Embryology & Development Nervous System
Nervous system develops from the dorsomedial embryonic ectoderm (outermost layer), at about 21-22 days of age (embryo is <4mm in length).
Development of the nervous system follows a number of overlapping sequences:
1) Formation of the neural tube and neural crest from the ectoderm (neurulation)
• dorsomedial part of embryonic ectoderm initially thickens called neuroectoderm or neural plate
• half of ectoderm becomes neuroectoderm
• lateral margins of neuroectoderm elevate neural groove eventually make contact to become neural tube (lumen is called neurocoele)
• small groups neural cells migrate from surface ectoderm and neural tube to form a strip of ectoderm, called the neural crest gives rise to peripheral nervous system
• the central nervous system derive from neural tube
• the peripheral nervous system derive from the neural crest
• ends of neural tube remain open for a loner period of time (if posterior fails to close spina bifada; if anterior anencephaly)
2) Proliferation of cells within the neural tube
• single layer of simple epithelium of neural tube becomes stratified following cell divisions (these neuroepithelial cells divide rapidly)
• some cells become post mitotic then pushed to externally, superficial to those that are still mitotic to form the intermediate zone (later becomes the gray matter of spinal cord)
• most of the postmitotic cells are neuroblasts
• when all cells become postmitotic simple ependyma
3) Migration of neuroblastic cells
• when neuroblast becomes post mitotic it elongates and migrates along a radial glial ells (to become ‘nerve fibre’)
• migration zone is external to intermediate zone – the marginal zone (later becomes white matter of spinal cord)
• when migration complete, radial glial cells either develop into astrocytes or degenerate
• when cells stop migrating, those with similar properties aggregate
4) Formation of axonal and dendritic processes of nerve cells
• neurons next differentiate form cytoplasmic processes necessary for impulse transmission
• growing tip of axon is called the ‘growth cone’
5) Formation of connections between nerve cells
• nerve cell adhesion molecules (N-CAM) – appears to be a ‘glue’ that is important in development of neuron (can be blocked by antibodies)
• synapses form (synaptogenesis) when axon reaches its target (under influence of mechanical and chemical factors to find target – called ‘path finding’)
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