Cytology, General Histology and Embryology - V. K. Napkhanyuk 2002

Tissues
Nervous tissue
Histogenesis of nervous tissue

Nervous Tissue (textus nervosus) is a specialized tissue that serves as the primary structural element of The Nervous system. It regulates The activity of tissues and Organs, ensures their interaction and connection with the external environment, and coordinates the Functions, integration, and adaptation of the Organism as a whole.

Nervous tissue consists of Nerve Cells (neurocytes, Neurons) and associated neuroglial cells. Together, these structural elements form the unified morphological and functional basis of all Organs of the nervous system.

Nervous tissue develops from the dorsal thickening of the ectoderm, known as the neural plate. The neural plate sequentially transforms into the neural groove and then into the neural tube (Fig. 101). The neural tube separates from the overlying epidermal ectoderm. A portion of the neural plate cells does not become part of either the neural tube or the epidermal ectoderm; instead, they lie between them as a loose cluster known as the neural crest or ganglionic plate. These crest cells begin to migrate laterally and ventrally. The crest Cells of the HEAD region form the cranial nerve ganglia, with neural placodes serving as a secondary source of their formation.

The cells of the trunk region of the neural crest separate into two streams: superficial and deep.

The superficial stream spreads between the ectoderm and mesoderm, giving rise to Skin pigment cells.

The deep stream directs inward and passes ventrally between the somite and the neural tube, as well as among mesenchymal cells migrating from the somite. These cells form the neurons of the spinal and Autonomic nervous system ganglia, as well as neuroglia (lemocytes).

Thickenings of the ectoderm on the sides of the head are called neural placodes. They participate in The formation of the ganglia of the V, VII, and X pairs of Cranial Nerves.

In Cytology/cytology/16.html">Early stages of Embryogenesis, the neural tube is a pseudostratified neuroepithelium formed by ventricular or neuroepithelial cells.

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Fig. 101. Diagram of neural tube formation in a chick embryo (after A. G. Knorre):

a — neural plate stage; b — closure of the neural tube; c — Separation of the neural tube and ganglionic plate from the ectoderm; 1 — neural groove; 2 — neural folds; 3 — surface ectoderm; 4 — notochord; 5 — mesoderm; 6 — ganglionic plate; 7 — neural tube; 8 — mesenchyme

Ventricular cells are cylindrical cells whose apical processes border the lumen of the neural tube and are connected by Gap Junctions. The basal ends of these cells Touch the subapical limiting membrane.

These cells are characterized by cyclic nuclear translocation: the nuclei of premitotic cells lie deep within; during prophase, they approach the surface, karyokinesis occurs near the ventricular surface, and the daughter nuclei migrate inward again. During embryonic development, the proliferative activity of ventricular cells decreases, and after birth, it is no longer observed.

Morphologically, ventricular cells differentiate into various Cell types of the mature nervous tissue. Some of them give rise to neurocytes, while the rest develop into glial cells (ependymocytes, astrocytes, and oligodendrocytes).

In Brain regions where histogenesis is particularly intensive, ventricular cells lose their cylindrical shape and nuclear migration capacity, yet retain high proliferative activity. These cells are referred to as subventricular and extraventricular neurogerminative (cambial) cells. Subsequently, they give rise to certain types of neurocytes and glial cells. Sub- and extraventricular cells persist for some time after birth.