Biology Reference
In-Depth Information
CHAPTER
15
The Epith
elial State: a Brief
Overview
Most of the cell types that were discussed in previous chapters can demonstrate their main
characteristics individually. Even if they use cooperation and swarm intelligence in a real
embryo, mesenchymal cells retain their chief physiological characteristics when isolated in
culture. Epithelia and endothelia are different. Their most important characteristic is that
they adhere to one another to build almost impermeable sheets that separate the inside of
the body from the outside, and separate different compartments of the body from each other.
Epithelia are of great importance to the metazoan body plan, and epitheliumwas probably
the first true tissue type to evolve. One of the simplest of all multicellular animal bodies is
provided by the phylum placozoa, which for a while was thought to consist of only one
species, Trichoplax adherens, but which is now known to contain considerable and confusing
diversity and broad geographical location.
1
T. adherens has a flat and very flexible body about
3 mm in diameter (
Figure 15.1
). It consists mainly of two discs of epithelium: an upper one
that consists of a single cell type and a lower one that consists of ciliated cells and glandular
cells. These discs are joined at their edges, where cells expressing the animal's Hox gene,
Trox2, may act as stem cells capable of becoming either upper-edge or lower-edge epithe-
lium.
2
Between the discs are contractile star cells that can change the shape of the whole
multicellular body in a manner reminiscent of unicellular amoebae. That, apparently, is
more or less it; there is no body axis, no nervous system and no internal organs, although
since there is evidence for sexual reproduction as well as simple fission of the body, there
must be some means for producing gametes.
3
Metazoan life is, at its simplest, almost
completely epithelial.
The primacy of epithelium in phylogenetic history is reflected in developmental history.
The embryos of even 'advanced' metazoa such as ourselves form an outer epithelial covering
before making any other type of specialized tissue (
Figure 15.1
). The stage of early embryo-
genesis at which the body consists of an epithelial 'cyst', perhaps with some cells inside it, is
called the 'blastula' and is universal in metazoan development; indeed, passing through
a blastula stage is, according to some definitions, the essential qualification for membership
of the animal kingdom.
4
Although both evolutionary and developmental progress add many other cell types to the
body, the internal anatomy of 'higher' animals is still dominated by epithelial tissues. Most
substance exchange between compartments happens across epithelia and endothelia
and their net area in some organs, such as the mammalian lung, is often many orders of
