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1995), although much more study of the Cnidarians is
necessary to determine the degree to which the mech-
anisms for neurogenesis are common to all multicellu-
lar animals.
P 0
P 0
P 1
AB
AB
P 1
C. elegans
The development of C. elegans , a nematode worm,
highlights the shared lineage of the epidermal and
neural cell fates. These animals have been studied pri-
marily because of their simple structure (containing
only about a thousand cells), their rapid generation
time (allowing for rapid screening of new genetic
mutants), and their transparency (enabling lineage
relationships of the cells to be established). These
nematodes have a rigid cuticle that is made of col-
lagenous proteins secreted by the underlying cells of
the hypodermis. The hypodermis is analogous to the
epidermis of other animals, except that it is composed
of a syncytium of nuclei rather than of individual cells.
They have a simple nervous system, composed of only
302 neurons and 56 glial cells. These neurons are
organized into nerve cords instead of the nerve net
of the jellyfish. The nerve cords are primarily in the
dorsal and ventral sides of the animals, but there are
some neurons that run along the lateral sides of the
animal as well. The nematodes move by a series of
longitudinal muscles, and they have a simple digestive
system.
Nematodes have long been a subject for develop-
mental biologists' attention. Theodore Boveri studied
nematode embryology and first described the highly
reproducible pattern of cell divisions in these animals
in the late 1800s. Boveri's most famous student, Hans
Spemann, whose work on amphibian neural induction
will be described below, worked on nematodes for his
Ph.D. research. The modern interest in nematodes,
however, was motivated by Sydney Brenner, a molec-
ular biologist who was searching for an animal that
would allow the techniques of molecular genetics to be
applied to the development of metazoans (Brenner,
1974).
Because of the stereotypy in the pattern of cell divi-
sions, the lineage relationships of all the cells of C.
elegans have been determined (Sulston et al., 1983). The
first cleavage produces a large somatic cell, the AB
blastomere, which gives rise to most of the hypoder-
mis and the nervous system and the smaller germline
P cell, which in addition to the gonads will also gen-
erate the gut and most of the muscles of the animal
(Figure 1.4). Subsequent cleavages produce the germ
cell precursor, P4, and the precursor's cells for the rest
of the animal: the MS, E, C, and D blastomeres (Figure
1.4), and these cells all migrate into the interior of the
AB p
AB p
LR
EMS
AB a
LR
P 2
AB a
P 2
EMS
AB
MS
E
C
P 3
AB
AB
C
AB
MS
P 3
E
D
P 4
Hypodermis and
nervous system
Body, muscles, gut,
somatic gonad
C
E
E
D
Germ
line
P 4
MS MS
FIGURE 1.4 The nervous system shares a common cellular
lineage with the ectoderm. The cell divisions that generate the C.
elegans nematode worm are highly reproducible from animal to
animal. The first division produces the AB blastomere and the P1
blastomere. The germ line is segregated into the P4 blastomere
within a few divisions after fertilization. The subsequent divisions
of the AB blastomere go on to give rise to most of the neurons of
the animal, as well as to the cells that produce the hypodermis—the
epidermis of the animal.
embryo, while the AB-derived cells spread out over
the outside of the embryo completing gastrulation
(Figure 1.5). The next phase of development is charac-
terized by many cell divisions and is known as the
proliferation phase . Then an indentation forms at the
ventral side of the animal marking the beginning of
the morphogenesis stage, and as this indentation pro-
gresses, the worm begins to take shape (Figure 1.5). At
this point, the worm has only 556 cells and will add
the remaining cells (to the total of 959) over the four
larval molts. The entire development of the animal
takes about two days.
The neurons of C. elegans arise primarily from the
AB blastomere, in lineages shared with the ectoder-
mally derived hypodermis. An example of one of these
lineages is shown in Figure 1.5. The Abarpa blastomere
can be readily identified in the 100-min embryo
through its position and lineal history. This cell then
goes on to give rise to 20 additional cells, including 9
neurons of the ring ganglion. The progeny of the
Abarpa blastomere, like most of the progeny of the AB
lineage, lie primarily on the surface of the embryo
prior to 200 min of development. At this time, the cells
on the ventral and lateral sides of the embryo move
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