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vided into finer and finer regions over time. The drive
toward specialization seems to be fundamental to
biology at all levels, from cells, tissues, organisms,
and biological communities, and the cerebral cortex,
arguably the basis for human preeminence, is no
molecules are critically involved in setting up these
patterns. These diffusible signaling molecules act to
restrict the expression of specific transcription factors,
which go on to regulate the expression of downstream
target genes specific for the regional identity of part of
the nervous system. One particularly well-conserved
class of transcription factors, the Hox genes, is impor-
tant in establishing and maintaining the regional iden-
tity of cells and tissues along the anterior-posterior axis
of vertebrates throughout the hindbrain and likely the
spinal cord. This conceptual framework holds true for
vertebrates and invertebrates, and indeed, many of the
molecular systems for generating specific parts of
the nervous system have been highly conserved over
the millions of years of evolution and considerable
morphological diversity of animals.
The understanding of how the basic pattern of the
nervous system is established has been put on a solid
molecular ground in the past decade. One of the basic
principles that has emerged from this work is that
graded concentrations of antagonizing diffusible
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