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BRAIN DEVELOPMENT AND DEVELOPMENTAL DISABILITIES
Dr. J. M. Rao,
M.B., B.S., D.P.M., M.R.C.Psych., F.R.C.P.
Associate Professor, Department of Psychiatry, Developmental Disabilities
Program, The University of Western Ontario, London, Ontario
Children and adults with developmental (intellectual) and learning
difficulties may exhibit delayed cognitive development. Some may
suffer from profound delays, while others may show minor difficulties
in learning. We know, from our current state of knowledge that
the functional organisation of the brain, as it develops, may
be disrupted at various stages, starting from when the foetal
brain is in its early stages of development to post birth, until
adolescence. Various genetic and environmental factors may cause
such disruption in the regulation of the orderly development of
the brain.
It is, therefore, of interest to briefly look at some of the
developmental stages in the foetal brain and also look at how
a normal brain develops after birth. This gives us a frame of
reference for understanding many of the syndromes that unfortunately
represent maldevelopment of the brain.
a) Initial Stages in Brain Development
About the 18th day of gestation, the nervous system begins
as a plate of tissue. The edges of this plate fold to form a tube,
much as one may fold a rolled sheet of dough. The two ends of
the tube then close by the 26th day. This closed tube eventually
develops into the brain and spinal cord. Disturbances to this
process result in errors in the neural tube closure.
The following are some examples:
Anencephaly: In this condition, the head end of the tube has
not closed, resulting in the absence of major parts of the skull,
brain and brain stem. These foetuses do not survive.
Myelomeningocele: The tail end of the neural tube fails to develop
appropriately, resulting in deficiency in the axial skeleton.
These infants survive but have major deficits.
b) Middle Stages of Development
During this stage, intense growth of the neural tube takes
place resulting in the formation of the face and forebrain. The
most intense activity takes place during the 5th and 6th weeks:
Much of the nerve tracts associated with the eye and nose develop,
as well as the cranial nerves, basal ganglia (inner parts of the
brain) and the two cerebral hemispheres. In the 2nd and 3rd months,
other structures such as the hypothalamic plate (the centre that
controls endocrines, temperature and appetite) develop. Most important,
the bridge between the hemisphere called the corpus callosum develops,
to be completed by 20 weeks of gestation.
Understandably, disruptions at this stage may cause profound
damage; however, some damage may not even be detected during life.
Some examples of detectable damage follow below:
Haloprocencephaly: In this condition, the forebrain is severely
malformed, resulting in profound retardation.
Agenesis of Corpus Callosum: In this condition, the bridge between
the two brain hemispheres is defective, resulting in problems
in the communication between the two parts of the brain. Depending
on the extent of the damage, the brain may compensate for these
defects and the individual may be able to function, albeit with
some difficulties.
c) Later Stages of Development
From between two and four months, the basic structure of the
brain starts growing. Nerve cells, receptors and the supporting
structures start proliferating. Disorders at this stage may result
in small brains called micrencephaly or excessive deranged proliferation
called macrencephaly. Various degrees of cognitive difficulties
result from this.
Apart from growth during this stage, millions of nerve cells
move to various specific regions of the brain and settle down.
This migration is at its peak around the 5th month. Disturbances
can result in serious malformations of the brain to patches of
nerve cells found in abnormal places. Seizures are frequent clinical
events. Carbon monoxide poisoning at around 22 weeks of gestation
may cause some types of migrational problems.
From about the 5th month to several years after birth, the brain
goes through an organisational phase. During this stage, the nerve
cells and receptors grow in numbers initially but after two years
of age are gradually pruned until adolescence to nearly 50% of
the original number. This process is very important and enables
the infant to adapt to living in the world with all its various
demands. This process enables appropriate connections to develop
in the circuitry in the brain. The organisation of the brain is
dependent on a certain structure within the brain called the subplate.
This structure provides a docking site for the connections that
flow from the rest of the brain to the future cortex. These neurones
also wait in the subplate, like passengers in a railway station,
and in the process, mature and get ready for an onward journey.
Eventually, they travel to a specific layer of the cortex (the
outer brain) which is now differentiating into a six layered structure.
If the subplate neurones are damaged during the 2nd and 3rd trimesters
of pregnancy, the subsequent organisation of the neural network
in the brain is severely disrupted. These networks are the basic
wiring needed for cognitive development.
The growth of the brain continues, as alluded to above, even
after birth. However, after birth, it is more a process of pruning
the excessive wiring, and the development of extensive connections
between the neurones and synapses. This process of dendritic connections
(dendrites are wiring that sprout from the head of the nerve cell)
is most active during and after adolescence. At the same time,
the density of receptors in layer II and III of the cortex increases.
These are the layers that are involved in information processing.
Localised disruption to this process will only become evident
clinically after several years, around adolescence. This explains
why in some children cognitive abilities decline in mid or later
childhood. Disorders of the organisation of neural connections,
as described above, are associated with mental retardation with
or without seizures; Down's syndrome, Angelman syndrome, infantile
autism, Duchene's muscular dystrophy, etc.
Another important development is myelination. Myelin is a sheath
that wraps around the neuronal wiring to insulate and facilitate
faster transmission of electrical potentials. This process begins
prior to birth and continues until adulthood in an orderly fashion.
For example, sensory pathways are myelinated first and motor pathways
after. The rear lobe (occipital) that is concerned with vision
is myelinated first and the frontal lobe (the executive brain)
later. This explains why in children sensory efficiency develops
before motor efficiency and why complex problem solving and such
cognitive abilities develop much later. Disorders of myelination
lead to conditions such as cerebral white matter hypoplasia, amino
and organic acidopathies, congenital hypothyroidism, periventricular
leukomalacia .
Conclusion
I have outlined, very briefly, some of the milestones in the
development of the brain and the conditions that may arise if
there is disruption of such events.
There are exciting developments in the field of developmental
biology and medicine that are gradually unravelling the mystery
of brain development. We are gaining an understanding of what
may interrupt this complex process of development. Eventually,
this knowledge may lead to biological, psychological and social
interventions that may provide us more effective preventive and
interventional strategies. While on one hand, such complexity
is overwhelming, on the other hand, the knowledge that we are
acquiring can be seen as the dawn of a new era of hope.
This article first appeared in the Clinical Bulletin of the
Developmental Disabilities Programme at the University of Western
Ontario Vol 13 - No.4 December 2002
Recommended reading list
1) Miller JC & Freidhoff AJ (1988) Prenatal neurotransmitter
programming of postnatal receptor function. Progress in Brain
Research; 73, 509-22.
2) Doupe AJ, Patterson PH & Landis SC (1985a) Environmental
influences in the development of neural crest derivatives: glucocorticoids,
growth factors and chromaffin cell plasticity. Journal of Neuroscience;
5:21, 19-42.
3) Oster-Granite ML & Ebner FF (1996) Developmental processes
and the pathophysiology of mental retardation. Mental Retardation
and Developmental Disabilities; 2:197-208.
4) Greenough WT (1986) What is special about development? Thoughts
on the bases of experience-sensitive synaptic plasticity. In Greenough
WT, Juraska A (eds.) Developmental Neuropsychobiology. New York:
Academic Press, p. 387-407.
5) Harris JC (1995) Developmental neuroanatomy. In Developmental
Neuropsychiatry, Vol. 1. New York: Oxford University Press, p.
26-48.
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