Effects of Parathyroid Hormone on Nasopremaxillary Growth under Different Masticatory Loading Conditions in Growing Mice

Abstract

It is well accepted that reduced masticatoiy function resulted from a soft diet with low physical consistency causes motphologic and histologic alterations in the craniofacial skeleton in growing animals. It is also assumed that these alterations are associated with reduced proliferative activity of osteoblasts on the bone surface, indicating a significant role of mechanical stimuli mediated by various local growth factors such as insulin-like growth factor-I (IGF-I) and parathyroid hormone (PTH). A previous study demonstrated that PTH accelerated the effect of IGF-I on bone formation. This study was thus conducted to examine the effect of PTH on the induction of osteoclasts and osteoblasts and the subsequent nasopremaxillaiy growth under different mechanical loading conditions from mastication. In animals treated with PTH, the nasal bone length increased significantly in the solid-diet group. No significant differences in these dimensions were found between the solid-diet mice injected with physiological saline and the granulated-diet group injected with PTH. It is shown that PTH induces nasal and premaxillaiy growth, and the effects may be exerted if mechanical loading from mastication is sufficient. it is also demonstrated that the effects of PTH are more potent in the ostcogenic activity than in the osteoclastic one, in terms of more significant expression of osteoblasts than osteoclasts.It is indicated that physical exercise or the related mechanical loading has a positive effect on bone remodeling to increase bone mass, and contrarily defect of mechanical stress during space flight decreases bone mass. Dehority er oi. reported that unloading decreased the number of osteoblasts, the rates of mineral apposition and bone formation and inhibited periosteal and cancellous bone formation (6). In our previous study, different masticatory loadAbbreviufioris: PTH; parathyroid hormone, IGF-I; insulin-like growth factor-I, GH; growth hormone, bFGF; basic fibroblast growth factor. *Corresponding author to Dr Chiyoko Tokimasa, at the above address. Japan. Tel: +81-82-257-5686; Fax: +81-82-25'?-5687; E-meil: tokichi@hiroshima-u.ac.jp ings induced by varying physical consistencies of diet caused morphologic and histologic alterations in the nasopremaxillaiy complex in growing mice (31). Furthermore, morphometric analyses demonstrated that changes in masticatoiy function affected not only the local bone morphology in the vicinity of muscle attachments but also the overall morphology of the upper viscerocranium (23, l4). The zygomatic width was significantly smaller with a significantly larger gonial angle of the mandible in rats fed a soft diet (1, 27, 23, 22). In addition, reduced masticatoiy function significantly affected the dimension of facial sutures (17, 21) and morphology of the nasopreniaxillary skeleton and cortical bone formation (1 l).it is well accepted that mechanical loading inhibits bone resorption and increases in vivo bone forma-