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 image below shows the derivatives of each of the germ layers
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Summary: Derivatives of the Germ Layers. - Ectoderm (lining the amniotic cavity) – there are several separate parts forming different structures:
- Surface ectoderm (‘external’ structures):
- Epidermis, hair, nails, enamel of teeth,
- Cutaneous and mammary glands
- Placodes
- Inner ear/Lens/Olfactory apparatus
- Cranial Nerve ganglia
- Neuroectoderm which is made up by cells of the neural plate
- Neural crest (cranial and sensory ganglia and nerves, medulla of adrenal glands, pigment cells brachial arch cartilages, head mesenchyme)
- Neural tube: CNS, retina, pineal body, posterior pituitary
- Surface ectoderm (‘external’ structures):
- 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.
- Forms internal structures, especially epithelial lining of respiratory and digestive tracts.