this insertion is the umbilicus or navel).
The function of the umbilical cord is to allow the transport of substances, nutrients, gases and wastes, between the fetus and the mother’s body.
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