MATURATION OF THE CENTRAL NERVOUS SYSTEMBoth neurons and glia develop from the original neurectoderm forming the
neural tube.
Microgliaare the exception: they develop from the monocyte-macrophage
lineage of mesodermal (bone marrow) origin and migrate into the CNS.
Inductionof regional differences in the developing CNS is regulated
by retinoic acid (vitamin A). Overexposure of the cranial region to retinoic
acidcan result in “caudalization,” i.e., development more similar to the
spinal cord.
During development, the spinal cord and presumptive brainstem
develop three layers: (1) a germinal layerorventricular zone, (2) an
intermediate layercontainingneuroblastsand comprising gray matter,
and (3) a marginal zonecontaining myelinated fibers (white matter).
Other layers are added in the cerebrum and cerebellum by cell migra-
tion along glial scaffolds.
The notochord induces the establishment of dorsal-ventral polarity
in the neural tube. Ventral portions of the tube will become the basal plate
and give rise to motor neurons, whereas the dorsal portions become the
alar plates, derivatives of which subserve sensory functions.
Meninges are formed by mesoderm surrounding the neural tube with
contributions to the arachnoid and pia from neural crest.
Defects in the CNS may result from several causes including high mater-
nal blood glucose levels and vitamin A overexposure and often involve bony
defects (e.g., spina bifida and anencephaly). Defects are most common in the
regions of neuropore closure. Folic acid, also known as folate, is a B-vitamin
that can be found in some enriched foods and vitamin supplements.Women
who take folate before pregnancy have a decreased risk of neural tube defects
(NTDs) including spina bifida and anencephaly. The U.S. Public Health Ser-
vice recommends that all women who could possibly become pregnant get
400 μg (or 0.4 mg) of folic acid every day. This could prevent up to 70% of
NTDs. Folic acid is found in some foods, such as enriched breads, pastas,
rice, and cereals (some with 100% of the daily requirement).
Fetal alcohol syndrome (FAS) is the most common cause of mental
retardation; FAS includes the triad of growth retardation, characteristic facial
dysmorphology and neurodevelopmental abnormalities. Alcohol rapidly
crosses the placenta and the fetal blood-brain barrier. Damage is dependent
on gestational age, alcohol dosage, and pattern of maternal alochol abuse.
Altered neural crest cell migration, differentiation and programmed cell death
High-Yield Facts 5