The Questions of Developmental Biology

The chick, on the other hand, has 14 cervical vertebrae, 7 thoracic vertebrae, 12 or 13
(depending on the strain) lumbosacral vertebrae, and 5 coccygeal (fused tail) vertebrae. The
researchers asked, Does the constellation of Hox gene expression correlate with the type of
vertebra formed (e.g., cervical or thoracic) or with the relative position of the vertebrae (e.g.,
number 8 or 9)?
The answer is that the constellation of Hox gene expression predicts the type of vertebra
formed. In the mouse, the transition between cervical and thoracic vertebrae is between vertebrae
7 and 8; in the chick it is between vertebrae 14 and 15. In both cases, the Hox-5 paralogues are
expressed in the last cervical vertebrae, while the anterior boundary of the Hox-6 paralogues
extends to the first thoracic vertebra. Similarly, in both animals, the thoracic-lumbar transition is
seen at the boundary between the Hox-9 and Hox-10 paralogous groups. It appears there is a code
of differing Hox gene expression along the anterior-posterior axis, and this code determines the
type of vertebra formed.
The Dorsal-Ventral and Left-Right Axes in Mammals
The dorsal-ventral axis
Very little is known about the
mechanisms of dorsal-ventral axis
formation in mammals. In mice and
humans, the hypoblast forms on the side
of the inner cell mass that is exposed to
the blastocyst fluid, while the dorsal axis
forms from those ICM cells that are in
contact with the trophoblast. Thus, the
dorsal-ventral axis of the embryo is, in part, defined by the embryonic-abembryonic axis of the
blastocyst. This axis (wherein the embryonic region contains the ICM while the abembryonic
region is that part of the blastocyst opposite the ICM) may be determined within the oocyte, as it
develops perpendicularly to the animal-vegetal axis of the newly fertilized egg (Figure 11.41;
Gardner 1997). As development proceeds, the notochord maintains dorsal-ventral polarity by
inducing specific dorsal-ventral patterns of gene expression in the overlying ectoderm (Goulding
et al. 1993).
The left-right axis
The mammalian body is not symmetrical. Although the heart begins its formation at the
midline of the embryo, it moves to the left side of the chest cavity and loops to the right). The
spleen is found solely on the left side of the abdomen, the major lobe of the liver forms on the
right side of the abdomen, the large intestine loops right to left as it traverses the abdominal
cavity, and the right lung has one more lobe than the left lung.
Mutations in mice have shown that there are two levels of regulating the left-right axis: a
global level and an organ-specific level. Some genes, such as situs inversus viscerum (iv),
randomizes the left-right axis for each asymmetrical organ (Hummel and Chapman 1959; Layton
1976). This means that the heart may loop to the left in one homozygous animal, but loop to the
right in another (Figure 11.43). Moreover, the direction of heart looping is not coordinated with
the placement of the spleen or the stomach. This can cause serious problems, even death. The
second gene, inversion of embryonic turning (inv), causes a more global phenotype.