Fig: Spina Bifida Occulta
Picture from The Developing Human. Keith L. Moore. 1982, p. 385.
Neural migration takes place in the cerebral cortex of the brain. The cerebral
cortex is a thin layer of grey matter, ranging from 1.5 to 4.5 mm in thickness
that cover most of the brain structures. The development of the cerebral
cortex occurs from the inside out during embryonic life. New cortical cells
surround the ventricles in the middle of the brain, and what are to be the
innermost cells arise first. Cells to be located further outward in the
cortex arise later, and must migrate past cells located in the inner cortex.
The cells are guided in their migration by glial fibers extending perpendicular
to the cortical surface: these are called radial glia. Errors in the timing
of organization of this developmental process can result in a number of
very serious birth defects such as:
Epilepsy
This seems to result from the excessively irregular rhythm of nerve impulses
in the central nervous system. When this dysrhythmia passes from the
central nervous system along the motor nerves, convulsions of the muscles
result, accompanied by loss of consciousness. If the dysrhythmia is
retained within the central nervous system, and therefore does not affect
the muscles, temporary unconsciousness, unaccompanied by convulsions, may
occur.
Epilepsy also results from a condition called microdysgenesis in which areas
of disordered migration are evident. In this condition, the cerebral cortex
cannot develop the appropriate relationships between the various cortical
cell types.
Lissencephaly
This is a 'smooth brain' condition where the brain has few or none of the
normal convolutions and is associated with profound retardation.
Heterotopias
This condition is a diffuse abnormality in the organization of the cortical
layer, may result in a range of symptoms including behavioral and learning
difficulties.
Epilepsy
Foundation of America
Comprehensive
Epilepsy Center
Brain Mapping Center
Epilepsy
Management
Epilepsy
Resources
Family
Village
George
Law's Lissencephaly Page
Dev.
Disorders of the cerebral Hemisphere
Sources:
1. American Zoologist. "Neural crest: contributions to
the development of the vertebrate head." Ann C. Graveson. Sept 1993
v33 n4 p424(10).
2. BioEssays. "Mechanisms of neural crest cell migration."
Marianne Bronner-Fraser. April 1993 v15 n4 p221(10).
3. Human Development. Phyllis C. Martin and Elizabeth
Lee Vincent. The Ronald Press Company, 1960 p365.
Spinal Cord Defects
Neural tube defects are malformations of the spinal cord. The brain and spinal cord develop from a single pancake of cells that first folds into a tube. There are many possibilities for defects in the development of the neural tube. Here are just a few:
Spina Bifida
Malformations involving the caudal end of the neural
tube and the vertebral arches are referred to as spina bifida (divided spine).
This term describes nonfusion of the vertebral arches common to all types
of spina bifida. These conditions range from clinically significant
types to minor, clinically unimportant types.
Spina Bifida Occulta---This
is a vertebral defect resulting from failure of the two halves of the vertebral
arch to fuse, usually in the sacral, lumbar, and cervical regions.
This defect occurs in about 10 percent of people. In its most minor
form, there is no defect in the skin, and the only evidence of its presence
may be a small dimple with a tuft of hair. Spina bifida occulta produces
no clinical symptoms.
Spina Bifida Cystica---Severe
types of spina bifida, involving protrusion of the spinal cord and/or the
meninges through the defect in the vertebral arch, are often referred to
collectively as spina bifida cystica because of the cyst-like protrusion,
or sac, that is associated with these malformations. When the sac
contains meninges and cerebrospinal fluid, the condition is called spina
bifida with meningocele. The spinal cord and spinal roots are in their
normal position, but there may be spinal cord abnormalities. If the
spinal cord and/or nerve roots are included in the sac, the malformation
is called spina bifida with meningomyelocele. Meningomyelocele is
a more common and a very much severe malformation than meningocele.
Spina bifida cystica occurs about once in every 1000 births.
Association
for Spina Bifida
Spina
Bifida Home Page
Home
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Columbia_Presbyterian
Medical Center
Abnormal development of the brain is not uncommon, owing to the complexity of its embryological history. Most major congenital malformations of the brain result from defective closure of the rostral neuropore during the fourth week and involve the overlying tissues (future meninges and calvaria). The factors causing the faulty development may be primarily either genetic or environmental in nature.
Microcephaly
In this uncommon condition, the calvaria is small,
but the face is normal-sized. Generally, these infants are grossly
mentally retarded because the brain is small and underdeveloped. The
cause of this condition is often uncertain; some cases appear to be genetic
in origin, and others seem to be associated with environmental factors.
Hydrocephalus
This condition is the overproduction of cerebrospinal
fluid (CSF), obstruction of its flow, or interference with its absorption
resulting in an excess of CSF. Hydrocephalus often results from congenital
aqueductal stenosis, in which the cerebral aqueduct is narrow or consists
of several minute channels. Blockage of CSF circulation results in
dilation of the ventricles superior to the obstruction and in pressure on
the cerebral hemispheres. This squeezes the brain between the ventricular
fluid and the bones of the cranium. In infants, the internal pressure
results in expansion of the brain and the calvaria because the sutures and
fontanelles are still open. Hydrocephalus usually refers to internal hydrocephalus,
in which all or part of the ventricular system is enlarged.
Anencephaly
A condition in which the nervous tissue undergoes
degeneration until most of it is replaced in fetuses by a spongy, vascular
mass consisting mostly of hindbrain structures. Although anencephaly
means "without a brain", a rudimentary brain stem and traces of
the basal ganglia are usually present. Anencephaly is a common malformation,
occurring about once in every 1000 births, and is about four times more
common in females than males. Sustained extrauterine life is impossible
in infants bornwith anencephaly. Infants afflicted with this defect
survive for a few hours after birth, at most.
Sources:
1. JAMA. "Folate levels and neural tube defects."
Leslie E. Daly et. al. Dec. 6, 1995 v274 n21 p1698(5).
2. Science News. "Closing in on neural tube defects."
Kathy A Fackelman. July 10, 1993 v144 n2 p30.
3. The Developing Human. 3rd ed. Keith L. Moore. W.
B. Sauders Co., 1982.
4. Developmental Anatomy. Leslie Brainerd Arey. W.
B. Saunders Co., 1966.