Chapter 10: Pregnancy - Human Reproduction PDF

CHAPTER 10 Pregnancy After Fertilization Shortly after fertilization the zygote divides through mitosis into two blastomeres. These blastomeres go through a number of successive mitotic divisions until the blastocyst is formed. It is the blastocyst that eventually implants into the endometrium and...

CHAPTER 10 Pregnancy After Fertilization Shortly after fertilization the zygote divides through mitosis into two blastomeres. These blastomeres go through a number of successive mitotic divisions until the blastocyst is formed. It is the blastocyst that eventually implants into the endometrium and begins the pathway of differentiation and growth to create an embryo. https://www.the-scientist.com/features/new-techniques-detail-embryos-first-hours-and-days-30175 Pregnancy Through most definitions, pregnancy begins at fertilization and carries forward for 38 weeks. Encompasses all the changes required for the zygote to develop into a fully functional human. Weeks 1-3 are considered preembryo, 3-8 embryo, and beyond fetal. https://en.wikipedia.org/wiki/Pregnancy Signs of Pregnancy Often referred to as presumptive signs. Most common first sign is a missed menstrual cycle. Other symptoms include: morning sickness, breast tenderness/swelling, darkening of areola, fatigue, abdominal cramping, urination changes, smell sensitivity. https://www.clearblue.com/am-i-pregnant/early-pregnancy-signs Testing for Pregnancy Pregnancy tests are testing for the hormone human chorionic gonadotropin (hCG). This hormone is secreted by the syncytiotrophoblast and from the placenta. In office tests are often immunoassay style tests, looking for a reaction between anti-hCG antibodies and hCG in a blood/urine sample. https://americanpregnancy.org/getting-pregnant/hcg-levels/ Timing of Pregnancy How one determines the timing of a pregnancy is dependent on who/what is the action at the beginning. It is important to have a clear definition of what is the starting point when speaking with medical professionals. Human Reproductive Biology, 4th Ed. (2014) pg. 176 Early Pregnancy Shortly after a missed menstrual period (with or without a positive pregnancy test) a medical appointment should be made. At this initial consultation the medical professional will look for the following information: • Medical history (this includes menstrual history, previous pregnancies, etc) • Childhood diseases and any other applicable infectious illnesses • Overall health assessment • Genetic concerns for pregnant individual and sperm donor A pap smear and pelvic exam will be done, along with blood work and any other diagnostic tests the medical teams deems necessary. Process of Pregnancy Pregnancy can be broken down into a number of sequential processes. • Fertilization, zygote formation, and early divisions • Implantation • Embryogenesis (gastrulation) • Embryo development • Fetal development • Labor and birth https://ocwmg.com/ob-gyn-services/obstetrical-care/prenatal-care-timeline/ Morula Through several (~5) divisions, the zygote multiplies into a solid ball of cells (32) called a morula. These divisions occur as it travels down the uterine tube. The morula is still surrounded by the zona pellucida from the original oocyte. https://www.nationalgeographic.com/science/art icle/the-morula-solution Blastocyst Around day 3-4 after fertilization, the pre-embryo enters the uterus and has now divided into a blastocyst. Some cellular organization has begun in the blastocyst and the interior space is a fluid filled chamber called the blastocoel. Around the outside (under the zona pellucida) is a layer of cells called the trophoblast. An inner cell mass is formed which will eventually develop into the embryo. Human Reproductive Biology, 4th Ed. (2014) Figure 10.1 pg. 178 Maternal proteases break down the zona pellucida around day 6. Early Implantation Implantation generally begins between day 7-10 post fertilization. The trophoblast layer divides into an exterior syncytiotrophoblast and interio cytotrophoblast. The syncytiotrophoblast invades the endometrium. Cells in this trophoblast layer have lost their cell membranes and opening communicate. Human Reproductive Biology, 4th Ed. (2014) Figure 10.3 pg. 180 Late Implantation Cells of the uterine stroma begin to rapidly multiply and create a covering over the implanted blastocyst. This is known as the deciduoma response. Implantation is completed and fetal and carrier tissues merge to share nutrients and past wastes. Human Reproductive Biology, 4th Ed. (2014) Figure 10.3 pg. 180 Immune Response As the fetal tissues are foreign to the carrier, why aren’t they attacked by the immune system? https://www.sciencedirect.com/topics/medicine-and-dentistry/human-leukocyte-antigen Normally detection of foreign cells occurs through histocompatibility proteins on white blood cells called human leukocyte antigens (HLAs). The placenta doesn’t have a large cohort of regular HLA’s but does have some others which are not found in tissues outside the placenta. Early Embryonic Development Shortly after implantation the pre-embryo begins to differentiate into two tissues layers: epiblast and hypoblast . Following this division is gastrulation. This creates the three primary germ layers. Ectoderm – nerves, skin + derivatives Mesoderm – skeleton, muscles, circulatory system, notocord Endoderm - internal organs + membranes https://ecampusontario.pressbooks.pub/humananatomyandp hysiology/chapter/28-2-embryonic-development/ Embryonic Membranes 3 of the 4 embryonic membranes derive from the inner cell mass. Yolk sac – endoderm lining of the blastocoel. Supplies blood cells to embryo Amnion – grows around developing embryo. Filled with fluid. Allantois – small pouch at posterior tail region. Blood vessels formed here contribute to the umbilical cord. Chorion – derived from the cytotrophoblast. Creates the chorionic villi of the placenta. Human Reproductive Biology, 4th Ed. (2014) pg. 185 The Placenta An organ derived of both fetal and carrier tissues which allows gas and waste exchange between the developing fetus and carrier. Initially formed as extensions of cytotrophoblast pierce through the syncytiotrophoblast into the uterine stroma. Enzymes from the syncytiotrophoblast breakdown small blood vessels in the uterine strome creating sinusoids. https://www.merckmanuals.com/en-ca/home/women-s-healthissues/normal-pregnancy/stages-of-development-of-the-fetus Exchange happens via the sinusoids. Exchange between Fetus and Carrier Exchange between the fetus and carrier is limit to products less than 500 molecular weight. Restricts the movement of bacteria, but not viruses. Some stem cells from the carrier are passed to the developing fetus and some cells of the fetus pass to the carrier. Creates microchimerism in either system. Linked to some autoimmune disorders, polymorphic eruption, and preeclampsia. Human Reproductive Biology, 4th Ed. (2014) Box 10.3 pg. 181 Twin Pregnancies Generally in dizygotic twins the embryos implant separately and then two separate placentas and amnions are formed. Sometimes they implant so close that a fused placenta is created. In monozygotic twins generally share a placenta but often have separate amnions. Human Reproductive Biology, 4th Ed. (2014) Figure 10.7/8 pg. 181 Genetic Imprinting Imprinting occurs during throughout the human genome and affects which allelic form of a gene is expressed. Imprinted genes are expressed differentially depending on their source (egg or sperm). Human Reproductive Biology, 4th Ed. (2014) Box 10.2 pg. 181 Some genes are only expressed when derived from the egg, others from the sperm. Both egg and sperm genetic material are required for proper embryogenesis. Controlled through methylation which can silence or increase expression.