The Questions of Developmental Biology

Kessel and Gruss (1991) found this to be the case. Wild-type mice
have 7 cervical (neck) vertebrae, 13 thoracic (ribbed) vertebrae, and 6
lumbar (abdominal) vertebrae, in addition to the sacral and caudal (tail)
vertebrae. In embryos exposed to RA on day 8 of gestation (during
gastrulation), the first one or two lumbar vertebrae were transformed into
thoracic (ribbed) vertebrae, while the first sacral vertebra often became a
lumbar vertebra. In some cases, the entire posterior region of the mouse
embryo failed to form (Figure 11.39E). These changes in structure were
correlated with changes in the constellation of Hox genes expressed in these tissues. For example,
when RA was given to embryos on day 8, Hoxa-10 expression was shifted posteriorly, and an
additional set of ribs formed on what had been the first lumbar vertebra. When posterior Hox
genes were not expressed at all, the caudal part of the embryo failed to form.
Retinoic acid probably plays a role in axis specification during normal development, and
the source of retinoic acid is probably the node (Hogan et al. 1992; Maden et al. 1996). It is
possible that the specification of mesoderm cells depends on the amount of time spent within the
high retinoic acid concentrations of the node: the more time spent in the node, the more posterior
the specification. This pattern can be demonstrated in culture, as embryonal carcinoma cells
express more "posterior" Hox genes the longer they are exposed to retinoic acid (Simeone et al.
1990). Giving RA exogenously would mimic the RA concentrations normally encountered only
by the posterior cells. The evidence points to a Hox code wherein different constellations of Hox
genes, activated by different retinoic acid concentrations, specify the regional characteristics
along the anterior-posterior axis.
Comparative anatomy.
A new type of comparative embryology is now emerging, and it is based on the
comparison of gene expression patterns. Gaunt (1994) and Burke and her collaborators (1995)
have compared the vertebrae of the mouse and the chick. Although the mouse and the chick have
a similar number of vertebrae, they apportion them differently. Mice (like all mammals, be they
giraffes or whales) have only 7 cervical vertebrae. These are followed by 13 thoracic vertebrae, 6
lumbar vertebrae, 4 sacral vertebrae, and a variable (20+) number of caudal vertebrae (Figure
11.40).