Biology Reference
In-Depth Information
create a series of small steps of expansion in random directions. This is equivalent to a stat-
istician's 'drunken walk'; the distance from the origin that is travelled by a drunkard who has
made n steps, each of unit length but in a random direction, is on average not zero but
n.
Second, Hertwig's rule would mean that once one cell divides and its daughters begin to
grow, their neighbours will be stretched along the direction of the first cell's division. They
would therefore divide the same way, and by this positive feedback the tissue would be
expected to be subject to runaway distortions. Embryos probably defend themselves from
this by three methods: their cells can rearrange to relieve tissue stresses, their tissues are
really bounded by other tissues so that growth is not unconstrained, and a combination of
competition for mitogens and contact inhibition of proliferation will prevent too many
mitoses occurring in the same place.
In case this section of the topic leaves a false impression, it must be stressed that many
morphogenetic events of animal development take place without orientated cell division
and, indeed, without cell division at all. Even where mitoses do seem to be orientated consis-
tently, their orientation may not actually be helpful to morphogenesis and cell movement
may be required to abrogate the deleterious morphological effect that the orientation of
mitoses would otherwise have. An example of this is seen in the cnidarian Hydra, in which
the directions of mitoses in the two layers of the body wall are different and would be
expected to generate an awful mess if cell rearrangement were not able to sort everything
out.
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It should be borne in mind, therefore, that orientated cell division is just one of
many morphogenetic mechanisms available to animals and that, even if present, it may
work with or against other processes that are taking place.
O
ORIENTATED CELL DIVISION IN PLANTS
Orientated cell division is also seen in plants. Indeed, it is particularly important for organ-
isms inwhich cell rearrangement is impossible. From the earliest stages of plant development,
the form of the embryo emerges directly from the pattern of orientated cell divisions that
creates the characteristic dumbbell-shaped, octant and suspensor morphology (
Figure 23.14
).
Orientated cell division is also critical to the setting up of the characteristic anatomy of
growing roots and shoots, a fact that can be demonstrated by mutants of Arabidopsis thaliana.
Roots grow by the addition of cells that derive from a population of 'initials' in the meristem
just under the root cap; initials have the characteristics that would earn them the name 'stem
cells' in animals, but that phrase is not used in botany because it would be an obvious source
of confusion. There are four sets of initials,
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each of which produces cells that give rise to
a different concentric layer of the root: one set gives rise to pericycle and vascular tissue,
one to columnella, one to the cortex/endodermis endoderm and the other to the epidermis.
The cortex and endodermis derive from the same daughter of an initial cell; this daughter
divides to produce one cell destined to be cortex and one destined to be endoderm. This divi-
sion is periclinal so that it produces one outer cell, destined to contribute to cortex, and one
inner cell destined to contribute to the endodermis (the nomenclature of division planes is
illustrated in
Figure 23.15
). Each of these cells then undergoes a series of anticlinal divisions
to produce a series of progenitor cells that occupy a one-cell-thick arc of the root tip. These
then go on to divide and elongate to produce the root.
