The Questions of Developmental Biology

7. In birds, gravity is critical in determining the anterior-posterior axis, while pH differences
appear crucial for distinguishing dorsal from ventral. The left-right axis is formed by the
expression of nodal on the left side of the embryo, which signals pitx2 expression on the left side
of developing organs.
8. Mammals undergo holoblastic rotational cleavage, characterized by a slow rate of division, a
unique cleavage orientation, lack of divisional synchrony, and the formation of a blastocyst.
9. The blastocyst forms after the blastomeres undergo compaction. It contains outer cells the
trophoblast cells that become the chorion, and an inner cell mass that becomes the amnion and
the embryo.
10. The chorion forms the fetal portion of the placenta, which functions to provide oxygen and
nutrition to the embryo, to provide hormones for the maintenance of pregnancy, and to provide
barriers to the mother's immune system.
11. Mammalian gastrulation is not unlike that of birds. There appear to be two signaling
centers one in the node and one in the anterior visceral endoderm. The latter is critical for
generating the forebrain, while the former is critical in inducing the axial structures caudally from
the midbrain.
12. Hox genes pattern the anterior-posterior axis and help to specify positions along that axis. If
Hox genes are knocked out, segment-specific malformations can arise. Similarly, causing the
ectopic expression of Hox genes can alter the body axis.
13. The homology of gene structure and the similarity of expression patterns between Drosophila
and mammalian Hox genes suggests that this patterning mechanism is extremely ancient.
14. The mammalian left-right axis is specified similarly to that of the chick.
*In mammalian development, Hensen's node is usually just called "the node," despite the fact that Hensen discovered
this structure in rabbit embryos.
There are numerous types of placentas, and the extraembryonic membranes form differently in different orders of
mammals (see Cruz and Pedersen 1991). Although mice and humans gastrulate and implant in a similar fashion, their
extraembryonic structures are distinctive. It is very risky to extrapolate developmental phenomena from one group of
mammals to another. Even Leonardo da Vinci got caught (Renfree 1982). His remarkable drawing of the human fetus
inside the placenta is stunning art, but poor science: the placenta is that of a cow.
Hoxa-10 is also important in specifying the axial pattern of the genital ducts. Knockouts of Hoxa-10 create mice
wherein the upper region of the uterus is transformed into tissue resembling the oviduct. This region coincides with the
anterior limit of Hoxa-10 expression in the wild-type Müllerian duct (Benson et al. 1996).
This gene was discovered accidentally when Yokoyama and colleagues (1993) made transgenic mice wherein the
transgene (for the tyrosinase enzyme) was inserted randomly into the genome. In one instance, this gene inserted itself
into a region of chromosome 4, knocking out the existing inv gene. The resulting homozygous mice had laterality
defects.