FoundationalConceptsNeuroscience

After conception, the fertilized egg begins to divide and form multiple
cells—embryonic stem cells—with the capacity to continue dividing
and to differentiate into any type of cell in the body. Embryonic stem
cells in the developing nervous system differentiate into neural pro-
genitor cells, which are on track to become various types of neurons or
glia. The formation of nerve cells and glial cells from neural progeni-
tor cells is called neurogenesis and gliogenesis.
Occurring in tandem with neurogenesis and gliogenesis is cell
migration—as they differentiate, cells move around to occupy specific
locations. Developing neurons grow out their dendrites and axons
and differentiate so as to use one or another particular molecule for
neurotransmitter signaling. As the neurons mature they also begin to
wire together, forming synapses, a process called synaptogenesis. And
thus the remarkable symphony that is neuronal development: neuro-
genesis, gliogenesis, migration, and synaptogenesis—all happening
for billions of cells in the growing brain.
Within three weeks of conception in a developing human embryo,
a group of cells begins to fold and form a structure called the neural
tube (Fig. 10.2). The entire central nervous system will develop from
this folded piece of tissue as it grows and differentiates.
Human conception to human birth requires approximately nine
months of development within the mother’s womb. By the third
month after conception brain growth really takes off. Convolutions of
the cerebral cortex, the gyri and sulci, begin to form during the final
two months before birth, reflecting rapid expansion of the density
and connectivity of cells in the cerebrum. The expanding volume of
the brain is due to the proliferation of neurons and glia from dividing
stem cells, as well as the growth and branching of dendrites and axons
and the formation of trillions of synaptic connections between cells.