The Questions of Developmental Biology

the aortic arch arteries and the septum between the aorta and the pulmonary artery (Waldo et al.
1998; see Figure 13.10). In the chick, the cardiac neural crest lies above the neural tube region
from rhombomere 7 through the portion of the spinal cord apposing the third somite, and its cells
migrate into pharyngeal arches 3, 4, and 6. The cardiac neural crest is unique in that if it is
removed and replaced by anterior cranial or trunk neural crest, cardiac abnormalities (notably the
failure of the truncus arteriosus to separate into the aorticand pulmonary arteries) occur. Thus, the
cardiac neural crest is already determined to generate cardiac cells, and the other regions of the
neural crest cannot substitute for it (Kirby 1989; Kuratani and Kirby 1991).
In mice, the cardiac neural crest cells are peculiar in that they express the transcription
factor Pax3. Mutations of Pax3 result in persistent truncus arteriosus (the failure of the aorta and
pulmonary artery to separate), as well as defects in the thymus, thyroid, and parathyroid glands
(Conway et al. 1997). Congenital heart defects in humans and mice often occur with defects in
the parathyroid, thyroid, or thymus glands. It would not be surprising if all of these were linked to
defects in the migration of cells from the neural crest.
*The BMP signal may be amplified by other signals coming from the presumptive ectoderm. Fibroblast growth factors
and Wnt proteins may be essential for maintaining the BMP-initiated events (see Mayor et al. 1997; LaBonne and
Bronner-Fraser 1998).
The pharyngeal. or branchial, arches are outpocketings of the head and neck region into which neural crest cells
migrate (see Figure 13.1). The pharyngeal pouches form between these arches and become the thyroid, parathyroid, and
thymus.
Tooth Development
During the morphogenesis of any organ, numerous dialogues are occurring between the
interacting tissues. In epithelial-mesenchymal interactions, the mesenchyme influences the
epithelium; the epithelial tissue, once changed by the mesenchyme, can secrete factors that
change the mesenchyme. Such interactions continue until an organ is formed with organ-specific
mesenchyme cells and organ-specific epithelia. Some of the most extensively studied interactions
are those that form the mammalian tooth. Here, the neural crest-derived mesenchyme cells
become the dentin-secreting odontoblasts, while the jaw epithelium differentiates into the
enamel-secreting ameloblasts. A summary of recent research that correlates mesenchyme
induction and differentiation in the mammalian tooth is shown in Figure 13.9.