Biology Reference
In-Depth Information
condensates then signal back to the epidermis to induce its invasive growth to form the
shaft of a hair follicle. Further signalling events cause the differentiation of the hair follicle
into its different functional components (these signalling systems are beyond the scope of
this chapter, but have been reviewed well elsewhere).
23
Before their condensation, cells of the dermis are separated by a matrix that consists of
proteins, such as interstitial collagens, and of large glycosaminoglycans, notably hyaluronic
acid. Hyaluronic acid is an uncharged glycosaminoglycan that consists of alternating resi-
dues of N-acetylglucosamine and glucuronic acid. The carbohydrate chains tend to adopt
an extended conformation and make complexes with water to produce a space-filling,
gel-like structure with a viscosity about 5000 times that of water alone.
26
Long chains of
hyaluronic acid are a feature of extracellular matrix molecules such as aggrecan. Before
condensation, the interstitial matrix of the dermis is rich in hyaluronic acid but, as conden-
sation takes place, hyaluronic acid disappears between the condensing cells and is absent
from areas of condensation once they have formed.
27
This is probably because condensing
cells, but not neighbouring cells uninvolved in condensation, express the cell-surface hyal-
uronidase, CD44.
The expression of the CD44 hyaluronidase by condensing cells suggests that one mecha-
nism of condensation may simply be destruction of the most space-filling component of the
matrix that normally keeps cells apart. As long as there were to be positive pressure from
neighbouring tissues, this mechanism alone would be expected to bring cells closer together
(
Figure 14.7
). The importance of hyaluronic acid destruction to condensation can be demon-
strated by injecting a soluble bacterial hyaluronidase into the dermis; mesenchymal cells at
the injection site clump together as if making a condensate.
27
This observation strongly
supports the model of condensation by elimination of matrix, but does not prove it conclu-
sively because it remains possible that the effect of destroying hyaluronic acid is biochemical
rather thanmechanical. CD44 is a 'receptor' for hyaluronic acid and other glycosaminoglycans
FIGURE 14.7
Model for condensation of dermal cells by elimination of interstitial hyaluronic acid. The model
depends on there being external pressure present always, to drive the eventual coalescence of cells no longer being
kept apart by matrix.
