Myelinogenesis

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The first area of the cerebral cortex to myelinate - nissl stain parasaggital section of a 7 month old human fetus (Flechsig 1921)

Myelinogenesis is the process of sequential myelination or development of a myelin sheaths around a nerve fibres of the parts of the central nervous system. The order of development approximates the evolutionary development of the central nervous system.^{[citation needed]}

Paul Flechsig spent most of his career studying and publishing the details of the process in the cerebral cortex of the human. This takes place mostly between two months before and after birth. He identified 45 separate cortical areas and, in fact, mapped the cerebral cortex by the myelination pattern. The first cortical region to myelinate is in the motor cortex (part of Brodmann's area 4), the second is the olfactory cortex and the third is part of the somatosensory cortex (BA 3,1,2).

The last areas to myelinate are the anterior cingulate gyrus (F#43), the Inferior temporal lobe (F#44) and the Dorsolateral prefrontal cortex (F#45).]]

"In the cerebral convolutions, as in all other parts of the central nervous system, the nerve-fibres do not develop everywhere simultaneously, but step by step in a definite succession, this order of events being particularly maintained in regard to the appearance of the medullary sub- stance. In the convolutions of the cerebrum the investment with medullary substance (myelinisation) has already begun in some places three months before the maturity of the foetus, whilst in other places numerous fibres are devoid of medullary substance even three months after birth. The order of succession in the convolutions is governed by a law identical with the law which I have shown holds good for the spinal cord, the medulla oblongata, and the mesocephalon, and which may be stated somewhat in this way- that, speaking approximately, equally important nerve-fibres are developed simultaneously, but those of dissimilar importance are developed one after another in a succession defined by an imperative law (Fundamental Law of Myelogenesis). The formation of medullary substance is almost completed in certain convolutions at a time when in some it is not even begun and in others has made only slight progress..." (Flechsig 1901 Lancet)