Healthcare and Medicine Reference
In-Depth Information
CHAPTER
1
Neural Induction
DEVELOPMENT AND
EVOLUTION OF NEURONS
system will give us insight into its evolution. It is also
wise to remember, as Dobzhansky pointed out, that
“nothing in biology makes sense except in the light of
evolution.”
Almost as early as multicellular animals evolved,
neurons have been part of their tissues. Metazoan
nervous systems range in complexity from the simple
nerve net of the jellyfish to the billions of specifically
interconnected neuron assemblies of the human brain.
Nevertheless, the neurons and nervous systems of all
multicellular animals share many common features.
Voltage-gated ion channels are responsible for action
potentials in the neurons of hydras, as they are in
people. Synaptic transmission between neurons in
nerve nets is basically the same as that in the cerebral
cortex in humans (Figure 1.1). This topic describes the
mechanisms responsible for the generation of these
nervous systems, highlighting examples from a variety
of organisms. Despite the great diversity in the
nervous systems of various organisms, underlying
principles of neural development have been main-
tained throughout evolution.
It is appropriate to begin a book concerned with the
development of the nervous system with an evolu-
tionary perspective. The subjects of embryology and
evolution have long shared an interrelated intellectual
history. One of the major currents of late-nineteenth-
century biology was that a description of the stages of
development would provide the key to the path of
evolution of life. The phrase “ontogeny recapitulates
phylogeny” was important at the start of experimen-
tal embryology (Gould, 1970). Although the careful
study of embryos showed that they did not resemble
the adult forms of their ancestors, it is clear that new
forms are built upon the structures of biological pre-
decessors. One aim of this topic is to show how an
understanding of the development of the nervous
EA RLY EMBRYOLOGY OF METAZOA NS
The development of multicellular organisms varies
substantially across phyla; nevertheless, there are
some common features. The cells of all metazoans
are organized as layers. These layers give rise to the
various organs and tissues, including the nervous
system. These layers are generated from the egg cell
through a series of cell divisions and their subsequent
rearrangements (Figure 1.2). The egg cells of animals
are typically polarized, with an “animal pole” and a
“vegetal pole.” This polarity is often visible in the egg
cell, since the vegetal pole contains the yolk, the stored
nutrient material necessary for sustaining the embryo
as it develops. Once fertilized by the sperm, the egg
cell undergoes a series of rapid cell divisions, known
as cleavages. There are many variations of cleavage
patterns in embryos, but the end result is that a large
collection of cells, the blastula, is generated over a
relatively short period of time. In many organisms the
cells of the blastula are arranged as a hollow ball, with
an inner cavity known as a blastocoel. Those cells at
the vegetal pole will ultimately develop as the gut,
whereas those at the animal pole will give rise to the
epidermis and the nervous system. Cells in between
the animal and vegetal poles will generate mesoder-
mal derivatives, including muscles and internal skele-
tal elements. The rearrangement of this collection
of cells into the primary (or germ) layers is called
1
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