Healthcare and Medicine Reference
In-Depth Information
The next three sections will deal with the embryol-
ogy of several examples of metazoan development:
Cnidarians (hydra); nematode worms ( Caenorhabditis
elegans ); insects ( Drosophila melanogaster ), and several
vertebrates (frogs, fish, birds, and mammals). The
development of these animals is described because
they have been particularly well studied for historical
and practical reasons. However, one should take
these examples as representative, not as definitive.
The necessity of studying many diverse species has
become critical to the understanding of the develop-
ment of any one species.
Te ntacles
FIGURE 1.3 The nervous system shares a common cellular
lineage with the ectoderm. In very simple animals, like the hydra,
the neurons are derived from a precursor in the epidermis, known
as the interstitial cell, which can generate both neurons and other
sensory cells.
The development of the nervous system begins with
the segregation of neural and glial cells from other
types of tissues. The many differences in gene expres-
sion between neurons and muscle tissue, for example,
arise through the progressive narrowing of the poten-
tial fates available to a blast cell during development.
The divergence of neural and glial cells from other
tissues can occur in many different ways and at many
different points in the development of an organism.
However, the cellular and molecular mechanisms
that are responsible for the divergence of the neural
and glial lineages from other tissues are remarkably
of tentacles to capture food. The nervous system of
hydras and jellyfish is composed of bipolar neurons
organized as a network. The neurons coordinate the
activity of the animal via voltage-gated channels,
action potentials, and chemical and electrical synaptic
transmission. Thus, the basic features of the nervous
system have been around for at least 600 million years,
and appear to have been present in animals ancestral
to all metazoans except sponges.
Given the many similarities neurons have had since
they arose in evolution, it is worthwhile to consider
how they develop in these most primitive animals.
Cnidarians can reproduce either asexually or sexually,
and most biologists are familiar with the asexual
budding of hydras (Figure 1.3A). A bud forms as an
evagination from a region of the body wall known as
the bud zone. The bud elongates over the next two
days and then separates from the parent organism. The
neurons, like all the cells of these animals, arise from
multipotent progenitor cells in the epidermal layer,
known as the interstitial cells. The interstitial cells are
a heterogeneous collection of true stem cells and pro-
genitor cells differentiating along various cell-specific
pathways. Are these interstitial cells similar to the pre-
cursors of the neurons in other phyla? Unfortunately,
although the development of the nervous system has
been described in some detail, little is known of the
cellular and molecular mechanisms that give rise to
neurons in these animals. Therefore, it is difficult to
make direct comparisons between the Cnidarians
and other metazoans. However, genes related to the
proneural genes (see below) of Drosophila and verte-
brates have been discovered in hydra (Grens et al.,
The first generalization that can be made concern-
ing neural segregation is that the nervous system is
derived from the ectodermal germ layer in all
triploblastic (three-germ-layered) organisms. Most
of the organisms we will discuss in this topic are
triploblastic; that is, they have three distinct primary
layers. However, neurons are present in more primi-
tive diploblastic (two-layered) organisms such as jel-
lyfish and hydras. The jellyfish and hydras are among
the organisms that belong to the cnidarian phylum.
These animals are among the most primitive multicel-
lular animals, with no defined organs and only a tissue
level of organization for the different cell types. The
freshwater hydra is one of the more well-studied
examples of the phylum (Figure 1.3A). Hydras have an
outer layer, the epidermis, and an inner epithelium, the
gastrodermis (Figure 1.3B), and between these layers
is an extracellular matrix similar in composition to the
basement membranes of other animals. The gastric
cavity has a single opening that serves as both a mouth
and an anus, and the hydra uses the surrounding ring
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