--- module: 085-02 language: en chapter: 85 title: "Developmental Biology, Germ Layers, Folding, and Placentation" module_title: "Gastrulation, germ layers, body axes, neurulation, folding, and segmentation" source_sha256: 1ed6e9281a72d468fff7a576dabf411ed0cf4a6f8d51c67fe9584a4a4b3f5993 --- # Gastrulation to segmentation ## Gastrulation ### Bilaminar disc becomes trilaminar ### Primitive streak appears caudally ### Epiblast cells become motile mesenchyme #### First ingressing cells form endoderm #### Next cells form intraembryonic mesoderm #### Surface cells become ectoderm ### All embryo proper derives from epiblast ### Primitive node is a major organiser ### Notochordal process becomes notochord #### Defines axis, induces neural development #### Persists as nucleus pulposus ### Oropharyngeal and cloacal membranes lack mesoderm ### Failed limits: teratoma, caudal regression #### Midline defects: craniofacial, forebrain ## Germ-layer derivatives ### Surface ectoderm: epidermis, lens, enamel ### Neuroectoderm: neural tube and crest ### Placodes: sensory structures, cranial ganglia ### Mesoderm: connective tissue, muscle, blood, kidney #### Axial, paraxial, intermediate, lateral plate ### Endoderm: gut, airway, bladder epithelia #### Liver, pancreas, thyroid, parathyroid, thymus ### Mature organs combine lineages ## Neurulation ### Organiser signals block surface-ectoderm fate ### Neural plate, folds, groove ### Fusion starts near cervical region ### Cranial neuropore first, caudal shortly after #### Failure: anencephaly to spina bifida #### Folate lowers but does not abolish risk ### Neural tube becomes CNS and retina ### Alar plate sensory, basal plate motor #### Sonic hedgehog gradient from floor plate ## Neural crest ### Arises at neural-plate border ### Detaches during closure, migrates widely ### Peripheral neurons, Schwann cells, melanocytes ### Adrenal medulla, craniofacial cartilage and bone ### Conotruncal septation, enteric nervous system ### Defects combine multisystem anomalies ## Mesodermal segmentation ### Somites form in rhythmic sequence #### Somite number estimates age ### Sclerotome forms vertebrae and ribs ### Myotome forms skeletal muscle ### Dermatome forms dermis of back ### Resegmentation builds vertebral bodies #### Spinal nerves exit between vertebrae #### Myotomes bridge adjacent levels ### Intermediate mesoderm: pronephros to metanephros ### Lateral plate splits around coelom #### Somatopleure: body wall and limbs #### Splanchnopleure: gut wall and heart #### Blood islands form endothelium, blood cells ## Axis patterning ### Left-right set near primitive node #### Nodal cilia drive directional flow #### NODAL, LEFTY, PITX two on left #### Disruption: mirror image or heterotaxy #### Ciliary defects add respiratory disease ### HOX order matches expression domains ### Signals act by concentration and time #### Same signal, different outcome by history ## Embryonic folding ### Driven by neural tube and somite growth ### Cranial folding moves heart ventrally #### Foregut incorporated ### Caudal folding incorporates hindgut ### Lateral folds close body wall #### Except at umbilical ring ### Vitelline duct links midgut to yolk sac ### Defects: omphalocele, gastroschisis, remnants ## Gut, pharynx and timing ### Foregut: coeliac artery ### Midgut: superior mesenteric artery ### Hindgut: inferior mesenteric artery ### Each arch: cartilage, muscle, artery, nerve #### Pouches form thymus, parathyroids #### Clefts vanish except external meatus #### Explain neck cysts and fistulas ### Carnegie stages use morphology ### Embryonic period: high malformation risk ### Trace defects to failed transitions