Embryonic development of nervous system
A study of development of the nervous system helps to understand its complex organisation and the occurrence of various congenital anomalies.
The whole of the nervous system is derived from ectoderm except its blood vessels and some neuroglia elements.
The specific cel population of the early ectoderm, which gives rise to entire nervous system and special sense organs is termed neural ectoderm. The neural ectoderm later differentiates into three structures:
- Neural tube
- Neural crest cells
- Ectoderm placodes
The neural tube gives rise to the central nervous system, the neural crest cells from nearly all the peripheral nervous system and ectodermal placodes contribute to the cranial sensory ganglia, hypophysis and inner ear.
Stages of development of nervous system
Formation of Neural tube
In the early embryonic disc, at about 16th day of embryonic life, the ectoderm overlying the newly formed notochord thickens in the midline forming the neural plate. As somatic mesoderm develops on either side of notochord, the margins of neural plate are elevated as neural folds, as a result the centre of the plate sinks, creating the neural groove, the neural folds gradually move together towards the midline and finally fuse to form a cylindrical neural tube that loses its connection with the surface ectoderm. The process of neural tube formation is termed neurulation
The neural tube and surrounding amniotic cavity, therefore, remain temporarily in open communication with each other through these pores. The anterior neuropore closes in the middle of the 4th week at 18-20 somite stage and posterior neuropore closes at the end of 4th week at about 25 somite stage. By the time the neural tube is completely closed, it is divisible into an enlarged cranial part and an elongated caudal part which later on gives rise to brain and spinal cord.
Formation of neural crest cells
As the neural folds come together and fuse, the cells at the tips of neural folds break away from the neuroectoderm to from neural crest cells. The surface ectoderm of one side becomes continuous with the surface ectoderm of the opposite over the neural tube.
Thus the cells at the tips of neural folds do not participate in neural tube formation . The neural crest cells at first remain in the middle between the dorsal surface of the neural tube and the surface ectoderm, and them forms two-cell clusters dorsolaterally, one on either side of the neural tube.
The neural crest cells differentiate to form the cels of dorsal root ganglia, sensory of cranial nerves, autonomic ganglia, adrenal medulla, chromaffin tissue, melanocytes and schwann cells.
Formation of Ectodermal Placodes
Prior to the neural tube closure , the neural folds contains two types of cell populations:
- Neural crest cells
- Neuroepithelial cells
During neurulation, the neural crest cells are detached and neuroepithelial cells become incorporated into the surface ectoderm. These areas of neuroepithelial within the surface ectoderm are termed ectodermal placodes.
Development of spinal cord
The spinal cord develops from the caudal elongated part of the neural tube. The neural tube increase in thickness due to repeated mitosis of its epithelial lining. By the middle of 5th week of embryonic development, the transverse section of the recently closed neural tube reveal three distinct layers or zones. From within outwards these are:
- Matrix zone
- Mantle zone
- Marginal zone
Development of Brain
The brain develops from the enlarged cranial part of the neural tube. At about the end of 4th week, the enlarged cephalic part shows three distinct dilatations called primary brain vesicles. Craniocaudlly, these are:
- Prosencephalon
- Mesencephalon
- Rhombencephalon
Prosencephalon
The diencephalon develops from the median portion of the pos-encephalon. Its cavity is called third ventricle. The primitive diet-cephalon consists of two thick lateral walls, a thin root and floor plates. Each lateral wall presents a sulcus, the hypothalamic sulcus which appears to be the rostral continuation of sulcus limitants.
Mesencephalon
Morphology the midbrain is the must primitive of the brain vesicles. It generally retains a cylindrical form and its narrowed cavity forms the cerebral aqueduct which is continuous below with the fourth ventricle and above with the third ventricle.
Anterior to the cerebral aqueduct, the basal laminae give rise to the tegmentum and substantia nigra. The marginal layer of each basal lamina enlarges and forms the crus cerebri.
Rhombencephalon
The caudal part of the myelencephalon has a central canal and forms the closed part of the medulla oblongata. Rostrally the central canal expands as the cavity of the fourth ventricle, and thus the rostral part of myelencephalon forms the open part of the medulla oblongata.
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