The Questions of Developmental Biology

extracellular matrix (Figure 6.32). As we will see in later
chapters, fibronectin also has an important role in cell migration.
The "roads" over which certain migrating cells travel are paved
with this protein. Fibronectin paths lead germ cells to the gonads
and lead heart cells to the midline of the embryo. If chick
embryos are injected with antibodies to fibronectin, the heartforming
cells fail to reach the midline, and two separate hearts
develop (Heasman et al. 1981; Linask and Lash 1988).
Laminin and type IV collagen are major components of a type of extracellular matrix called the
basal lamina (Figure 6.33). This basal lamina is characteristic of the closely knit sheets that
surround epithelial tissue. The adhesion of epithelial cells to laminin (upon which they sit) is
much greater than the affinity of mesenchymal cells for fibronectin (to which they must bind and
release if they are to migrate).
Like fibronectin, laminin plays a role in
assembling the extracellular matrix,
promoting cell adhesion and growth,
changing cell shape, and permitting cell
migration (Hakamori et al. 1984).
Integrins, the cell receptors for
extracellular matrix molecules.
The ability of a cell to bind to adhesive
glycoproteins depends on its expressing a cell
membrane receptor for the cell-binding site of
these large molecules. The main fibronectin
receptors were identified by using antibodies that
block the attachment of cells to fibronectin
(Chen et al. 1985; Knudsen et al. 1985).
The fibronectin receptor complex was found not
only to bind fibronectin on the outside of the cell,
but also to bind cytoskeletal proteins on the inside
of the cell. Thus, the fibronectin receptor complex
appears to span the cell membrane and unite two
types of matrices. On the outside of the cell, it
binds to the fibronectin of the extracellular matrix;
on the inside of the cell, it serves as an anchorage
site for the actin microfilaments that move the cell