Biology Reference
In-Depth Information
the mesenchyme derived from the other species, take up cells from that other species.
38
These
cells appear to become fully epithelial. Whether this mode of development takes place in the
nephric ducts of other species is currently a matter of debate.
THE FORCES THAT SHAPE AN EPITHELIUM
Epithelia are shaped by a combination of the forces they generate internally and the
various pressures that derive from the rest of the embryo and its environment. The attach-
ment of the cytoskeleton to the cell-cell junctions of epithelia has the consequence that forces
generated by, or experienced in, one cell are communicated to the rest of the epithelium and
the tissue therefore behaves as a mechanically integrated whole. The actin-myosin stress
fibres attached to cell-cell and cell-matrix junctions are, like all stress fibres, in tension (see
Chapter 8). The net effect of the stress fibres and junctions of all of the cells is to impart
a tension to the complete epithelial sheet that is analogous to the surface tension of soap
bubbles. Except when some cells of the epithelium behave differently to others to generate
specific additional forces
d
for example, by the cell wedging which will be described in
Chapter 18
d
the general shape of an epithelium might therefore be expected to follow the
same rules as soap bubbles, whose behaviour can be explained completely by surface
tension.
Surfaces under tension take up forms that minimize their surface area (Chapter 5). If a one-
dimensional object, such as an idealized length of wire, is stretched over a curve of radius
R and is under tension T (
Figure 15.5
), the inwards pressure per unit length that it exerts
is given by:
T
R
Similarly, if a two-dimensional surface is stretched over a two-dimensional surface of radii
R
1
and R
2
:
refs39,40
r
¼
T
R
T
R
r
¼
1
þ
2
In equilibrium, this inwards pressure must equal whatever outwards pressure is present
in the system
d
for example, that due to fluid contained inside an epithelium. If the surface is
the same everywhere (no special zones of cell wedging, and so on), then T will be the same
FIGURE 15.5
When a frictionless wire is stretched with tension T over a curve with radius R, it exerts a pressure
per unit length
r
¼
T/R.
