Chapter 7
Recent estimates suggest that there are about 100 billion neurons in the human brain. Neurons
each develop thousands of synapses, and in the end the brain contains a staggering 100 trillion
synaptic connections!
The fertilized egg, or zygote , contains complete genetic blueprints
for the new person in its 46 chromosomes. A period of rapid cell
division follows, and by the end of a week, the cell mass, now known
as a(n) embryo , shows three distinct cell layers. The nervous
system will develop from the ectoderm .
By the end of the 8th week of embryonic life, rudiments of all major
organ systems are apparent, and the head is about 50 % of the total
mass of the embryo. After 10 weeks, the embryo is called a(n) fetus .
The production of nerve cells is called neurogenesis . Although
neurons themselves do not divide, the cells that become neurons
(reproduce) through a process called mitosis and form a closely
packed zone of cells called the ventricular zone
Cells in the ventricular layer of the neural tube undergo division, or
mitosis, giving rise to daughter cells that will eventually leave this
layer and go on to form neurons and glial cells. Neurons themselves
do not divide, and very few are added after birth
In contrast to the situation with invertebrates such as C. elegans,
vertebrate neuronal development is regulated by cell – cell
interactions
The increasing weight of the brain during postnatal development has
traditionally been attributed to increases in the size of neurons, the
branching of dendrites , the birth of glial cells, and the wrapping of
myelin around axons. However, we now know that neurogenesis also
occurs in humans through adulthood . Before becoming neurons proper, newly born nerve cells move to
new positions. Cells tend to move away from the ventricular layer
where they were born, creeping along spokes formed by radial glial
cells. Alternatively, some neurons move so that they are
perpendicular to these glial cells, while still others form a rostral
stream to supply the olfactory bulbs .
Molecules making up one important specialized family of molecules
involved in the guidance of migrating cells and growing cell processes
are the cell adhesion molecules (CAMs). In the adult, these
molecules may also guide regenerating axons after trauma.
Newly born cells, having migrated to their new positions, do not
resemble neurons until they begin to express the genes that code for
the specialized proteins that make up a neuron. This process is
referred to as cell differentiation
The process of induction, whereby some spinal cord cells become
motoneurons, is a consequence of the release of a protein
messenger called Sonic hedgehog from a structure called the
notochord .
Growth cones at the tips of axons and dendrites give rise to two kinds
of outgrowths—the threadlike filopodium and the sheetlike
lamellipodium . These outgrowths adhere t
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