Cell Types And Their Development

Cell types and their development  Nerve cells. o Neuroblasts arise exclusively by division of NE cells. o Initially have a central process extending to the lumen (transient dendrite), but when they migrate into the mantle layer, this process disappears, and neuroblasts are temporarily round and apolar. o Further differentiation produces two new cytoplasmic processes that appear on opposite sides, forming bipolar neuroblasts. o The process at one end elongates rapidly to form the primitive axon, whilst the process at the other branches to form the primitive dendrite. This forms a multipolar neuroblast. o With further development, a neuron is formed. o Axons of neurons in the basal plate break through the marginal zone and become the ventral motor root of the spinal nerve. o Axons of neurons in the alar plate penetrate into the marginal layer of the cord, where they ascend to either higher or lower levels to form association neurons.  Supporting cells. o Majority of primitive form, gliablasts, are formed by NE cells after production of neuroblasts ceases. o Gliablasts migrate from the NE layer to the mantle and marginal layers. o In the mantle layer, they differentiate into protoplasmic astrocytes and fibrillar astrocytes. These sit between neurons and blood vessels, where they provide support and serve metabolic functions. o In the marginal layer, they may differentiate into oligodendroglial cells which form myelin sheaths around the ascending and descending axons in the marginal layer. o In the second half of development, there is also production of a third type of supporting cell, the microglial cell. This cell serves as phagocytosis mechanism and is derived from vascular mesenchyme when blood vessels grow into nervous system.  Neural crest cells. o Ectodermal in origin. o Appear along the edge of the neural folds and extend throughout the length of the neural tube. o Migrate laterally and give rise to sensory ganglia of the spinal nerves. o After further development, neuroblasts of the sensory ganglia form two processes. o Centrally growing process penetrates the dorsal portion of the neural tube, and in the spinal cord, end in the dorsal horn or ascend through the marginal layer to one of the higher brain centres. These processes are collectively known as the dorsal sensory root of the spinal nerve. o Peripherally growing process join fibres of the ventral motor roots and thus participate in the formation of the trunk of the spinal nerve. Eventually, these terminate in the sensory receptor organs.   Spinal nerves. o Motor nerve fibres arise from nerve cells in the basal plates (ventral horns). Fibres collect into bundles known as ventral nerve root. o Dorsal nerve roots form as collections of fibres originating from cells in the dorsal root ganglia. o Central processes of dorsal root form bundles that grow into the spinal cord opposite the dorsal horns. Distal processes join the ventral nerve roots to form a spinal nerve. o Spinal nerve divides into dorsal and ventral primary rami; dorsal innervate dorsal axial musculature, vertebral joints and the skin of the back, whereas ventral innervate the limbs and vertebral body wall and form the major nerve plexuses. Myelination. o Schwann cells are responsible for myelination of a peripheral nerves. Each cell myelinates only a single axon. They originate from neural crest, migrate peripherally and wrap themselves around axons, forming neurilemma sheath. o At the spinal cord, myelination is achieved by oligodendroglial cells. These can myelinate up to 50 axons.