Chapter 14: Brain Growth & Neural Development

Brain Growth & Neural Development overview explores the intricate biological mechanisms governing the growth and organization of the mammalian central nervous system, with a particular focus on the evolutionary milestones of the human brain. The process begins with the differentiation of the neural tube into three fundamental zones: the ventricular, mantle, and marginal layers, which eventually form the functional regions of the brain and spinal cord. Multipotent neuroepithelial cells serve as the foundational stem cells, transitioning into radial glia that function as both primary progenitors and structural scaffolds for migrating neurons. These cells undergo complex behaviors such as interkinetic nuclear migration, where the nucleus shifts position during the cell cycle to facilitate division at the apical surface. The chapter distinguishes between various cell types, including neurons that transmit electrical signals via axons and dendrites, and macroglia like oligodendrocytes and astroglia that support and insulate neural pathways. A critical aspect of cortical development is the "inside-out" migration pattern, where later-born neurons travel past their predecessors to form the superficial layers of the six-layered neocortex. This migration is regulated by molecular signals such as Reelin, which coordinates cell adhesion through N-cadherin and integrins. The decision for a stem cell to remain a progenitor or differentiate is heavily influenced by the plane of cell division and the asymmetrical inheritance of factors like Par-3 and Notch signaling. Human brain development is uniquely characterized by gyrencephaly—the folding of the cerebral cortex—which is driven by the expansion of outer radial glia and human-specific genetic regulators like ARHGAP11B and the HAR1 noncoding RNA. Furthermore, humans retain high fetal neuronal growth rates well into early childhood, a process known as hypermorphosis. The maturation process continues through adolescence, involving significant synaptic pruning and increased myelination in the frontal cortex, which refines cognitive and emotional processing.