Biology Reference
In-Depth Information
CHAPTER
4
Morphogenesis of Individual Cells:
A Brief Overview
Most examples of morphogenesis at the levels of tissue, organ and organism emerge
from the morphogenesis of cells that constitute those higher-level structures.
)
An under-
standing of the mechanisms that control and change the morphologies of individual cells
is therefore a pre-requisite to a deep understanding of larger-scale morphogenesis. Also,
some of the most spectacular examples of morphogenesis that occur during development
and adult life take place not at the scale of tissues and organs but rather at the scale of indi-
vidual cells.
The static shapes of cells vary from simple spheres to cubes, rods, disks, branched trees
and even hollow tubes. In some cases, a specific cell shape is dictated mainly by the
physiological function of the cell: in other cases, changes of cell shape are required to
change the shape of a developing tissue. The purpose of this brief chapter is to give an
overview of the types and developmental roles of morphogenetic changes in individual
cells; readers already familiar with the histology of both animals and plants should feel
free to skip it. Chapters 5 and 6 will delve more deeply into the molecular mechanisms
involved.
FLATTENING AND ELONGATION OF CELLS
Drawings of a 'typical' animal cell, of the kind that appear near the beginning of most
biology textbooks,
1
e
4
usually depict a spherical or cubic entity, all axes of which are
equal. Many cells really are like this; for example, lymphocytes in peripheral mammalian
blood are approximately spherical
5
and epithelial cells in the distal tubule of the
mammalian kidney are approximately cuboidal
1
(
Figure 4.1
a). A change in the ratio of
length to width is a simple and common example of change of cell shape. For example,
epithelial cells may flatten considerably in their apico-basal axis (
Figure 4.1
.b); this flat-
tening is often seen where substances have to travel across the epithelium, as in the
alveoli of the lung, and is presumably an adaptation to minimize the diffusion path.
)
The main exceptions are those driven by accumulation of fluid or extracellular matrix; cell-level behaviour
is still important but the cells involved may not change their own shapes.
