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Chapter 28 – Section 2 Hormonal Changes During Pregnancy • Human chorionic gonadotropin (hCG) • Secreted by trophoblast cells, later the chorion • Prompts corpus luteum to continue secretion of progesterone and estrogen • hCG levels rise until the end of the second month, then decline as the placenta begins to secrete progesterone and estrogen Copyright © 2010 Pearson Education, Inc. Human chorionic gonadotropin Estrogens Progesterone Gestation (weeks) Ovulation and fertilization Birth Figure 28.6 Placentation • Formation of the placenta from embryonic and maternal tissues 1. Embryonic tissues • Mesoderm cells develop from the inner cell mass and line the trophoblast • Together these form the chorion and chorionic villi Copyright © 2010 Pearson Education, Inc. Placentation 2. Maternal tissues • Decidua basalis (stratum functionalis between chorionic villi and stratum basalis of endometrium) develops blood-filled lacunae Copyright © 2010 Pearson Education, Inc. Placentation • The chorionic villi • Grow into blood-filled lacunae (intervillous spaces) • Vascularized by umbilical arteries and veins • Lie immersed in maternal blood Copyright © 2010 Pearson Education, Inc. Endometrium Lacuna (intervillous space) containing maternal blood Maternal blood vessels Proliferating syncytiotrophoblast Chorionic villus • Ectoderm Chorion • Mesoderm Amnion • Endoderm Forming body stalk Cytotrophoblast Amniotic cavity Bilayered embryonic disc • Epiblast • Hypoblast Endometrial epithelium Amniotic cavity Primary germ layers Yolk sac Allantois Extraembryonic mesoderm Chorion being formed Lumen of uterus (a) Implanting 71/2-day blastocyst. (b) 12-day blastocyst. Implantation The syncytiotrophoblast is eroding is complete. Extraembryonic the endometrium. Cells of the mesoderm is forming a discrete embryonic disc are now separated layer beneath the cytotrophoblast. from the amnion by a fluid-filled space. Extraembryonic coelom (c) 16-day embryo. Cytotrophoblast and associated mesoderm have become the chorion, and chorionic villi are elaborating. The embryo exhibits all three germ layers, a yolk sac and an allantois, which forms the basis of the umbilical cord. Figure 28.7 (a-c) Placentation • Placenta is fully formed and functional by the end of the third month • Placenta also secretes human placental lactogen, human chorionic thyrotropin, and relaxin Copyright © 2010 Pearson Education, Inc. Decidua basalis Maternal blood Chorionic villus Umbilical blood vessels in umbilical cord Amnion Amniotic cavity Yolk sac Extraembryonic coelom Lumen of uterus Chorion Decidua capsularis (d) 41/2-week embryo. The decidua capsularis, decidua basalis, amnion, and yolk sac are well formed. The chorionic villi lie in blood-filled intervillous spaces within the endometrium. The embryo is now receiving its nutrition via the umbilical vessels that connect it (through the umbilical cord) to the placenta. Figure 28.7d Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic cavity Umbilical cord Decidua capsularis Uterus Extraembryonic coelom Lumen of uterus (e) 13-week fetus. Figure 28.7e Placentation • Maternal and embryonic blood supplies do not intermix • Embryonic placental barriers include: • Membranes of the chorionic villi • Endothelium of embryonic capillaries Copyright © 2010 Pearson Education, Inc.