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Lecture 10

DEV2011: Lecture 10 summary

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Department
Medicine
Course Code
DEV2011
Professor
Various

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LECTURE 10 Implantation: Implantation is the very early stage of pregnancy at which the embryo adheres to [1] the wall of the uterus. At this stage of prenatal development, the embryo is a blastocyst. It is by this adhesion that the fetus receives oxygen and nutrients from the mother to be able to grow. In humans, implantation of a fertilized ovum is most likely to occur about 9 days after ovulation, ranging between 6 to 12 days. Amnion: The amnion is a membrane building the amniotic sac that surrounds and protects an embryo. The primary role of this is the protection of the embryo for its development. It stems from the extraembryonic somatic mesoderm on the outer side and the extraembryonic ectoderm on the inner side. When first formed, the amnion is in contact with the body of the embryo, but about the fourth or fifth week fluid (liquor amnii) begins to accumulate within it. This fluid increases in quantity and causes the amnion to expand and ultimately to adhere to the inner surface of the chorion, so that the extra-embryonic part of the coelom is obliterated. The liquor amnii increases in quantity up to the sixth or seventh month of pregnancy, after which it diminishes somewhat; at the end of pregnancy it amounts to about 1 liter. The amniotic fluid allows the free movements of the fetus during the later stages of pregnancy, and also protects it by diminishing the risk of injury from without. It contains less than two percent solids, consisting of urea and other extractives, inorganic salts, a small amount of protein, and frequently a trace of sugar. Yolk Sac: The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk. It functions as the developmental circulatory system of the human embryo, before internal circulation begins. It is the first element seen in the gestational sac during pregnancy, usually at 5 weeks gestation. The yolk sac is situated on the ventral aspect of the embryo; it is lined by extra-embryonic endoderm, outside of which is a layer of extra- embryonic mesenchyme, derived from the mesoderm. Allantois: It is a part of a developing amniote's conceptus (which consists of all embryonic and extra-embryonic tissues). It helps the embryo exchange gases and handle liquid waste. This sac-like structure is primarily involved in nutrition and excretion, and is webbed with blood vessels. The function of the allantois is to collect liquid waste from the embryo, as well as to exchange gases used by the embryo. In placental mammals, the allantois is part of and forms an axis for the development of the umbilical cord. The human allantois is an endodermal evagination of the developing hindgut, which becomes surrounded by the mesodermal connecting stalk. The connecting stalk forms the umbilical vasculature. The fetal bladder is connected to the allantois via the urachus, which removes nitrogenous waste from the fetal bladder. The allantois is vestigial and may regress; yet the homologous blood vessels persist as the umbilical arteries and veins connecting the embryo with the placenta. Human Placenta: The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. The umbilical cord inserts into the chorionic plate (has an eccentric attachment). Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons. In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species. The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium. The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development. The syncytiotrophoblast (otherwise known as syncytium), thereby contributes to the barrier function of the placenta. The placenta grows throughout pregnancy. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy. Trophoblast: Trophoblasts are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. They are formed during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg. This layer of trophoblasts is also collectively referred to as "the trophoblast", or, after gastrulation, the trophectoderm, as it is then contiguous with the ectoderm of the embryo. Trophoblasts are specialized cells of the placenta that play an important role in embryo implantation and interaction with the decidualised maternal uterus. The core of placental villi contain mesenchymal cells and placental blood vessels that are directly connected to the fetal circulation via the umbilical cord. This core is surrounded by two layers of trophoblast; a single layer of mononuclear cytotrophoblast that fuse together to form the overlying multinucleated syncytiotrophoblast laye
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