YR1 Lecture 1H - Intro to Embryology - Dr Elizabeth O'Connor 2022 (2).pdf

COPYRIGHT COMMONWEALTH OF AUSTRALIA Copyright Regulations 1969 WARNING This material has been reproduced and communicated to you by or on behalf of University of Western Sydney pursuant to Part VB of the Copyright Act 1968 (the Act). The material in this communication may be subject to copyright und...

COPYRIGHT COMMONWEALTH OF AUSTRALIA Copyright Regulations 1969 WARNING This material has been reproduced and communicated to you by or on behalf of University of Western Sydney pursuant to Part VB of the Copyright Act 1968 (the Act). The material in this communication may be subject to copyright under the Act. Any further reproduction or communication of this material by you may be the subject of copyright protection under the Act. Do not remove this notice. 1 Introduction to Embryology Dr. Elizabeth O’Connor [email protected] Learning objectives Describe in detail the cleavage and implantation phases of embryogenesis Describe the process of gastrulation Identify the relationship between embryonic folding and the development of major organs Reference: Moore and Persaud. The Developing Human Why study embryology? 3-4% of children born will have a defect due to abnormal embryonic processes Children with repaired defects can continue to have lifelong health implications Understanding normal developmental processes can help us understand how things go wrong ie. cancer It underlies specialties of obstetrics and pediatrics It is helpful in understanding/remembering human anatomy Stem cell therapies have their basis in embryological developmental processes Phases of embryogenesis Gametogenesis Fertilisation Yr2 Sem2 – Endocrinology and Reproduction Cleavage/blastulation/implantation Gastrulation Focus of this lecture Neurulation Organogenesis Associated with individual system blocks Positional terminology Anterior = ventral (belly) Posterior = dorsal (back) Rostral/cranial/cephalic = towards the head Caudal = towards the tail Positional terminology Developmental processes can be tracked through sectional views Periods of Development Prenatal Period Postnatal Period Week 1 Fertilisation and early cell division Fertilisation http://www.histology.leeds.ac.uk/female/fertilisation.php Occurs in the ampulla of the oviduct Sperm provides sex-determining chromosome (X or Y), egg provides mitochondria Egg is surrounded by glycoprotein layer – zona pellucida (ZP) – and cellular layer – corona radiata Sperm binding to ZP triggers chemical reactions (acrosome and cortical reactions) to allow only one sperm to penetrate through ZP and deposit nuclear contents Cleavage (d2-3) Fertilised egg (zygote) undergoes progressive divisions Still encased within ZP Mass of cells = morula (L. mulberry) Individual cells known as blastomeres Although cell number increases, overall size of morula does not http://virtualhumanembryo.lsuhsc.edu/demos/stage2/figpages/HTML/st2f-1.htm Blastulation (d4) Morula undergoes compaction -> blastocyst Some cells centrally, some peripherally Peripheral cells = trophoblasts (- -> placenta) Central cells = inner cell mass (ICM) (- -> embryo proper) Cavity formed by trophoblast cells creating a basolateral Na+ gradient; water follows Zygote reaches uterus Hatching (d5) ZP is enzymatically digested Blastocyst hatches from ZP in preparation for implantation Nutrients absorbed from uterine secretions /ICM http://www.embryology.ch/anglais/evorimplantation/furchung03.html https://embryology.med.unsw.edu.au/embryology/index.php/Blastocyst_Development Overview so far… Implantation (d6-10) Embryo makes contact with uterine wall (apposition) ICM aligns closest to the wall Embryo penetrates uterine wall (adhesion) Trophoblast cells and ICM undergo differentiation /ICM Day 6 Implantation (d6-10) Trophoblast: Ø Syncytiotrophoblast – invading front (- -> placenta) Ø Cytotrophoblast – cells remaining around embryo ICM (~7 days): Ø Epiblast – embryo proper (all germ layers) Ø Hypoblast – primitive endoderm, future extraembryonic structures /Epiblast Day 7 End of Week 1 Key concepts in Week 1 Fertilization occurs Zygote undergoes o Cleavage (cellular division) -> morula o Compaction -> blastocyst Encased within ZP and travelling down oviduct to uterus Upon reaching uterus, zygote hatches out of ZP Begins implantation Begins specialized differentiation Clinical considerations Ectopic pregnancy - Embryo implantation outside of uterus (usually in oviduct) - Unilateral lower abdominal/pelvic pain (DDx appendicitis), cramping, vaginal bleeding Chemical pregnancy - Very early miscarriage; fertilized egg that has failed to develop - Late period, cramping, tissue loss Blighted ovum - Fertilized egg in which embryo proper has not developed - Diagnosis by U/S; induce miscarriage/wait for natural Miscarriage - Up to 50% of fertilized embryos miscarry - Most occur in the first 12 weeks of pregnancy - Cramping, vaginal bleeding, tissue loss Week 2 Development of nutrient support systems Embryonic development At the end of week 1/start of week 2, we become bilaminar Epiblast and hypoblast = embryonic disc Amnion Chorion Syncytiotrophoblast Day 9 Placental development Day 12 Syncytiotrophoblast continues to invade, eventually making contact with uterine capillaries Syncytiotrophoblast produced βhCG (maintain ovarian hormone function) Implanted embryo looking from surface of endometrium Extraembryonic development Structures that form in week 2 – amnion and cavity, umbilical connecting stalk/allantois, chorion and sac Amnion – initially “dorsal” extension of epiblast with fluid cavity abutting the epiblast layer Amnion grows and folds with embryo Amniotic cavity expands to fill chorionic cavity (amnion and chorion will