Keywords
neural development, neural crest cells, congenital malformations
Chapter Outline
The Neural Tube and Neural Crest Give Rise to the Central and Peripheral Nervous Systems, 8
The Sulcus Limitans Separates Sensory and Motor Areas of the Spinal Cord and Brainstem, 8
The Neural Tube Has a Series of Bulges and Flexures, 8
Growth of the Telencephalon Overshadows Other Parts of the Nervous System, 10
Adverse Events During Development Can Cause Congenital Malformations of the Nervous System, 10
Defective Closure of the Neural Tube Can Cause Spina Bifida or Anencephaly, 11
Defective Secondary Neurulation Can Cause a Distinctive Set of Abnormalities, 11
The Prosencephalon Can Develop Abnormally Even if Neural Tube Closure Is Complete, 11
Understanding a little bit about the embryology of the brain helps clarify the way it’s put together in adults. The central nervous system (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 of cells 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 peripheral nervous system (PNS; and much more). These include the sensory neurons of spinal and most cranial nerve ganglia, postganglionic autonomic neurons, the enteric nervous system, and the Schwann cells of peripheral nerves and ganglia ( Fig. 2.2 ). The neural tube goes on to form the CNS.
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.
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 ).
The Neural Tube Has a Series of Bulges and Flexures
There are three primary vesicles.
There are five secondary vesicles.
The cavity of the neural tube persists as a system of ventricles.
The prosencephalon and rhombencephalon each divide into two secondary vesicles , so there is a total of five secondary vesicles. The prosencephalon forms the telencephalon (“end-brain”) and the diencephalon (“in-between-brain”). The telencephalon gives rise to the two cerebral hemispheres, whose cavities become the lateral ventricles . The diencephalon gives rise to the thalamus, hypothalamus, retina , pineal gland, and several other structures; its cavity becomes the third ventricle . The mesencephalon remains undivided as the midbrain; its cavity persists as the cerebral aqueduct , which interconnects the third and fourth ventricles. The rhombencephalon forms the metencephalon and the myelencephalon , which together give rise to the cerebellum, as well as the pons and medulla parts of the brainstem, and enclose the fourth ventricle. This ventricular arrangement is shown schematically in Fig. 2.5 .
