The Questions of Developmental Biology

The mechanism of intramembranous ossification involves bone morphogenetic proteins
and the activation of a transcription factor called CBFA1. Bone morphogenetic proteins
(probably BMP2, BMP4, and BMP7) from the head epidermis are thought to instruct the neural
crest-derived mesenchymal cells to become bone cells directly (Hall 1988). The BMPs activate
the Cbfa1 gene in the mesenchymal cells. Just as the myogenic bHLH family of transcription
factors is competent to transform primitive mesenchyme cells (or just about any other cell) into
muscle-forming myoblasts, the CBFA1 transcription factor appears to be able to transform
mesenchyme cells into osteoblasts. Ducy and her colleagues (1997) found that the mRNA for
mouse CBFA1 is severely restricted to the mesenchymal condensations that form bone, and is
limited to the osteoblast lineage. The protein appears to activate the genes for osteocalcin,
osteopontin, and other bone-specific extracellular matrix proteins.
Confirmation and extension of this
conclusion was obtained from gene targeting
experiments wherein the mouse Cbfa1 gene was
knocked out (Komori et al. 1997; Otto et al. 1997).
Mice homozygous for this deletion died shortly after
birth without taking a breath, and their skeletons
completely lacked bone. The mutants had only the
cartilaginous skeletal model (Figure 14.12). In these
mice, both endochondral and intramembranous
ossification had been eliminated. The osteoblasts
were in an arrested state of development, expressing
neither osteocalcin nor osteopontin.
Mice that were heterozygous for Cbfa1 showed skeletal defects similar to those of a
human syndrome called cleidocranial dysplasia (CCD). In this syndrome, the skull sutures fail to
close (adults retain the fontanel associated with young infants), growth is stunted, and the clavicle
(collarbone) is often absent or deformed. When DNA from patients with CCD was analyzed,
each patient had either deletions or point mutations in the CBFA1 gene. Control individuals did
not have such mutations. Therefore, it appears that cleidocranial dysplasia is caused by
heterozygosity of the CBFA1 gene (Mundlos et al. 1997).
Endochondral ossification