Category : 11th Class
Nervous system begins develop early in third week of development from ectoderm. Nervous tissue also develop from ectoderm except microglial cell, develop from mesoderm. The central nervous system of vertebrates includes the brain and the spinal cord. These are derived from a longitudinal mid-dorsal ectodermal thickening of the embryo, called the medullary or neural plate. This neural plate or neural groove is converted by fusion into a closed mid-dorsal longitudinal neural tube lying above the notochord. Histologically, the embryonic neural tube exhibits three zones of cells.
(1) Germinal layer : These are actively dividing cells lining the neural canal. They form the connective tissue lining of neural canal, called ependyma, and ventricles of brain.
(2) Mantle layer : It consists of embryonic neurons or nematoblasts, forming the gray matter.
(3) Marginal layer : It consists of nerve fibres, mostly surrounded by fatty myelin sheaths, and forms the white matter. Neurons and fibres are supported by a special connective tissue of ectodermal origin, the neuroglea, cells of which become increasingly abundant and diversified in higher vertebrates.
Development of brain
The anterior end of embryonic neural tube is already enlarged forming the embryonic brain, called encephalon. By differential growth and two constrictions, it is divided into a linear series of three primary cerebral vesicles, termed the forebrain, midbrain and hindbrain. These give rise to the three major divisions of the adult brain - (1) prosencephalon (forebrain), (2) mesencephalon (midbrain), and (3) rhombencephalon (hindbrain). These further become subdivided into 5 subdivisions. Prosencephalon divides into an anterior telencephalon and posterior diencephalon; the mesencephalon remain unchanged. The rhombencephalon divides into an anterior metencephalon and a posterior myelencephalon. Ultimately, telencephalon develops into cerebral hemisphere and basal ganglia and houses lateral ventricle. Diencephalon develops into thalamus, hypothalamus, and pineal gland and houses the third ventricle. Mesencephalon develops into mid brain and houses cerebral aqueduct. Metencephalon develops into pons and cerebellum; and myelencephalon develops into medulla oblongata, houses 4th ventricle. The area of neural tube inferior to myelencephalon gives rise to spinal cord.
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