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Genesis and Migration
The human brain is made up of an enormous
number of neurons and glial cells. The sources of all
these neurons and glia are the cells of the neural tube,
described in the previous chapters. Neurogenesis and
gliogenesis, the generation of neurons and glia during
development, occurs in many different ways in the
various regions of the embryo. Part of the complex
process of making the brain includes the proper migra-
tion of neurons and glia from their site of origin to their
final position in the adult brain. Precisely orchestrated
cell movements or migrations are an integral part of
what is collectively known as histogenesis in the brain.
This chapter describes the cellular and molecular prin-
ciples by which the appropriate numbers of neurons
and glia are generated from the neural precursors, and
gives an overview of some of the complex cellular
migration processes involved in the construction of
the brain.
The number of cells generated in the developing
nervous system is likely regulated at several levels. In
some cases, the production of neurons or glia may be
regulated by apparent intrinsic limits to the number of
progenitor cell divisions essentially, a “cellular clock.”
The level of proliferation and ultimately the number of
cells generated can also be controlled by extracellular
signals, acting as mitogens, promoting progenitor cells
to reenter the cell cycle and mitotic inhibitors that
induce progenitor cells to exit from the cell cycle.
However, it must also be remembered that the number
of neurons and glia in the mature nervous system is a
function not only of cell proliferation, but also of cell
death. There are many examples of neuronal overpro-
duction and subsequent attrition through pro-
grammed cell death; this process will be described in
Chapter 7.
In the many small invertebrates, like the nematode,
the lineages of the cells directly predict their numbers.
Since most of the mitotic divisions are asymmetric,
the final number of cells that are produced during
embryogenesis depends on the particular pattern of
cell divisions and the number of cells that die through
programmed cell death. The regulation of these divi-
sions does not appear to depend on interactions with
surrounding cells, but rather it is an intrinsic property
of the lineage. The lineage of these cells also predicts
the particular types of neurons that are generated from
a particular precursor, and it appears that the infor-
mation to define a given type of cell resides largely in
factors derived directly from the precursors.
In the Drosophila central nervous system, neuronal
number is also highly stereotypic. The neuroblasts of
the insect CNS delaminate from the ventral-lateral
ectoderm neurogenic region in successive waves (see
Chapter 1). In Drosophila , about 25 neuroblasts delam-
inate in each segment, and they are organized in four
columns and six rows. The pattern is basically the
same for other insects and other arthropods, but the
number of neuroblasts is dependent on the species
(Doe and Smouse, 1990). Once the neuroblast segre-
gates from the ectoderm, it then undergoes several
asymmetric divisions, giving rise to approximately
five smaller ganglion mother cells. Each ganglion
mother cell then divides to generate a pair of neurons.
These neurons make up the segmental ganglia of the
ventral nerve cord and have stereotypic numbers and
types of neurons.
In the vertebrate, the situation gets considerably
more complex. The neural tube of most vertebrates is
initially a single layer thick. As neurogenesis proceeds,
the progenitor cells undergo a considerable number of
cell divisions to produce a much thicker tube, with
several layers. A section through the developing spinal
cord is shown in Figure 3.1, and this basic structure is
present throughout the developing central nervous
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