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Weeks 8: Embryology (Overview (Core mechanisms in embryogenesis (Folding…

- - - - Embryo in uterus
        
        Followed by fatal period
    - - Embryo in egg
  - - - Ionising radiation
      - Rubella virus
      - Chemical agents
- - - - Acrosome reaction allows sperm to penetrate zona pellucida
        
        Sperm and oocyte membranes bind and fuse, activating calcium waves within oocyte
        
        Meiosis resumption
        
        Completion of metaphase II and extrusion of second polar body
        
        Polyspermy block
        
        Cortical granules fuse with oocyte membrane, releasing enzymes into subzonal space
        
        Rendering zona resistant to proteolytic enzymes of sperm
        
        Cleaves/digests binding sites on zona
        
        Reduces sperm-binding properties of oocyte membrane
  - - - Genetic material from male/female fuse, restoring diploidy (46 chromosomes) and forming new individual (zygote)
        
        Chromosomal sex established and mitosis initiated to yield 2-cell embryo (blastomere)
- - - - Cells are polarised with basal nucleus and apical organelles
        
        Important for blastocyst cell differentiation
        
        Outer trophoblast: forms embryonic/fetal part of placenta
        
        Inner cell mass: forms embryo
        
        Blastocoele: fluid-filled cavity
        
        Must hatch from zona pellucida to implant in endometrium of uterus
- - - - Syncytiotrophoblast erodes the endometrial connective tissue and secretes hCG (human chorionic gonadotrophin)
        
        Rescues corpus luteum and maintains progesterone secretion to maintain pregnancy (hCG is basis of hormonal pregnancy test)
        
        Completes Day 10
  - - - Trophoblast and inner cell mass both differentiate into two layers
        
        Day 8
        
        Embryonic disc comprising epiblast and hypoblast
        
        Day 9
        
        Hypoblast migrates to form primary yolk sac (ventral to bilaminar embryo)
        
        Day 12
        
        1 more item...
        
        Extraembryonic mesoderm derived from hypoblast
        
        Later: secondary (definitive) yolk sac
        
        Dorsal-ventral axis established
        
        Amniotic cavity formed within epiblast (dorsal to embryo)
    - - Primitive streak
        
        Primitive node
        
        Primitive groove
        
        Primitive pit
- - - - Day 17
        
        Mesoderm migrates laterally and cranially
        
        No mesoderm at oropharyngeal nor cloacal membranes
        
        Days 16-22: Cells that pass through primitive pit form notochord
        
        Mesodermal condensation of cells derived from the primitive node that lies in the midline
        
        Functions mostly in first six weeks
        
        Induction of vertebral bodies from mesoderm
        
        Median hinge point during formation of neural tube
        
        Induction of nervous tissue from ectoderm
        
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        1 more item...
- - - - Tube separates from surface ectoderm to form epidermis of skin
        
        .
        
        Neural tube closure
        
        Day 21
        
        Neural groove closes over like zipper
        
        Day 22
        
        Neuropores (cranial and caudal openings) close
        
        Otherwise, neural tube defects form
        
        Walls of neural tube expand, fold, and increase in size differentially to form brain and spinal cord
      - Neural groove becomes neural canal
- - - - Parietal mesoderm
      - Visceral mesoderm
        
        .
        
        Day 21
        
        Cavities encompassed into body as embryo folds
        
        Peritoneal
        
        Pericardial
        
        Pleural
  - - - Somites
        
        Dermatome: connective tissue of skin (dermis)
        
        Myotome: skeletal muscles of body, head, limbs
        
        Sclerotome: bones of axial skeleton
    - - Voluntary muscles of
        face, jaw, throat
- - - - Heart bulge from cardiogenic mesoderm
      - Initially cranial to oral membrane
      - Results in primitive gut lined by endoderm
        
        Ectoderm now covers outside of embryo
        
        Remainder comprises mesoderm/cavities