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Te lencephalon
Alar plate
Alar plate
Basal plate
Alar plate
Basal plate
Basal plate
Hensen's node
Primitive streak
FIGURE 2.18 The fate maps for amphibian, avian, and mammalian neural tubes. The basic forebrain
regions are common to all vertebrates; however, the basic pattern has been elaborated upon to generate the
wide diversity of brains that are found in vertebrates. The rhombomeric and prosomeric organization of the
mouse brain can already be recognized at this early stage by the pattern of expression of certain genes.
understanding the moments in development when
molecular mechanisms are actively directing a specific
region of the neural tube to its specific fate (i.e., is
“specified”). To determine at what point in develop-
ment this “specification” occurs, pieces of the neural
plate are transplanted to ectopic locations in the
embryo. If the transplanted cells give rise to a partic-
ular brain region, we say that it has already been
specified. For example, a piece of the anterior neural
plate, near the eye, is transplanted to the presumptive
flank of another embryo. After sufficient developmen-
tal time has passed, the embryos are analyzed for the
type of neural tissue that developed from the graft. In
this case the finding is that, as early as late gastrula, a
particular region of the neural plate will always give
rise to anterior brain, including the eye. This occurs
regardless of where the tissue is placed in the host
animal. A number of embryologists carried out these
types of experiments using various regions of the
neural plate as the donor tissue, and the results con-
sistently demonstrate that at some point in develop-
ment, the cells of the neural plate take on a regional
identity that cannot be changed by transplantation to
some other place in the embryo. The fact that different
regions of the neural plate are already committed to a
particular fate has been extended in recent years by the
observations that a number of genes are expressed in
highly specific regions of the developing nervous
system. In many cases the domain of expression of a
particular transcription factor corresponds to that
region of the neural tube that will ultimately give rise
to one of the five brain vesicles, and the gene may con-
tinue to be expressed in that brain region throughout
its development.
Many embryologists have taken advantage of the
patterns in gene expression in the forebrain to gain
insight into the basis of its organization. In what has
become known as the prosomeric model of forebrain
development, it is proposed that there are longitudinal
and transverse patterns of gene expression that subdi-
vide the neural tube into a grid of different regional
identities (Puelles and Rubenstein, 1993). The expres-
sion of some of these genes is shown for the mouse
embryo at two different stages of development (Figure
2.19). In many cases, the boundary of expression of a
particular gene corresponds closely to the morpholog-
ical distinctions between the prosomeres. For example,
two genes of the emx class are expressed in the telen-
cephalon, one in the anterior half of the cerebral hemi-
spheres ( emx1 ) and the other in the posterior half of
the hemispheres ( emx2 ). Thus, the telencephalic lobes
can be divided into anterior and posterior segments on
the basis of the pattern of expression of these two
genes. Analysis of the expression patterns of addi-
tional genes has led to the conclusion that the prosen-
cephalon can be subdivided into six prosomeres
(Figure 2.19). They are numbered from caudal to
rostral, and so prosomere 1 is adjacent to the mesen-
cephalon. P2 and P3 subdivide what is traditionally
known as the diencephalon, and P4, P5, and P6 sub-
divide the telencephalon.
While the studies of regional expression of tran-
scription factors present a model of brain organization
and evolution, the functional analyses of homeo-
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