EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
135. Early Development of The Nervous System
Review of MEDICAL EMBRYOLOGY Book by BEN PANSKY, Ph.D, M.D.
- General considerations
- THE NERVOUS SYSTEM, AS A WHOLE, including the spinal cord, brain, and peripheral nerves, is derived from ectoderm
- The primordial structure, which gives rise to the nervous system or neural ectoderm, appears very early in development, around day 1 It develops from the ectoderm in the dorsomedian region of the embryo, above the mesoderm, and cephalic to Hensen's node
- The mesoderm, formed during gastrulation, induces formation of the neural ectoderm from the overlying ectoderm
- During development, the neural ectoderm and the remaining ectoderm separat The latter forms the surface ectoderm, which gives origin to the epidermis and certain sense organs
- Neurulation is the transformation of the ectoderm overlying the notochord into a neural tube, which is flanked by 2 longitudinal formations, the neural crests
- INDUCTION OF THE NERVOUS SYSTEM
- The various phases of neurulation are induced by the notochord and the parachordal mesoderm (inductors) from the overlying competent ectoderm. Induction is also essential in the subsequent development of the nervous system
- The activity of the inductors is not uniform, and there is a cephalocaudal gradient
- Thus, the trunk areas of the notochord and parachordal mesoderm induce the formation of the spinal cord, while their extremities are inducing the middle and posterior parts of the brain (the mesencephalon and rhombencephalon)
- The prosencephalon, on the other hand, is induced by the prochordal plate, a small circular area of columnar entodermal cells found in front of the notochord
- The prochordal plate is an entodermal structure firmly attached to the overlying ectoderm, forming the oropharyngeal or buccopharyngeal membrane
- The prochordal plate indicates the site of the mouth and serves as an organizer of the head region, giving rise to mesenchyme in the head region and to the entodermal layer of the oropharyngeal membrane
- During its development, the neural tube, in turn, induces the formation of the posterior arch of the vertebrae and of the cranial vault
- It also plays an important role in the development of the face, the eye, and the nose
- Since either induction or competence can be defective at each stage of the developmental process, many abnormalities can occur as a result of such a complex mechanism of development
