Biology Reference
In-Depth Information
made until the 32-cell stage. Each protein heads to sites of cell contact immediately but only
when the ZO1
protein is present is the intercellular path (the path that allows medium to
pass through spaces between cells) sealed.
29
Desmosomal proteins are expressed from the
16-cell stage. Their expression is independent of E-cadherin function,
28
but does depend on
the cell concerned not being surrounded by other cells and therefore not being deep inside
the embryo:
30
deep cells will become the inner cell mass and are not, therefore, destined to
be epithelial. In a similar manner, only cells that are not surrounded by others assemble
tight junctions.
31
Presumably, free edges generate a permissive signal that is suppressed
when they make contact with other cells. As adhesion proceeds, E-cadherin-rich contact
sites recruit the actin cytoskeleton and its associated proteins to the developing junctions.
Of these, actin-associated proteins, spectrin, ankyrin and fodrin are particularly important
because they form a network that can bind membrane proteins, such as Na
þ
/K
þ
ATPase,
which are localized to the basolateral membranes in mature epithelia.
32
This may be one
mechanism by which these proteins are recruited away from membranes that are not
making E-cadherin-mediated adhesions and towards those that are.
33
Membranes that
cannot form any E-cadherin-mediated adhesions will be those facing the outside of the
embryo, which will form the apical domain. Adhering E-cadherin also recruits a complex
of the proteins Sec6 and Sec8. The Sec6/8 complex acts as a docking receptor for vesicles
carrying basolateral proteins from the Golgi complex,
34
andactsasanothermechanism
to drive polarization of the cells.
35
This connection between cell adhesion and polarization
means that the forming epithelium can organize its own polarity according to the environ-
ment in which it finds itself
d
another example of adaptive self-organization.
The polarization of pumps such as the Na
þ
/K
þ
ATPase means that, once the tight
junctions have formed, cells can pump fluid inwards and inflate the blastocoel cavity. The
epithelial cells of the trophectoderm, the outer layer of the embryo, continue to divide as
development proceeds. If they divide tangentially, both daughters are epithelial while, if
they divide radially, the daughter still facing the free edge remains epithelial but the inner
one loses its epithelial nature and joins the inner cell mass.
As well as forming from blastomeres early in development, epithelia sometimes form from
mesenchymal cells during organogenesis. In the examples of mesenchyme-to-epithelium
transition most closely studied, such as formation of epithelial nephrons from the mesen-
chyme of kidneys in vertebrates, the process is similar to that found in early embryos. Cells
down-regulate their expression of typically mesenchymal proteins and express the adhesion
and cytoskeletal systems characteristic of epithelia, again in a temporal succession.
36
These
proteins, particularly cadherins, promote cell-cell adhesion and polarization so that cells
in contact with each other organize themselves into a polarized epithelial cyst.
37
The growth of an epithelium is generally achieved by proliferation of cells within that
epithelium, and the role of controlled proliferation in driving morphogenesis is described
in Chapter 23. Some epithelia, however, also grow by recruitment of mesenchymal cells in
a process of continuous mesenchyme-to-epitelial transition. An example is the nephric
duct of at least some vertebrates. The nephric duct is the drainage system for the pronephric
and mesonephric kidneys, which are temporary embryonic structures in reptiles, mammals
and birds, but are retained as permanent kidneys in fish and amphibians. Experiments using
chimaeric embryos that include tissues from the frogs Xenopus laevis and Xenopus borealis
demonstrate that a nephric duct derived from one species will, when migrating through
a
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