The Questions of Developmental Biology

Axis Formation in the Chick Embryo
While the formation of the body axes is accomplished during gastrulation, their
specification occurs earlier, during the cleavage stage.
The role of pH in forming the dorsal-ventral axis
The dorsal-ventral (back-belly) axis is critical to the formation of the hypoblast and to the
further development of the embryo. This axis is established when the cleaving cells of the
blastoderm establish a barrier between the basic (pH 9.5) albumin above the blastodisc and the
acidic (pH 6.5) subgerminal space below it. Water and sodium ions are transported from the
albumin through the cells and into the subgerminal space, creating a membrane potential
difference of 25 mV across the epiblast cell layer (positive at the ventral side of the cells). This
process distinguishes two sides of the epiblast: a side facing the negative and basic albumin,
which becomes the dorsal side, and a side facing the positive and acidic subgerminal space fluid,
which becomes the ventral side. This axis can be reversed experimentally either by reversing the
pH gradient or by inverting the potential difference across the cell layer (reviewed in Stern and
Canning 1988).
The role of gravity in forming the anterior-posterior axis
The conversion of the radially symmetrical blastoderm into a bilaterally symmetrical
structure is determined by gravity. As the ovum passes through the hen's reproductive tract, it is
rotated for about 20 hours in the shell gland. This spinning, at a rate of 10 to 12 revolutions per
hour, shifts the yolk such that its lighter components lie beneath one side of the blastoderm. This
tips up that end of the blastoderm, and that end becomes the posterior portion of the embryo the
part where primitive streak formation begins (Figure 11.13; Kochav and Eyal-Giladi 1971; Eyal-
Giladi and Fabian 1980).
It is not known what interactions cause this portion of the blastoderm to become the
posterior margin and to initiate gastrulation. The ability to initiate a primitive streak is found
throughout the marginal zone, and if the blastoderm is separated into parts, each having its own
marginal zone, each part will form its own primitive streak (Figure 11.14; Spratt and Haas 1960).
However, once a posterior marginal zone (PMZ) has formed, it controls the other regions of the
margin. Not only do these PMZ cells initiate gastrulation, but they also prevent other regions of
the margin from forming their own primitive streaks (Khaner and Eyal-Giladi 1989; Eyal-Giladi
et al. 1992).