Homeobox genes and growth factors in regulation of craniofacial and tooth morphogenesis.

Homeobox genes encode a special group of transcription factors that regulate gene expression in the developing embryo. The so-called Hox-cluster genes were first discovered in the Drosophila (fruit fly). They specify the identity of body segments and their patterning along the anteroposterior axis. Other homeobox-containing genes appear to regulate patterning of the head and face. The function of the Msx-1 homeobox gene has been shown to be necessary for tooth development. In general, it is thought that special combinations of homeobox genes specify the patterning of individual structures. Bone morphogenetic proteins (BMPs) are growth factors belonging to the family of transforming growth factor-beta (TGF-beta). BMPs regulate bone and cartilage development, and individual BMPs have been shown to contribute to the shaping of various skeletal elements. BMPs regulate bone and dentin formation also postnatally, and they have therapeutic potential in reparative osteogenesis and odontogenesis. BMPs also act as inductive signals between tissue layers in the embryo, and they regulate the expression of several transcription factors, including homeobox-containing genes. BMP-4 has been identified as an epithelial inductive signal in tooth development. As it is produced by early dental epithelium and regulates tooth-specific gene expression in the dental mesenchyme, including Msx-1 expression, it may be an important signal for the initiation of tooth development.