Unit 5 Chapter 28 Pregnancy and human development Learning Objectives At the end of the discussion students should be able to: 1. Summarize all the stages of human development. 2. Define the specific embryological terms. 3. Explain the embryonic development till the fetus. 4. Enlist the derivatives of embryonic germ layers Lecture outlines Human development from egg to zygote. 1. Ovulation 2. Fertilization 3. Sperm transport and capacitation 4. Effects of capacitation 5. Acrosomal reaction 6. Completion of meiosis and fertilization 7. Formation of zygote. Lecture outlines Embryonic development involving 1. 2. 3. 4. 5. Cleavage and blastocyst formation Implantation Placentation Gastrulation Formation of embryonic germ layers. Embryonic germ layer derivatives Human development (From egg to zygote) Oocyte female reproductive cell Ovulation release of the oocyte(egg) from the ovary. The oocyte is viable for 12 to 24 hours after it is cast out from ovary. (Sperm is viable 24 to 48 hours after ejaculation) Human development (From egg to zygote) Fertilization Fusion of the sperm with the oocyte to form the fertilized egg the zygote. EGG SPERM Sperm transport and capacitation (From egg to zygote) Millions of sperms enter into vaginal canal during copulation. Only few thousand reaches to the uterine tubes by reverse peristalsis. Capacitation The process by which sperm membranes become fragile and it became able to penetrate the oocyte. Sperm transport and capacitation (From egg to zygote) Effects of capacitation 1. capacitation takes 6 to 8 hours to be completed. 2. Mobility of the sperm enhanced 3. Membranes of the sperm become fragile. Acrosomal reaction and sperm penetration (From egg to zygote) Acrosomal reaction involves 1. 2. The breakdown of plasma membrane and Acrosomal membrane. Release of Acrosomal enzymes to digest the holes in the zona pelluccida Acrosomal reaction and sperm penetration (From egg to zygote) Sperm head approaches the oocyte Oocyte and sperm membranes fuse Only one sperm is allowed to penetrate the oocyte (monospermy) Acrosomal reaction and sperm penetration (From egg to zygote) Sperm head approaches the oocyte An acrosomal process forms and binds to receptors Oocyte and sperm membranes fuse Only one sperm is allowed to penetrate the oocyte Completion of Meiosis II and Fertilization (From egg to zygote) As sperm nucleus moves toward the oocyte nucleus it swells to form the male pronucleus Ovum nucleus swells to become a female pronucleus Membranes of the two pronuclei rupture and the chromosomes combine and produce the diploid zygote Embryonic Development Cleavage Mitotic divisions of zygote results in the formation of 16 or more cells called morula At day 3 or 4, the embryo of ~100 cells (blastocyst) has reached the uterus Blastocyst: fluid-filled hollow sphere Trophoblast cells Participate in placenta formation Inner cell mass Becomes the embryonic disc (embryo and three of the embryonic membranes) Implantation Blastocyst floats for 2–3 days Implantation begins 6–7 days after ovulation The implanted blastocyst is covered over by endometrial cells Implantation is completed by the twelfth day after ovulation Placentation Formation of the placenta from embryonic and maternal tissues Placenta is fully formed and functional by the end of the third month Placenta also secretes human placental lactogen, human chorionic thyrotropin, and relaxin Embryonic Development: Gastrula to Fetus Germ Layers During implantation, the blastocyst starts to convert to a gastrula Inner cell mass develops into the embryonic disc (subdivides into epiblast and hypoblast) The three primary germ layers and the extraembryonic membranes develop Gastrulation Occurs in week 3, in which the embryonic disc becomes a three-layered embryo with ectoderm, mesoderm, and endoderm Begins with appearance of primitive streak, a raised dorsal groove that establishes the longitudinal axis of the embryo The primitive tissues from which all body organs derived Ectoderm nervous system and skin epidermis Endoderm epithelial linings of the digestive, respiratory, and urogenital systems Mesoderm forms all other tissues Summary Key points 1. 2. 3. Ovulation- release of the egg from the ovary. Fertilization - Fusion of the sperm with the oocyte to form the fertilized egg the zygote. Capacitation - The process by which sperm membranes become fragile and it became able to penetrate the oocyte. 4. Acrosomal reaction - involves the breakdown of plasma membrane and Acrosomal membrane to allow the sperm to unite with an egg. 5. Zygote formed by union of male and female pronuclei. 6. Cleavage - Mitotic divisions of zygote results in the formation of 16 or more cells called morula. 7. Blastocyst: fluid-filled hollow sphere 8. Trophoblast cells - Participate in placenta formation 9. Inner cell mass - Becomes the embryonic disc 10. Implantation – attachement of the zygote with utrine wall. 11. Placentation - Formation of the placenta from embryonic and maternal tissues 12. Gastrulation – formation of three germ layers. 1. 2. 3. Ectoderm Mesoderm Endoderm ANY QUESTIONS ?????????