a1 Department of Anatomy and Cell Biology, Emory University, Atlanta, GA 30322, USA
During the past decade growing evidence has suggested that cell-cell communication via gap junctions is crucial for early developmental processes (Warner et al., 1984; Guthrie & Gilula, 1989; Serras et al., 1989). It has been shown that embryos of mice (Kalimi & Lo, 1988), teleosts (Kimmel et al., 1984), insects (Warner & Lawrence, 1982; Ruangvoravat & Lo, 1992) and molluscs (Serras et al., 1989) become regionally organized into restricted domains of junctionally connected cells that share developmental potential. In the mouse gastrula, for example, dye-coupling experiments have demonstrated that cells within a developmental compartment have a high degree of coupling whereas cells across compartmental boundaries have reduced coupling (Kalimi & Lo, 1988). Classic experiments (Townes & Holtfretter, 1955; Steinberg, 1963) demonstrated that as cells begin to differentiate along common pathways, they develop selective adhesion properties and the ability to sort themselves from unlike neighbours (for review see Edelman 1988; Edelman et al., 1990). More recently a multitude of specific cell adhesion molecules (CAMs) have been identified that mediate these processes and trigger the cytoplasmic events that drive further differentiation (Edelman et al., 1990; Albelda, 1991; Geiger & Ayalon, 1992).