Biology Reference
In-Depth Information
CHAPTER
23
Morphogenesis by Orientated
Cell Division
Mitotic division can do more than simply expand the cell number in, and the volume of,
a tissue. When cleavage planes have a specific orientation, mitosis can be used to expand
a tissue in a particular direction and thus to create new form. The very earliest stages of
animal development tend to be where the importance of accurately orientated cell division
is most obvious. Following fertilization, most animals enter a phase of cleavage in which
the large, unicellular zygote is divided into increasing numbers of smaller cells with no
change in overall volume of the embryo. In some phyla, the precise spatial pattern of
cleavage seems to be important in setting up the body plan. Most animals use one of two
basic patterns of cleavage: radial and spiral (
Figure 23.1
).
In radial cleavage, mitotic spindles are aligned precisely along some embryo axes and
perpendicular to others, so that cleavage planes are always parallel to, or transverse to, the
meridian plane. In spiral cleavage, mitotic spindles are at an oblique angle to the animal-
vegetal axis and also oblique to the meridian plane, and they alternate between being
oblique-left and oblique-right: the result is a spiral arrangement of cells. In at least some
animals with spiral cleavage, whether the alternation begins left or begins right is critical
to the body plan; the third cleavage division is right-handed in most snails and the spiral
in the shell of the adult animal is also right-handed. If the cleavage of the third division is
left-handed, so is the adult body. This correlation is shown most dramatically by the snail
Lymnaea peregra, which makes either right-handed or left-handed shells under the control
of a single gene. The right-handed allele is dominant and cytoplasm from right-handed
eggs can convert left-handed ones to right-handed cleavage and adult body plan. This
strongly suggests a causal relationship between the direction of spiral cleavage in the early
embryo and the chirality of the adult body,
1
but unless it can be shown that the product of
the right-promoting allele affects only the orientation of cell division, the observations cannot
prove causality. The cleavage planes of radially cleaving embryos may be less important;
experimental manipulations of the radially cleaving embryo of the frog Xenopus laevis can
alter the primary cleavage plane without altering the future axis of the embryo.
2
Unfortunately, very little is known about how orientation is controlled during embryonic
cleavage divisions. Some theoretical work has been done on models in which furrows are
placed to minimize free energy in the system.
3,4
Some produce realistic arrangements of cells
