Anatomy & Physiology

Anatomy & Physiology Study Guide around the blastocyst and the placenta develops. Embryogenesis is the formation of a viable embryo. • The blastocyst consists of an outer trophoblast and an inner cell mass. • Implantation occurs about seven days after fertilization as the blastocyst adheres to the uterine lining. • As the trophoblast enlarges and spreads, maternal blood flows through open lacunae. After gastrulation, there is an embryonic disc composed of endoderm, ectoderm, and an intervening mesoderm. It is from these germ layers that the body systems differentiate. • Germ layers help form four extraembryonic membranes: the yolk sac, amnion, allantois, and chorion. • The yolk sac is an important site of blood cell formation. The amnion encloses fluid that surrounds and cushions the developing embryo. The base of the allantois later gives rise to the urinary bladder. Circulation within the vessels of the chorion provides a rapid-transit system that links the embryo with the trophoblast. • Chorionic villi extend outward into the maternal tissues, forming an intricate, branching network through which maternal blood flows. As development proceeds, the umbilical cord connects the fetus to the placenta. The syncytial trophoblast synthesizes human chorionic gonadotropin (hCG), estrogens, progesterone, human placental lactogen (hPL), placental prolactin, and relaxin. • The first trimester is critical because events in the first 12 weeks establish the basis for organogenesis (organ formation). During the second and third trimesters, maternal organ systems support the developing fetus, and the uterus undergoes structural and functional changes: • In the second trimester, the organ systems increase in complexity. During the third trimester, many of the organ systems become fully functional. The fetus undergoes its largest weight gain in the third trimester. At the end of gestation, the fetus and the enlarged uterus displace many of the mother’s abdominal organs. • The developing fetus is totally dependent on maternal organs for nourishment, respiration, and waste removal. Maternal adaptations include increases in respiratory rate, tidal volume, blood volume, nutrient and vitamin intake, and glomerular filtration rate, as well as changes in the size of the uterus and mammary glands. • Progesterone produced by the placenta has an inhibitory effect on uterine muscles; its calming action is opposed by estrogens, oxytocin, and prostaglandins. At some point, multiple factors interact to produce labor contractions in the uterine wall. Labor consists of the dilation, expulsion, and placental stages: • The goal of true labor is parturition, the forcible expulsion of the fetus. Labor can be divided into three stages: the dilation stage, the expulsion stage, and the placental stage. Premature labor may result in premature delivery. • Twin births are either dizygotic (fraternal) or monozygotic (identical). Achieve Page 362 of 368 ©2018