The Questions of Developmental Biology

On their lateral sides, each cell has another cell for a neighbor. Fink and McClay found that the
prospective ectoderm and endoderm cells (descendants of the mesomeres and macromeres,
respectively) bind tightly to one another and to the hyaline layer, but adhere only loosely to the
basal lamina (Table 8.2).
Table 8.2. Affinities of mesenchymal and nonmesenchymal cells to cellular and extracellular
components a
Dislodgment force (in dynes)
Cell type Hyaline Gastrula cell monolayers Basal lamina
16-cell-stage micromeres 5.8 × 10 -5 6.8 × 10 -5 4.8 × 10 -7
Migratory-stage mesenchyme cells 1.2 × 10 -7 1.2 × 10 -7 1.5 × 10 -5
Gastrula ectoderm and endoderm 5.0 × 10 -5 5.0 × 10 -5 5.0 × 10 -7
Source: After Fink and McClay 1985.
a Tested cells were allowed to adhere to plates containing hyaline, extracellular basal lamina, or cell monolayers. The
plates were inverted and centrifuged at various strengths to dislodge the cells. The dislodgement force is calculated
from the centrifugal force needed to remove the test cells from the substrate.
The micromeres originally display a similar pattern of binding. However, the micromere
pattern changes at gastrulation. Whereas the other cells retain their tight binding to the hyaline
layer and to their neighbors, the primary mesenchyme precursors lose their affinity for these
structures (which drops to about 2% of its original value) while their affinity for components of
the basal lamina and extracellular matrix (such as fibronectin) increases a hundredfold. This
change in affinity causes the micromeres to release their attachments to the external hyaline layer
and their neighboring cells and, drawn in by the basal lamina, migrate up into the blastocoel
(Figures 8.18, 8.19). These changes in affinity have been correlated with changes in cell surface
molecules that occur during this time (Wessel and McClay 1985).
As shown in Figure 8.18, there is a heavy concentration of extracellular material around
the ingressing primary mesenchyme cells (Galileo and Morrill 1985; Cherr et al. 1992). Once
inside the blastocoel, the primary mesenchyme cells appear to migrate along the extracellular
matrix of the blastocoel wall, extending their filopodia in front of them (Galileo and Morrill 1985;
Karp and Solursh 1985). Several proteins (including fibronectin and a particular sulfated
glycoprotein) are necessary to initiate and maintain this migration (Wessel et al. 1984; Sugiyama
1972; Lane and Solursh 1991; Berg et al. 1996).