Biology Reference
In-Depth Information
CHAPTER
14
Co
ndensation of Ce
lls
Once migrating cells have completed their journey, their next step is often to 'condense'
together to form a compact mass of cells that subsequently forms a new structure such as
a bone or a hair follicle. There are several mechanisms for condensation and these may
work separately or in concert, depending on the system concerned. They include directed
migration, local proliferation, changes of cell adhesion and elimination of interstitial matrix.
Migration, such as that used by
D. discoideum
, and proliferation, are covered in Chapters 7
and 22, respectively. This chapter concentrates instead on the main mechanisms that drive
cell condensation in developing animals; enhanced cell adhesion and local elimination of
matrix.
COND
ENSATION THROUGH ENHANCED CELL ADH
ESION
The balance between whether cells remain spaced out in their extracellular matrix or asso-
ciated closely with one another is largely controlled by the balance between the cells' expres-
sion of cell-cell adhesion molecules and cell-matrix adhesion molecules. One simple way in
which cells can be induced to aggregate together is therefore for them to express more, or
more adhesive, cell-cell adhesion molecules, or to express fewer cell-matrix adhesion mole-
cules, or both. Provided that the cells move enough for chance encounters between them to
take place, switching on the expression of a cell-cell adhesion molecule may be all that is
required to cause them to form a single mass, which will grow as more cells expressing
the cell arrive and are added.
Developing limb bones provide a good example of condensation that is driven, at least in
part, by changes in the expression of cell-cell adhesion molecules. Limb bones develop from
cartilaginous predecessors, and these develop from condensation of mesenchymal cells.
1
An
local increase in cell density is achieved without cell proliferation in this system,
2
which
makes the rˆ le of other mechanisms particularly clear. Before condensation begins, cells of
the mesenchyme are separated by matrix-rich spaces and show little interest in making
contact with each other. As they condense, however, they display an increased tendency to
make contact so that the membranes of adjacent cells in the condensate touch over much
of their area.
3
This change in adhesive properties is due primarily to the cells activating
the expression of the homophilic cell-cell adhesion molecule, Cadherin 2. Shortly before
condensation begins, Cadherin 2 is expressed by scattered cells in the region that will become
