Insights into the Role of Cell–Cell Junctions in Physiology and Disease - International Review of Cell and Molecular Biology - page 193

used; additionally, connexin genes were disrupted to assess downstream
developmental effects. Still, few conclusions can be drawn regarding the spe-
cific role for connexins during preimplantation development, given some-
times conflicting results ( Davies et al., 1996; Houghton, 2005; Houghton
et al., 2002 ). For example, neither connexin knockouts nor pharmacological
inhibition of connexins significantly disrupted development; in contrast,
antibody-mediated disruption of connexin perturbed compaction and blas-
tocyst development ( Kidder and Winterhager, 2001; Vance and

Wiley, 1999 ).
Human preimplantation embryos predominantly express connexin 43;
however, connexin expression patterns were found to vary among differ-
ent (human) embryos of normal morphology ( Bloor et al., 2004; Hardy
et al., 1996 ). Furthermore, evidence suggests that gap junctions could
potentially control tight junction assembly and cell lineage divergence
( Houghton, 2005; Toyofuku et al., 1998 ). Protein kinase C signaling
could be involved in these processes, but different lines of evidence
demonstrate that there is still clarification needed in specifying its role
( Eckert and Fleming, 2008 ).

3.6. Cell junctions and the cytoskeleton in development
Embryonic morphogenesis requires a constant and precisely executed rem-
odeling of cell junctions. This process is accompanied by dramatic cytoskel-
etal remodeling that facilitates the changes in cell shape and motility ( Jamora
and Fuchs, 2002 ). In preimplantation mammalian embryos, actin
cytoskeleton-associated adherens junctions and tight junctions appear at
the eight-cell stage, while the intermediate filament-associated desmosomal
emergence occurs at the 32-cell stage ( Eckert and Fleming, 2008 ). A rich
body of evidence points to a precisely controlled cross-junctional force bal-
ance between intercellular adhesion and cortical tension in governing the
dynamics of cell junctions ( Lecuit and Lenne, 2007; Paluch and
Heisenberg, 2009 ). Some important features of junctional mechanics during
morphogenesis have been characterized ( Hayashi and Carthew, 2004; Kafer
et al., 2007; Rauzi et al., 2008 ). To some extent, embryonic morphogenesis
results largely from epithelial sheet stretching, contraction, migration, and
deformation ( Stepniak et al., 2009 ). The mechanical forces generated by
dynamic rearrangements of cell junctions and the cytoskeleton thereby facil-
itate the morphological changes that transform flat cell sheets into three-
dimensional structures ( Jamora and Fuchs, 2002 ).