Extraembryonic Structures

• The decidua reaction is a progesterone-induced endometrial cell conversion to secretory decidual.
• After birth, the urachus becomes a fibrous cord, known as the Median Umbilical Ligament.
• As the bladder enlarges, the allantois involutes to form a thick tube, called the Urachus.
• The fetal membranes are the placenta, umbilical cord, amnion, yolk sac, and allantois.
• The chorion is a component of the fetal membranes.
• The amnion, yolk sac, and allantois are components of the fetal membranes.
• The decidua is the term for the uterine lining (endometrium) during a pregnancy, which forms the maternal part of the placenta.
• The decidua forms highly characteristic cells and envelops the conceptus during gestation.
• The decidua is shed during delivery and periodically with menstruation.
• The decidua has three regions named according to their relation to the implantation site: the decidua basalis, the decidua capsularis, and the decidua parietalis.
• The placenta is a fetomaternal organ with two components: a fetal portion that develops from the chorionic sac with villi formation and a maternal portion that is derived from the endometrium (decidua basalis).
• The villi (from fetus) and decidua basalis (from mother) and intervillous spaces between them form the placenta.
• The intervillous space containing blood is derived from the lacunae that developed in the syncytiotrophoblast during the second week of development.
• By day 11 to 13, cytotrophoblastic proliferations that bud into the overlying cytiotrophoblast.
• The fetal blood in the chorionic villi is separated from the maternal blood, in the intervillous spaces, by Placental Barrier which is composed of the endothelium of the fetal blood vessels, the villous stroma, the cytotrophoblast.
• Stem chorionic villi (anchoring villi) are attached firmly to the decidua basalis through the cytoplasmic shell and anchor the chorionic sac to the decidua basalis.
• Oxytocinase, Monoamino oxidase, Insulinase, Histaminase and Alkaline phosphatase are enzymes produced by the placenta.
• Fetal poorly oxygenated blood leaves the fetus, passes through the umbilical arteries to the placenta, these arteries divide into chorionic arteries, the well-oxygenated fetal blood in the fetal capillaries passes into the umbilical vein, this vessel carries oxygen-rich blood to the fetus.
• The transfer of nutrients from the mother to the fetus is achieved by simple diffusion (water and electrolytes), facilitated diffusion (glucose), active diffusion (amino acids), pinocytosis (large protein molecules and cells).
• The epithelium of the placenta is continuous with the amnion adhering to the fetal surface of the placenta.
• By day 21, the mesodermal core differentiates into connective tissue and blood vessel.
• Waste products of the fetus as urea are passed to maternal blood by simple diffusion through the placenta.
• Oxygen and carbon dioxide pass across the placenta by simple diffusion, fetal hemoglobin has more affinity and carrying capacity than adult haemoglobin.
• The placenta is discoid in shape, with a diameter of 15 - 20 cm and a weight of 500 g.
• Primary stem villi B begin to proliferate and invade the center of each primary stem villus on day 16 of embryonic development.
• The maternal surface of the placenta has a cobblestone appearance and slightly bulging villous areas - cotyledons - which are separated by grooves.
• The umbilical cord is attached to the fetal surface of the placenta.
• The yolk sac’s endoderm, derived from epiblast, gives rise to the trachea, bronchi, lungs, and digestive tract.
• Blood development first occurs in the 3rd week of development.
• The amniotic fluid provides a protective buffer for the embryo or fetus, allows room for fetal movements, and assists in the regulation of fetal body temperature.
• Polyhydroamnios is characterized by a volume greater than 2000 ml, and can be associated with esophageal atresia.
• The yolk sac produces blood for the embryo until the liver is formed during the sixth week.
• The yolk sac’s wall forms the primordial germ cells.
• The intraembryonic portion of the allantois runs from the umbilicus for the urinary bladder.
• The allantois is a small diverticulum that arises from the caudal part of the yolk sac during the third week and develops and grows into the connecting stalk.
• The yolk sac plays a role in the transfer of nutrients to the embryo during the 2nd and 3rd weeks.
• Rupture of the amnion may cause various fetal anomalies that constitute the amniotic band syndrome (ABS) or the amniotic band disruption complex (ABDC).
• During the 4th week, the endoderm of the yolk sac is incorporated into the embryo as the primitive gut.
• The yolk sac’s dorsal portion forms the primitive gut.
• After the folding of embryo, the allantois is connected with the dilated terminal part of the hindgut called cloaca.