Biology Reference
In-Depth Information
FIGURE 4.1
The flattening and elongation of epithelial cells. (a) Depicts epithelial cells of cuboidal form, for
example in the distal tubule of kidneys. (b) Depicts flattened epithelial cells, for example in the alveoli of the lung.
(c) Depicts elongated epithelial cells, for example in an embryonic placode (Chapter 18). The cross-hatched lines
depict a basement membrane.
Alternatively, cells can elongate in the basal-apical axis to become long and thin, as they
do in the placodes of vertebrate embryos
6
(
Figure 4.1
c).
PRODUCTION OF CELL PROCESSES
One of the most common types of cell morphogenesis is the formation of processes.
Reasons for this include increasing surface area, connecting with other cells, exploring the
environment and sensing vibration. The production of microvilli by epithelial cells of the
small intestine or of the renal proximal tubule is an example of process formation to increase
the surface area available for substance exchange (
Figure 4.2
a). Cell processes that are
produced for the purposes of connecting cells together can be temporary or permanent.
Temporary processes are often seen when cells are about to adhere (
Figure 4.2
b); the
processes from each cell push against the membrane of the other and their interdigitation
provides a large surface area for the establishment of adhesions (Chapter 17), although as
the adhesions mature, the processes disappear. A morphologically similar interdigitation
of processes
d
but one that is permanent
d
is used by glomerular podocytes to create the
main filtration element of mammalian kidneys; podocytes produce many processes that lie
parallel to those of other cells, and a fine filter develops between them where their
membranes fuse
7
(
Figure 4.2
c).
FIGURE 4.2
Examples of cell morphogenesis by the production of processes (part 1). (a) Depicts an absorptive
cell typical of gut and renal epithelia. (b) Depicts two epithelial cells meeting and forming adhesions, although the
scale of the processes involved has been exaggerated for clarity: in reality there are many more, much finer
processes. (c) Depicts the interdigitating processes of adjacent podocytes in the glomerulus of a mammalian kidney.
