Development of the Nervous System

Published on 16/03/2015 by admin

Last modified 16/03/2015

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2 Development of the Nervous System

Understanding a little bit about the embryology of the brain helps clarify the way it’s put together in adults. The CNS starts out as a simple ectodermal tube that develops some folds and bulges. The cavity of the tube persists as the ventricles, and the folds and bulges determine the shape and layout of many parts of the CNS.

The Neural Tube and Neural Crest Give Rise to the Central and Peripheral Nervous Systems

Cells of the neural crest grow at the apex of each neural fold. When the neural folds fuse to form the neural tube, the neural crest becomes a detached layer between the neural tube and the surface ectoderm (Fig. 2-1). Neural crest cells migrate from there, and go on to form most neurons and glial cells of the PNS (and much more). These include the sensory neurons of spinal and most cranial nerve ganglia, postganglionic autonomic neurons, and the Schwann cells of peripheral nerves and ganglia (Fig. 2-2). The neural tube goes on to form the CNS.

Figure 2-1 Neural groove and neural tube.

Figure 2-2 Cell lineages in the nervous system. (Most think that microglia are derived from monocyte-related stem cells rather than the neural tube, but this is not known for sure.)

The Sulcus Limitans Separates Sensory and Motor Areas of the Spinal Cord and Brainstem

The sulcus limitans is a longitudinal groove that develops in the lateral wall of the embryonic spinal cord and extends into the rhombencephalon (the embryonic medulla and pons, as discussed a little later). It separates two groups of neuronal cell bodies, the alar plate (dorsal to the sulcus limitans in the spinal cord) and the basal plate (ventral to the sulcus limitans in the spinal cord). The alar and basal plates go on to become sensory and motor structures, respectively (Fig. 2-3). The spinal alar plate becomes the posterior horn, where primary sensory neurons terminate. The spinal basal plate becomes the anterior horn, where the cell bodies of motor neurons live.

Figure 2-3 Development of the alar (AP) and basal (BP) plates of the embryonic spinal cord into sensory and motor regions of the mature spinal cord. *Central canal of the spinal cord (the remnant of the cavity of the neural tube, where the sulcus limitans used to be); DRG, dorsal root ganglion cell.

The walls of the neural tube are spread apart in the rhombencephalon, forming the floor of the fourth ventricle, so in the medulla and pons the alar plate ends up lateral to the basal plate (Fig. 2-4). The same development into sensory and motor structures occurs, however, so cranial nerve sensory nuclei are lateral to cranial nerve motor nuclei in the adult brainstem (see Fig. 12-2).

Figure 2-4 Spreading apart of the walls of the neural tube in the medulla and pons results in the alar (AP) and basal (BP) plates winding up in the floor of the fourth ventricle. CNG, Cranial nerve ganglion cell; DRG, dorsal root ganglion cell.