Biology Reference
In-Depth Information
FIGURE 15.1
The dominance of epithelia in very simple animals, such as Trichoplax adherens, and in the early
stages of even complex animals such as ourselves.
magnitude larger than the area of the outer surface of the organ itself. To achieve a large area,
epithelia may be folded, for example in the mammalian large intestine, or they may form
highly branched tubes as they do in the lung. The histological appearance of most organs
is dominated by the epithelial structures that they contain; epithelial morphogenesis is there-
fore very important to the organogenesis as a whole.
The epithelial state is relatively easy to define in anatomical terms, implying a sheet of
closely apposed cells that have an innate apico-basal polarization across the plane of the
sheet. Defining the epithelial state in molecular terms is, however, more difficult as there
appear to be no genes that are expressed in epithelia but nowhere else.
5
There seem to be
two reasons for this. The first is that there is a great variety of different epithelial tissues,
at least 200 in an adult human,
6
each of which is specialized for different functions
and expresses different subsets of genes. The second is that 'typically epithelial' proteins
tend to be expressed in some non-epithelial sites as well and cannot therefore be used to
define epithelia; examples of such non-epithelial expression of 'epithelial' proteins include
E-cadherin in the CNS,
7
desmosomal proteins in cardiac muscle,
8
cytokeratin in smooth
muscle,
9
ZO1 in astrocytes,
10
6 integrin in lymphocytes,
11
and collagen IV in embryonic
a
lung mesenchyme.
12
It is perhaps fitting that the epithelial state is best defined in anatomical rather than molecu-
lar terms, for the intimate association between epithelial cells is so central to the character of
the tissue that an isolated epithelial cell is arguably not an epithelial cell at all. The communal