eventually meet) Connecting stalk – extension by which embryo remains attached to placenta, umbilical vessels will form within Extraembryonic development Chorion – forms from hypoblast-derived extraembryonic mesoderm Cavity forming in extraembryonic mesoderm -> chorionic cavity (= extraembryonic coelom) Around embryo, chorionic cavity disappears as amniotic cavity grows In placenta, chorion develops into villi, which are bathed in maternal blood Hypoblast cells -> extraembryonic endoderm -> yolk sac Day 12 Day 13 /Chorionic cavity Eventual placental circulation CVS for prenatal testing samples chorionic villi Extraembryonic development Yolk sac – role in nutrient transfer in early implanted embryo via blood vessels that form in mesoderm walls Blood cell development until liver takes over in week 6 Blood vessels develop into umbilical blood vessels Secondary (definitive) yolk sac can be seen by ultrasound ~5 weeks pregnancy = 3 weeks development Day 13 Day 14 Extraembryonic development Prechordal plate – thickening of hypoblast cells at one site of embryonic disc Site of future mouth Important in location of future head region Day 14 End of Week 2 Key concepts in Week 2 Embryo completes differentiation to bilaminar disc Trophoblast expands dramatically to complete implantation Syncytiotrophoblast degrades endometrial tissue to seek nutrient supply Amnion/amniotic cavity; chorion/chorionic cavity; connecting stalk and yolk sac all form to support nutrient exchange and protect the developing embryo Thickened hypoblast region forms prechordal plate (future mouth region) Clinical considerations – similar to Week 1 Week 3 Formation of the germ layers and neurulation Gastrulation Begins middle of week 3 Process of becoming trilaminar -> forms primary germ layers o Endoderm o Mesoderm Germ layers o Ectoderm Begins with primitive streak o Coincides with first missed menstrual period o Defines positional differences (cranial/caudal; left/right; etc.) Primitive streak Begins as a depression in epiblast – primitive pit in primitive node Depression starts at caudal end and moves towards cranial end Depression made up of epiblast cells moving in and down towards hypoblast Days 15-16 Primitive streak Epiblast cells replace hypoblast completely -> (definitive) endoderm Epiblast cells remaining “on top” -> ectoderm Epiblast cells between ectoderm and endoderm -> mesoderm Primitive streak Germ layer derivatives Neurulation Begins in week 3 after gastrulation Tube-like structure – notochord – forms in mesoderm Notochord induces neural plate formation as thickening of overlying ectoderm 1. Notochord intercalated with endoderm -> patterning of future endoderm structures. Neural groove is induced in overlying ectoderm 2. Notochord partitions off from endoderm Days 18-19 3. Neural folds are visible as neural tube develops. Lateral mesoderm differentiates Neurulation Neural groove forms base of neural tube Neural folds each side fold up and over -> primitive CNS Days 19-21 Week 3 video A great video showing the processes of gastrulation and neurulation - based on the diagrams in Moore (and Persaud) https://www.youtube.com/watch?v=0OvWRZqXBt8 End of Week 3 Key concepts in Week 3 Gastrulation begins with formation of primitive streak at caudal end of epiblast The primitive streak defines the body axes Epiblast cells invaginate and form (definitive) endoderm and mesoderm layers Epiblast cells remaining in original outer layer are now called ectoderm The 3 germ layers (ectoderm, mesoderm, endoderm) define future derivatives Notochord (from mesoderm) forms and intercalates with endoderm Notochord repartitions from endoderm and induces overlying ectoderm to thicken and form neural tube => primitive CNS Clinical considerations Neural tube defects – from failure of various regions of the neural tube to fully close Risk factors – Folic acid (Vit B9) and Vit B12 deficiency, some anti-convulsant drugs, maternal obesity, genetics Eg failure of neural tube closure at the rostral neuropore - little to no brain matter, not (usually) compatible with life Eg failure of neural tube closure close to the brain - opening in skull through which brain protrudes Day 23 Clinical considerations Spina bifida – failure of closure of various parts of neural tube, most often near the caudal neuropore (lumbar region) Can be mild – gap in spine or malformation of tissue over spinal cord More serious versions - cyst-like protrusion though back +/- neural tissue Week 4 Embryo folding Embryo folding – sagittal plane Head/tail folding Embryo folding – transverse plane Lateral and ventral body wall folding Langman’s Medical Embryology Week 4 video A great video showing the processes of body folding in both planes - based on the diagrams in Moore (and Persaud) https://www.youtube.com/watch?v=4lGq4DkTNko End of Week 4…. Embryonic period for a further 4 weeks and then Fetal period More embryology to come in the future……. Prenatal Period Postnatal Period Summary Fertilization, cleavage and implantation occur during week 1 Formation of the bilaminar embryo (epiblast and hypoblast) occurs near the end of week 1/beginning of week 2 Much of the development in week 2 occurs in supporting structures – amnion, chorion, yolk sac etc Gastrulation occurs in the middle of week 3 – formation of 3 germ layers The germ layers (endoderm, mesoderm, ectoderm) form the basis of all body cell derivatives Ectoderm forms neural tube -> primitive CNS, induced by the notochord In week 4, the embryo folds in transverse plane (to form future body walls) and in sagittal plane (to position head/tail) Thank you! Thank you! Please take a moment to complete this anonymous lecture feedback survey. 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