Embryology & Development Nervous System

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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 ependyma3) 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 aggregate4) 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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