Chapter 23, End of Chapter, Review Questions, Exercise 1

During the genesis of neurons, neuronal structures develop by cell proliferation, migration, and differentiation. The neural progenitor cells form neurons and astrocytes in the layers of the telencephalic vesicles, which will form the cerebral cortex. The cell proliferation begins when the cells of the ventricular zone extend its processes through the marginal zone to the pia mater. The nucleus of the cell then travels into the marginal zone along the process while synthesizing deoxyribonucleic acid (DNA). The cell then retracts its extension and divides. The neurons reach the final destination by climbing along special types of glial cells called the radial glial cells during migration. 

Many of the daughter cells migrate large distances along spanning the ventricular zone and the pia mater. The neuroblasts migrate from the ventricular zone to the cortical plate. Neural precursor cells (immature neurons) move along the radial path covering the ventricular zone toward the brain surface. Some cells cross the cortex horizontally. The precursor cells that form the subplate cells of the cortex migrate early on during genesis, followed by cells that become the adult cortex. The latter cells cross the subplate, forming another layer called the cortical plate. The cells that arrive at the cortical plate first form the layer VI neurons, followed by layer V, succeeded by layer IV neurons and so forth. The new precursor cells are layered on top of the existing cortical plate, hence the cortical assembly is referred to as inside out. 

Once the neural migration is complete, the cells take on specific appearance and characteristics based on the gene expression. Neural differentiation begins when the precursor cells divide with uneven distribution of cell constituents. However, this differentiation continues even when the neural precursor arrives at the cortical plate. The layer V, VI differentiate into pyramidal cells even before layer II cells have migrated. In general, the neuronal differentiation succeeds astrocyte and oligodendrocyte differentiation.