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Embryology JAWAAD HUSSAINILO's u List the three developmental stages of life before birth and give their timings u Describe important early embryological events: cleavage, blastocyst formation u Briefly describe the process of implantation u Describe the extra-embryonic membranes, the amnion, the chorion and the yolk sac u Explain gastrulation and list the major derivatives of the three germ layers u Summarise the key steps in development between weeks 1-3 u Explain the concept of cell potencyPART 1 Week 1-3Week 1Cleavage u Zygote on day 1 u Oocyte fertilised by Sperm u Mix of female + Male genetic material u Surrounded by zona pellucida (glycoprotein coat) u Prevents premature implantation u Mitotic divisions u Overall size remains the same u First division produces two BLASTOMERES u After 4 days there are 16 blastomeres u At this point it is the MORULAMorula u Cells tightly compacted together u Coincides with entry of the cells into the uterus u These cells develop into blastocystBlastocyst Formation u First cell differentiation u Inner cell mass (embryoblast) u Eventually develops into embryo and extraembryonic structures u Trophoectoderm (Trophoblast) u Eventually develops into the placenta u Fluid enters via Zona Pellucida which fills the spaces between the cells of the inner cell mass u This results in a fluid filled cavityHatching u As the cells of the inner cell mass continue to proliferate, the blastocyst runs out of nutrients u The blastocyst must implant to acquire more nutrients u In order to implant, the blastocyst must break free of the zona pellucida u Fluid and cells build up until the blastocyst hatches out of the zona pellucidaWeek 2Further Differentiation u Trophoblast differentiates into: u Cytotrophoblast u Syncitiotrophoblast u Inner cell mass differentiates into: u Bilaminar disc u Epiblast (Amnion is a continuation of this) u Hypoblast (Yolk sac is a continuation of this) u Extra embryonic membranes will arise from these u What are the other two?Implantation u Day 7-12 u Trophoblast implants first u Syncitiotrophoblast is important uExcretes proteolytic enzymes to break down ECM uThis allows the passage of the blastocyst into the endometrium uThen secretes hCG: u maintains uterus u Stops menstruation from occuringWeek 3 Gastrulation u Bilaminar disc becomes the three germ layers u Cell migration and invagination u Primitive streak forms u Some cells replace Hypoblast u This forms the endoderm u Some cells migrate into the middle u This forms the mesoderm u Part of the mesoderm contribute towards the chorion u Remaining cells are called the Ectoderm u The three germ layers give rise to all tissues in the embryo u Notochord and neural tube are formedGerm Layer DerivativesPART 2 Definitions Summaries3 developmental stages u Preimplantation u Week 1 u Embryonic u Week 2-8 u Organogenesis u Foetal u Week 9-38 u Growth and DevelopmentExtraembryonic membranes u Amnion u Continuation of EPIBLAST u Cavity fills with fluid u Protects embryo from damage u Persists until birth u Yolk Sack u Continuation of HYPOBLAST u Nutrient transfer week 2-3 u Disappears week 20 u Chorion u Arises from TROPHOBLAST and EXTRAEMBRYONIC MESODERM u Foetal component of placenta u Filled by expanding amnionCell Potency u The more potent a cell, the larger the variety of cells it can differentiate into u Totipotent – All cells u Zygote u Pluripotent – Very many cells u Inner cell mass cells u Multipotent – Multiple cells u Haematopoietic cell uMany different blood cellsWeek 1-3 summary u Wk1-2 (pre-implantation) u Cleavage u Blastocyst formation u Wk2 – Implantation u Bilaminar disc development u Yolk sack and amnion development u Wk3 – Gastrulation u Trilaminar disc develops (3 Germ layers) u Notochord and neural tube arisePART 3 AbnormalitiesEctopic Pregnancy u Implantation outwith the uterine cavity u Possible locations: u Fallopian tubes u Ovaries u GI tract u Peritoneal wallTeratoma u Tumours arising from the three germ cell layers u Common tissues u Usually ectoderm u Hair / Teeth / Follicles u Mesoderm u CartilageResources u Teach me anatomy – Embryology u Sharon Sneddon's Lectures (Life before birth part 1)