Root development in mice lacking functional tissue non-specific alkaline phosphatase gene: Inhibition of acellular cementum formation

Tissue non-specific alkaline phosphatase (TNAP) is richly present in develo ping teeth including the cells of the periodontal ligament. Here, we invest igated tooth and root development in mice lacking the TNAP gene. Heterozygo us mutants were obtained from The Jackson Laboratory, Animal Resources (Bar Harbor, ME, USA) and bred. TNAP-deficient mice and their littermates were killed from 6 to 25 days after birth and their molar blocks processed for l ight and electron microscopy. It was observed that the eruption of the inci sors into the oral cavity was delayed for 2 to 3 days. Also, the onset of m ineralization of the mantle dentin in the roots of the developing molars wa s delayed for 2 to 3 days. Yet, dentin and enamel formation in the homozygo us mutants showed a more or less normal pattern, with the exception of loca lized enamel hypoplasias. The most conspicuous finding was the defective fo rmation of acellular cementum along the molar roots. Instead of a continuou s layer, the cementum was deposited as very thin and irregularly shaped pat ches around the bases of the periodontal ligament fibers. Sharpey's fibers were short and poorly developed. In contrast, the development of the alveol ar bone, the periodontal ligament, and the cellular cementum was seemingly unaffected. It is concluded that TNAP represents an essential factor in man tle dentin mineralization and in the formation of acellular cementum.