Introduction to Embryology PDF

Introduction to Embryology Fertilization and embryonic period up to week 3  Learning Outcomes: Name the phases of fertilization and describe the process Identify the stages of embryo formation and implantation Describe the details of events that happens in the first 3 weeks of embr...

Introduction to Embryology Fertilization and embryonic period up to week 3  Learning Outcomes: Name the phases of fertilization and describe the process Identify the stages of embryo formation and implantation Describe the details of events that happens in the first 3 weeks of embryonic period  Gametogenesis: Conversion of Germ Cells into Male and Female Gametes In meiotic cell division, diploid cells (with 2n number of chromosomes, for humans 46) divide two times(meiosis I and II) to produce 4 haploid cells (gametes), with n number of chromosomes, for humans 23). In spermatogenesis, each spermatogonium produces 4 active sperms. In oogenesis each primary oocyte produces on active ovum with 4 polar bodies  Fertilization Fertilization (the first embryonic stage) occurs as the genetic material (pronucleus) from the sperm and ovum merges to form a single fertilized zygote The phases of fertilization include the following: Phase 1, penetration of the corona radiata Phase 2, penetration of the zona pellucida Phase 3, fusion of the oocyte and sperm cell membranes Normally this event takes place in the uterine tubes, and the growing ball of cells has 4–5 days before it implants in the uterine cavity  The Embryonic Period The embryonic period is the first 8 weeks after fertilization. First week: From fertilization to implantation: As soon as the spermatocyte has entered the oocyte: 1. The oocyte finishes its second meiotic division and forms the female pronucleus. 2. The zona pellucida becomes impenetrable to other spermatozoa. 3. The head of the sperm separates from the tail, swells, and forms the male pronucleus The Embryonic Period Cleavage is the rapid mitotic division of the zygote, starting the first week of development Blastomeres are progressively smaller cells produced by cleavage 3-4 days after fertilization a solid ball of cells forms called the morula, followed a day later by the blastocyst (a hollow ball of cells that enters the uterine cavity) The Embryonic Period Implantation is the attachment of the blastocyst to the endometrium 7–8 days after fertilization The decidua is the portion of the endometrium that is modified after implantation  The Embryonic Period The blastocyst attaches to the endometrium oriented in such a way that the developing inner cell mass is directed toward the endometrium As the blastocyst burrows its way in, the endometrium responds by becoming more vascularized and the endometrial glands enlarge  Endometrium of uterus Endometrial gland Formation of exocoelomic membrane Cytotrophoblast Syncytiotrophoblast Bilaminar Amnion embryonic disc: Amniotic cavity Hypoblast Epiblast Blastocyst cavity Blood vessel Uterine cavity (a) Frontal section through endometrium of uterus showing blastocyst, about 8 days after fertilization Second Week of Development: Bilaminar Germ Disc At the early blastocyst stage, cells in the embryoblast differentiate into epiblast and hypoblast cells They become epiblast layer dorsally and hypoblast layer ventrally, adjacent to the blastocyst cavity (blastocele). Cells located more anteriorly in hypoblast layer differentiate into anterior visceral endoderm, which will be cranial side of embryo. Thus, by 4 to 6 days of gestation dorso-ventral and cranio-caudal axes have been established.  Endometrium of uterus Amniotic cavity Blood vessels Bilaminar Lacunae embryonic disc: Epiblast Hypoblast Yolk sac Cytotrophoblast Exocoelomic Syncytiotrophoblast membrane Uterine cavity (b) Frontal section through endometrium of uterus showing blastocyst, about 9 days after fertilization Second Week of Development: Bilaminar Germ Disc 1. The trophoblast differentiates into two layers: the cytotrophoblast and syncytiotrophoblast. 2. The embryoblast forms two layers: the epiblast and hypoblast. 3. The extraembryonic mesoderm splits into two layers: the somatic and splanchnic layers. 4. Two cavities form: the amniotic and yolk sac cavities.  Third Week of Development: Trilaminar Germ Disc The growth of the developing embryo continues that result in the formation of a trilaminar embryonic disc which develop to become the fetus and 4 extraembryonic membranes, the amnion , the yolk sac, the allantois, and the chorion Gastrulation, the process that establishes all three germ layers: The ectoderm is the superficial layer The mesoderm is in the middle layer The endoderm forms the inner layer  Neural plate Primitive Primitive streak node Notochordal process (outlined beneath ectoderm) TAIL END Oropharyngeal membrane Connecting stalk HEAD END Yolk sac Cloacal membrane Trilaminar embryonic disc (a) Dorsal and partial sectional views of trilaminar embryonic disc, about 16 days after fertilization  Trilaminar Germ Disc Ectoderm differentiates into the nervous system (brain and nerves), and the epidermis of the skin Mesoderm is a loosely organized connective tissue (mesenchyme) that matures to form blood, muscles, bones, Endoderm gives rise to the epithelial lining of the digestive tract, respiratory tract, and several other organs; also, the endothelial lining of blood vessels What do the Germ Layers Make? What do the Germ Layers Make? Extraembryonic membranes 4 extraembryonic membranes, the: Amnion is the innermost - it forms a protecting “bag of waters” (amniotic cavity) around the embryo Yolk sac becomes the site of early blood formation, degenerate and disappear later. Allantois helps form the umbilical cord Chorion is the outermost - it forms the fetal portion of the placenta Development of the Placenta The placenta is the site of nutrient and waste management, It also begins to take over the job of hormone secretion (hCG, estrogen, progesterone) as the corpus luteum in the ovary gradually atrophies.  Development of the Placenta The placenta is a unique organ developed by two individuals, the chorionic villi of the embryo and the decidua basalis of the endometrium of the mother. It acts as the respiratory, digestive, and urinary systems of embryo, yet prevents any mixing of fetal and mother's blood Any Questions?