The Questions of Developmental Biology

The mesodermal cells are thought to adhere to fibronectin through the vβ1 integrin
protein (Alfandari et al. 1995). Mesodermal migration can also be arrested by the microinjection
of antibodies against either fibronectin or the integrin subunit that serve as part of the fibronectin
receptor (D'Arribère et al. 1988, 1990). Alfandari and colleagues (1995) have shown that the v
subunit of integrin appears on the mesodermal cells just prior to gastrulation, persists on their
surfaces throughout gastrulation, and disappears when gastrulation ends. It seems, then, that the
synthesis of this fibronectin receptor may signal the times for the mesoderm to begin, continue,
and stop migration.
In addition to permitting the attachment of mesodermal cells, the fibronectin-containing
extracellular matrix appears to provide the cues for the direction of cell migration. Shi and
colleagues (1989) showed that salamander IMZ cells would migrate in the wrong direction if
extra fibronectin lattices were placed in their path. In Xenopus, convergent extension pushes the
migrating cells upward toward the animal pole. Fibronectin appears to delineate the boundaries
within which this movement can occur (see Figure 6.32). The fibronectin fibrils are necessary for
the head mesodermal cells to flatten and to extend broad (lamelliform) processes in the direction
of migration (Winklbauer et al. 1991; Winklbauer and Keller 1996). The importance of these
fibronectin fibrils is also seen in inviable interspecific hybrids. Delarue and colleagues (1985)
have shown that certain inviable hybrids between two species of toads arrest during gastrulation
because they do not secrete these fibronectin fibrils. It appears, then, that the extracellular matrix
of the blastocoel roof, and particularly its fibronectin component, is important in the migration of
the mesodermal cells during amphibian gastrulation.
Axis Formation in Amphibians: The Phenomenon of the Organizer
The Progressive Determination of the Amphibian Axes
Vertebrate axes do not form from localized determinants in the various blastomeres, as in
Drosophila. Rather, they arise progressively through a sequence of interactions between
neighboring cells. Amphibian axis formation is an example of regulative development. In Chapter
3, we discussed the concept of regulative development, wherein (1) an isolated blastomere has a
potency greater than its normal embryonic fate, and (2) a cell's fate is determined by interactions
between neighboring cells. Such interactions are called inductions (see Chapter 6). That such
inductive interactions were responsible for amphibian axis determination was demonstrated by
the laboratory of Hans Spemann at the University of Freiburg. The experiments of Spemann and
his students framed the questions that experimental embryologists asked for most of the twentieth
century, and they resulted in a Nobel Prize for Spemann in 1935. More recently, the discoveries
of the molecules associated with these inductive processes have provided some of the most
exciting moments in contemporary science.
The experiment that began this research program was performed in 1903, when Spemann
demonstrated that early newt blastomeres have identical nuclei, each capable of producing an
entire larva. His procedure was ingenious: Shortly after fertilizing a newt egg, Spemann used a
baby's hair taken from his daughter to lasso the zygote in the plane of the first cleavage. He then
partially constricted the egg, causing all the nuclear divisions to remain on one side of the
constriction. Eventually, often as late as the 16-cell stage, a nucleus would escape across the
constriction into the non-nucleated side. Cleavage then began on this side, too, whereupon
Spemann tightened the lasso until the two halves were completely separated. Twin larvae
developed, one slightly older than the other (Figure 10.17).