The Questions of Developmental Biology

mammary gland tissue. The binding of integrins to laminin is essential for the transcription of the
casein gene, and the integrins act in concert with prolactin (see Figure 6.21) to activate that gene's
expression (Roskelley et al. 1994; Muschler et al. 1999).
Recent studies have shown that the binding of integrins to the extracellular matrix can
stimulate the RTK-Ras pathway. When an integrin on the cell membrane of one cell binds to the
fibronectin or collagen secreted by a neighboring cell, the integrin can activate the tyrosine kinase
cascade through an adaptor protein-like complex that connects the integrin to the Ras G protein
(Figure 6.37A; Wary et al. 1998). Cadherins and other cell adhesion molecules can also transmit
signals by "hijacking" the FGF receptors (Williams et al. 1994b; Clark and Brugge 1995).
Cadherins, for example, are able to bind to the intramembrane region of FGF receptors and
thereby dimerize these receptors just like the normal FGF ligands (Figure 6.37B; Williams et al.
1994a; Doherty et al. 1995).
Direct transmission of signals through gap junctions
Throughout this chapter
we have been
discussing the reception
of signals by cell
membrane receptors. In
these mechanisms, the
receptor is in some
manner altered by binding the ligand, so that its
cytoplasmic domain transmits the signal into the
cell. Another mechanism transmits small,
soluble signals directly through the cell
membrane, for the membrane is not continuous
in all places. There are regions called gap
junctions that serve as communication channels
between adjacent cells (Figure 6.38A,B). Cells
so linked are said to be "coupled," and small
molecules (molecular weight