Question

Describe the development of the cerebral cortex and the rapid growth of brain mass in infancy.

Answer 1

The Central Nervous System (CNS), which consists of the brain and spinal cord, is in charge of how our bodies function. The CNS takes in, interprets and sends out signals.

The cerebral cortex is the largest, most complex structure of the brain, which is the last to stop growing, and resembles half of a shelled walnut.

Development of the cerebral cortex is a very dynamic process, involving a series of complex morphogenetic events. Following the division of progenitor cells in the ventricular zone, neurons undergo a series of morphological changes and migrate outward toward the cortical plate, where they differentiate and integrate into functional circuits. Errors at several of stages during neurogenesis and migration cause a variety of severe cortical malformations. A number of disease genes encode factors associated with the cytoskeleton, which plays a crucial role throughout cortical development. Methods for regulating gene expression coupled with imaging of subcellular structures have provided important insight into the mechanisms governing normal and abnormal brain development.

Rules that govern neuron migration in developing cerebral cortex # 1: Migrating neuron always assumes a position superficial or closer to the pia than the waves that preceded them.

Rules that govern neuron migration in developing cerebral cortex #2: Plexiform zone shrinks in size as cortical cell layers form within it.

Rules that govern neuron migration in developing cerebral cortex#3: What is left in the matrix zone (germinal matrix) becomes the ependymal lining of the lateral ventricle.

The process
1. undeveloped or undifferentiated part of the telencephalic neural tube consists of a plexiform zone and a layer of neuroblasts adjacent to the lateral ventricle known as the matrix zone.

2. noradrenergic axons arrive in the plexiform zone and induce a wave of neuroblast migration into the plexiform zone.

3. successive waves of neuroblasts follow during subsequent days always assuming a position closer to the pia than earlier waves.

4.each waveforms a cortical layer and five waves of migration occur during cortical neurogenesis

5. This inside-out process means that the first wave becomes layer VI when the cortex is formed and Layer II (closest to the pia) is the last wave of migrating neuroblasts.

6. Layer I (plexiform layer) is the oldest layer of adult cortex since it was there in the beginning and shrunk in width as the cortical cell layers formed within it.

7. Neuroblasts left in the matrix zone form the ependymal lining of the lateral ventricle.

Chronological age
Layer I: a cellular-oldest- it shrinks as cellular layers form.

Layer II the cellular- the 5th wave of migration

Layer III the cellular-4th wave of migration

Layer IV the cellular- the 3rd wave of migration

Layer V the cellular- 2nd wave of migration

Layer VI the cellular- 1st wave of migration

The order in which cortical (cortex) regions develop corresponds to the order in which various functions emerge in the infant.
For example.

A burst of synaptic growth occurs in the auditory and visual cortex areas in the 1st year. This is important because the 1st year is when infants make dramatic gains in auditory and visual perception and mastery of motor skills.

Though the production and migration of neurons are largely prenatal events, proliferation and migration of glial progenitors continue for an extended period after birth, and the differentiation and maturation of these cells continue throughout childhood. The full scope of neuron-glia interactions is still not fully defined, but it is clear that these interactions play an important role in the functional organization of neural circuits during postnatal life. Importantly, estimates of the developmental time course in humans of the postnatal processes outlined below are derived by extrapolation from data acquired in other species, often rodents, and from very limited human postmortem material. Unfortunately, the result is much remaining uncertainty about the temporal extent of proliferation, migration, differentiation, and regression during the postnatal period in humans, and about the timing of these processes relative to each other. In vivo brain imaging of children is providing important clues about the time course of age-related biological alterations in the brain, and provides an opportunity to link these changes to evolving behaviour.

Postnatal Proliferation and Migration

In the postnatal period, neurogenesis continues to only a very limited degree; however, in the subventricular zone, new neurons continue to emerge and migrate to the olfactory bulb, and neurons are also produced in the dentate gyrus of the hippocampus, where they migrate from the subgranular layer only as far as the nearby granular layer. These exceptional forms of neurogenesis appear to continue throughout adult life but produce only a small percentage of the neuronal population. In contrast, proliferation and migration of glial progenitors, while beginning prenatally, continue for a protracted period as oligodendrocytes and astrocytes differentiate; in fact, glial progenitors (particularly oligodendrocyte progenitor cells, or OPCs) appear to persist indefinitely in the adult brain in a wide anatomical distribution, and can differentiate in response to injury. Glial progenitors proliferate in the forebrain subventricular zone and migrate outward into the overlying white matter and cortex, striatum, and hippocampus, where they differentiate into oligodendrocytes and astrocytes. Unlike neural progenitors, glial progenitors continue to proliferate as they migrate.

Anyhow, proliferation and migration of glial precursors and differentiation of astrocytes and oligodendrocytes are largely postnatal processes. While there is little doubt that these processes play a critical role in the functional maturation of developing neural circuits, the full scope of their impact on neural dynamics may be much greater than was previously appreciated.