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BMP/dpp was critical in defining the neurogenic
region of the embryo. This section describes some of
the genes important in the next stage of nervous
system development, the formation of neuroblasts in
these neurogenic regions. Once again, the mechanisms
are conserved in vertebrates and invertebrates, and so
these mechanisms are very ancient ones. The produc-
tion of neural precursor cells will be described first in
Drosophila , where the mechanisms are best under-
stood. Though the same mechanisms also appear to
regulate this process in vertebrates, less is known
about this class.
In Drosophila, as described in the previous section,
the neural precursors or neuroblasts form by a process
that starts with their delamination: certain cells within
the neurogenic region enlarge and begin to move
to the inside of the embryo. Next, each neuroblast
divides to generate many progeny, known as ganglion
mother cells (GMCs). Each GMC then generates a pair of
neurons or glia (Figure 1.8). The neuroblasts form from
the neurogenic ectoderm in a highly stereotyped array,
and each neuroblast can be assigned a unique identity
based on its position in the array, the expression of a
particular pattern of genes, and the particular set of
neurons and glia that it generates (Doe, 1992). The first
neuroblasts to form are arranged in four rows along the
anterior-posterior axis and in three columns along the
medio-lateral axis (Figure 1.25). The types of neurons
and glia generated by a particular neuroblast depend
on its position in the array, and so each neuroblast
is said to have a unique identity. The next waves of
neuroblasts to form are also organized in rows
and columns, adjacent to the preceding waves of
neuroblasts. The genes involved in controlling the
identity of several of the neuroblasts have been defined
and will be discussed in Chapter 4; however, at this
point the mechanisms that control the segregation of
the neuroblasts from the ectoderm will be described.
Among the molecules that are intimately involved
in the segregation of the neuroblasts from the other epi-
dermal cells are the members of the achaete scute gene
complex. The achaete scute genes were identified for
their effects on the development of the bristles, or
chaete, on the fly, each of which contains a sensory
FIGURE 1.25 Neuroblast segregation in the Drosophila neurogenic region proceeds in a highly patterned
array. A. In this embryo stained with an antibody against achaete-scute (as-c) protein, clusters of proneural
cells in the ectoderm express the gene prior to delamination. B. A single neuroblast develops from each cluster
and continues to express the gene. The other proneural cells downregulate the as-c gene. (From Doe, 1992)
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