Neural System - Fetal

Introduction

Timeline of events in Human Neural Development

Human brain at three months (median sagittal section)

Human brain at four months (inferior surface)

Human brain at five months (outer surface)

During the fetal period there is ongoing growth in size, weight and surface area of the brain and spinal cord. Microscopically there is ongoing: cell migration, extension of processes, cell death and glial cell development.

Cortical maturation (sulcation and gyration) and vascularization of the lateral surface of the brain starts with the insular cortex (insula, insulary cortex or insular lobe) region during the fetal period. This cerebral cortex region in the adult brain lies deep within the lateral sulcus between the temporal lobe and the parietal lobe.

• sulcation - The process of brain growth in the second to third trimester which forms sulci, grooves or folds visible on fetal brain surface as gyri grow (gyration). Abnormalities of these processes can lead to a smooth brain (lissencephaly).
• gyration - The development of surface folds on the brain (singular, gyrus)

Insular Gyral and Sulcal Development

• 13-17 gestational weeks - appearance of the first sulcus
• 18-19 gestational weeks - development of the periinsular sulci
• 20-22 gestational weeks - central sulci and opercularization of the insula
• 24-26 gestational weeks - covering of the posterior insula
• 27-28 gestational weeks - closure of the laeteral sulcus (Sylvian fissure or lateral fissure)

(Data from: Afif A, etal., 2007)

Three-dimensional magnetic resonance imaging and image-processing algorithms have been used to quantitate between 29-41 weeks volumes of: total brain, cerebral gray matter, unmyelinated white matter, myelinated, and cerebrospinal fluid (grey matter- mainly neuronal cell bodies; white matter- mainly neural processes and glia). A study of 78 premature and mature newborns showed that total brain tissue volume increased linearly over this period at a rate of 22 ml/week. Total grey matter also showed a linear increase in relative intracranial volume of approximately 1.4% or 15 ml/week. The rapid increase in total grey matter is mainly due to a fourfold increase in cortical grey matter. Quantification of extracerebral and intraventricular CSF was found to change only minimally.

(Text - modified from Huppi etal., (1998) Quantitative magnetic resonance imaging of brain development in premature and mature newborns. Ann Neurol 43(2):224-235.)

Neural development will continue after birth with substantial growth, death and reorganization occuring during the postnatal period.

Links: Neuroscience - Regional specification of the developing brain