The Embryonic Period: Germ Layer Differentiation

Summary

  • During this period, the trilaminar disc develops into the foetus as the three germ layers (endoderm, mesoderm, and ectoderm) all develop to create different organ systems.
    • By 8 weeks, the major organ systems are developed from one of the three layers

Derivatives of the Germ Layers

  • The diagram below shows the positions of the three germ layers at the start of this period:
The three germ layers.

 

  • The image below shows the derivatives of each of the germ layers
  • Summary: Derivatives of the Germ Layers.
  • Ectoderm (lining the amniotic cavity) – there are several separate parts forming different structures:
    1. Surface ectoderm (‘external’ structures):
      • Epidermis, hair, nails, enamel of teeth,
      • Cutaneous and mammary glands
    2. Placodes
      • Inner ear/Lens/Olfactory apparatus
      • Cranial Nerve ganglia
    3. Neuroectoderm which is made up by cells of the neural plate
      1. Neural crest (cranial and sensory ganglia and nerves, medulla of adrenal glands, pigment cells brachial arch cartilages, head mesenchyme)
      2. Neural tube: CNS, retina, pineal body, posterior pituitary
  • Mesoderm as with the ectoderm there are several separate parts forming different things:
    • Head: skull, muscles and connective tissue of head, dentine
    • Paraxial: muscles of trunk and skeleton, dermis of skin, connective tissue
    • Intermediate: Urogenital system, including gonads, ducts and accessory glands
    • Lateral:
      • Connective tissues of muscle and viscera and limbs,
      • Serous membranes of pleura, pericardium and peritoneum
      • Blood and lymph cells – the cardiovascular and lymphatic systems,
      • Spleen
      • Adrenal cortex.
  • Endoderm (yellow):
    • Forms internal structures, especially epithelial lining of respiratory and digestive tracts.
      • I.e. the epithelial parts of trachea, bronchi, lungs, GI tract, liver and pancreas, urinary bladder, urachus, pharynx, thyroid, tympanic cavity, pharyngotympanic tube, tonsils, and parathyroid.