Ectoderm development

Summary

    • 4 groups of tissues come about from the ectoderm
      • Surface ectoderm (future epithelium and glands)
      • Neural tube tissue (future CNS)
      • Neural crest tissues (future melanocytes and PNS structures)
      • Ectodermal placodes (several tissues including the future visual/auditory/olfactory sensors)
  • The key process here is neurulation which divides the three

Surface ectoderm

  • This develops into the skin epidermis,  hair, nails, cutaneous and mammary glands, enamel of teeth, inner ear, lens
    • Essentially anything with keratin + glands
    • Additional info: this occurs in the presence of BMP-4
      • Inactivation of BMP4 leads to  neuralisation

Neurulation

  • The neuroectoderm forms through the process of neurulation
    • At this stage, the embryo is a lot longer than wider and is also larger cephalically
  • Process of Neurulation
    1. The ectoderm overlying the notochord is induced by it to become the neural plate                                                                                                                                                
    2. This develops 2 parts
      • Laterally becomes the more elevated neural folds
      • Midregion is the depressed neural groove                                                                                                 
    3. The folds rise dorsally and then approach each other
      • Fuse in the midline to form the neural tube – the future CNS
      • This lies above the notochord
      • (The otic and lens placodes are seen after closure -> develop into audio and visual vesicles)
Summary of the neurulation process

Development of the neural tube

The neural tube develops into the brain which has 3 main parts:

  • Forebrain
    1. Telencephalon (anterior)
      • Forebrain vesicles => later forms the Cerebral hemispheres
    2. Diencephalon (posterior) => forms the future optic system
      • Optic vesicle
      • Retinas -(stalks)
  •  Midbrain
    • Mesencephelon => future midbrain
    • Aqueduct of Sylvius => future ventricles for CSF drainage
  • Hindbrain
    • Metencephalon => cerebellum
    • Myelencephalon => Medulla oblongata – connects to spinal cord
The five vesicles of the brain.
The vesicles of the brain and their final structures in the brain.

Development of neural crest

  • During neurulation, the cells at the tips of the folds become the neural crests during migration
    • These detach after the neural tube forms and migrate into the mesoderm
    • Here, they become mesenchymal (connective tissue-like) and develop into variety of further tissues via 2 pathways:
      • Melanocytes
      • PNS nervous tissues
        • Craniofacial skeleton (esp viscerocranium via the pharyngeal arches)
        • The DRG , ANS ganglia, and CN ganglia
        • Enteric NS
        • Adrenal medulla
        • Schwann/glial cells
    • All of the above can be thought of as being connective tissue structures
In C), The NCCs can be seen to migrate and develop structures (e.g. the DRG, sympathetic ganglion, and enteric ganglia).

Ectodermal placodes

  • These are regions of thickened neuroectodermal epithelium that give rise to neurons and other structures (teeth, hair follicles)
    • There are many placodes , but certain are key
    • Essentially need to remember that these form the special sensory structures + some cranial nerve ganglia
  • Initially two bilateral ectodermal placodes develop upon neural tube closure in the cephalic region
    1. Otic placodes – future vestibular/cochlear apparatus
      • Invaginate to form the otic vesicles
    2. Lens placodes – future lens of the eye
      • Invaginate to form the optic vesicles
  • Olfactory (nasal) placode gives rise to the olfactory epithelium in the nose
    • Also involved in face (viscerocranium) development
  • There are also 4 placodes that form ganglia of the cranial nerves:
    1. Trigeminal placode forms the trigeminal ganglion involving only CNV1 and CNV2 – not CNV3
    2. Epipharyngeal placodes form the distal ganglia + nerve tissue of CNVII, IX, X – associate with the pharyngeal clefts hence the name
      1. Geniculate placode – forms geniculate ganglion + distal CNVII
      2. Petrosal placode – forms glossopharyngeal ganglion + distal CNIX
      3. Nodosal placode – forms nodose ganglion + distal CNX
  • You do not need to know this in huge detail for med school exams!

Summary of ectoderm differentiation

    • There are 4 different structures after ectoderm is developed
      • Surface ectoderm – develops into epidermis
      • Neural crests – develop into various mesenchyme including PNS
      • Neural Tube – develops into CNS
      • Ectodermal Placodes – develop into special sensory systems + some cranial nerve ganglia
This summarises the key structures of the ectoderm and where they end up. It shows the surface ectoderm, neural tube, and NCC structures.