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each other and form a clump instead of being enveloped by the somatic cells? If germ cells
were more adhesive than the somatic cells, they would be expected to segregate to form
a clump in the centre of the organ, as N-cadherin-expressing mesenchyme cells segregate
to the middle of the limb. Indeed, if primordial germ cells are engineered to overexpress
DE-cadherin this is precisely what happens. 19 If, on the other hand, somatic cells were
more adhesive, then they would be expected to segregate to the inside. The observed pattern
of cells, with germ cells enveloped by somatic cells, would be expected only if adhesions
between heterologous cells were stronger than those between homologous cells (or if they
featured lower interfacial tension). Possibly there are additional heterotypic cell-cell
adhesion molecules that ensure an especially strong interaction between germ cells and
somatic cells.
CONDEN SATION BY ELIMINATION OF INTERSTITIAL MATRIX
Mesenchyme cells are not really separated by 'space' as is implied by conventional histo-
logical diagrams, including many in this topic, but are instead embedded in, and separated
by, extracellular matrix. One way of bringing cells together into a tight condensate is to elim-
inate the matrix between themwhile maintaining it between uninvolved cells. An example of
this mechanism can be found in the mammalian dermis, where mesenchyme cells form
condensates as part of the early development of hair follicles.
Mammalian skin is a layered structure, in which a stratified epithelial epidermis (Chapter
15) is supported by a matrix-rich, mesenchymal dermis ( Figure 14.6 a). During foetal devel-
opment, the dermis secretes signals that induce the overlying epidermis to form a series of
evenly-spaced placodes. 21 The size and spacing of the placodes is under the control of the
dermis itself, so that dermis from a region of the body characterized by densely-spaced, thick
dark hair will induce formation of this type of hair even when it is transplanted under
epidermis at a site characterized by sparse, fine hair. This is true even in adults, as has
been shown by one scientist who transplanted dermis from his own scalp into the arm of
a female colleague! 22 The mechanism that controls spacing of placodes has not been eluci-
dated yet, but it is assumed that there is competition and lateral inhibition between
placode-promoting and placode-repressing signals, probably based on Wnt proteins. 23
Once formed, each of these placodes then signals back to the dermis, probably using a combi-
nation of PDGF 24 and Wnt proteins, 25 and causes the dermal cells to condense together. The
FIGURE 14.6 Cell movements in the early development of a single hair follicle.
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