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  <head>
    <title>085-02 Gastrulation, germ layers, body axes, neurulation, folding, and segmentation</title>
    <ownerName>Integrated Medical Foundations</ownerName>
  </head>
  <body>
    <outline text="Gastrulation to segmentation">
      <outline text="Gastrulation">
        <outline text="Bilaminar disc becomes trilaminar"/>
        <outline text="Primitive streak appears caudally"/>
        <outline text="Epiblast cells become motile mesenchyme">
          <outline text="First ingressing cells form endoderm"/>
          <outline text="Next cells form intraembryonic mesoderm"/>
          <outline text="Surface cells become ectoderm"/>
        </outline>
        <outline text="All embryo proper derives from epiblast"/>
        <outline text="Primitive node is a major organiser"/>
        <outline text="Notochordal process becomes notochord">
          <outline text="Defines axis, induces neural development"/>
          <outline text="Persists as nucleus pulposus"/>
        </outline>
        <outline text="Oropharyngeal and cloacal membranes lack mesoderm"/>
        <outline text="Failed limits: teratoma, caudal regression">
          <outline text="Midline defects: craniofacial, forebrain"/>
        </outline>
      </outline>
      <outline text="Germ-layer derivatives">
        <outline text="Surface ectoderm: epidermis, lens, enamel"/>
        <outline text="Neuroectoderm: neural tube and crest"/>
        <outline text="Placodes: sensory structures, cranial ganglia"/>
        <outline text="Mesoderm: connective tissue, muscle, blood, kidney">
          <outline text="Axial, paraxial, intermediate, lateral plate"/>
        </outline>
        <outline text="Endoderm: gut, airway, bladder epithelia">
          <outline text="Liver, pancreas, thyroid, parathyroid, thymus"/>
        </outline>
        <outline text="Mature organs combine lineages"/>
      </outline>
      <outline text="Neurulation">
        <outline text="Organiser signals block surface-ectoderm fate"/>
        <outline text="Neural plate, folds, groove"/>
        <outline text="Fusion starts near cervical region"/>
        <outline text="Cranial neuropore first, caudal shortly after">
          <outline text="Failure: anencephaly to spina bifida"/>
          <outline text="Folate lowers but does not abolish risk"/>
        </outline>
        <outline text="Neural tube becomes CNS and retina"/>
        <outline text="Alar plate sensory, basal plate motor">
          <outline text="Sonic hedgehog gradient from floor plate"/>
        </outline>
      </outline>
      <outline text="Neural crest">
        <outline text="Arises at neural-plate border"/>
        <outline text="Detaches during closure, migrates widely"/>
        <outline text="Peripheral neurons, Schwann cells, melanocytes"/>
        <outline text="Adrenal medulla, craniofacial cartilage and bone"/>
        <outline text="Conotruncal septation, enteric nervous system"/>
        <outline text="Defects combine multisystem anomalies"/>
      </outline>
      <outline text="Mesodermal segmentation">
        <outline text="Somites form in rhythmic sequence">
          <outline text="Somite number estimates age"/>
        </outline>
        <outline text="Sclerotome forms vertebrae and ribs"/>
        <outline text="Myotome forms skeletal muscle"/>
        <outline text="Dermatome forms dermis of back"/>
        <outline text="Resegmentation builds vertebral bodies">
          <outline text="Spinal nerves exit between vertebrae"/>
          <outline text="Myotomes bridge adjacent levels"/>
        </outline>
        <outline text="Intermediate mesoderm: pronephros to metanephros"/>
        <outline text="Lateral plate splits around coelom">
          <outline text="Somatopleure: body wall and limbs"/>
          <outline text="Splanchnopleure: gut wall and heart"/>
          <outline text="Blood islands form endothelium, blood cells"/>
        </outline>
      </outline>
      <outline text="Axis patterning">
        <outline text="Left-right set near primitive node">
          <outline text="Nodal cilia drive directional flow"/>
          <outline text="NODAL, LEFTY, PITX two on left"/>
          <outline text="Disruption: mirror image or heterotaxy"/>
          <outline text="Ciliary defects add respiratory disease"/>
        </outline>
        <outline text="HOX order matches expression domains"/>
        <outline text="Signals act by concentration and time">
          <outline text="Same signal, different outcome by history"/>
        </outline>
      </outline>
      <outline text="Embryonic folding">
        <outline text="Driven by neural tube and somite growth"/>
        <outline text="Cranial folding moves heart ventrally">
          <outline text="Foregut incorporated"/>
        </outline>
        <outline text="Caudal folding incorporates hindgut"/>
        <outline text="Lateral folds close body wall">
          <outline text="Except at umbilical ring"/>
        </outline>
        <outline text="Vitelline duct links midgut to yolk sac"/>
        <outline text="Defects: omphalocele, gastroschisis, remnants"/>
      </outline>
      <outline text="Gut, pharynx and timing">
        <outline text="Foregut: coeliac artery"/>
        <outline text="Midgut: superior mesenteric artery"/>
        <outline text="Hindgut: inferior mesenteric artery"/>
        <outline text="Each arch: cartilage, muscle, artery, nerve">
          <outline text="Pouches form thymus, parathyroids"/>
          <outline text="Clefts vanish except external meatus"/>
          <outline text="Explain neck cysts and fistulas"/>
        </outline>
        <outline text="Carnegie stages use morphology"/>
        <outline text="Embryonic period: high malformation risk"/>
        <outline text="Trace defects to failed transitions"/>
      </outline>
    </outline>
  </body>
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