Mandibular bone stiffening and increased bone calcium mass in rats permanently stunted by hypophysectomy

This investigation was designed to obtain information on the changes induced by hypophysectomy on biometric parameters, bone calcium mass, and material and architectural properties during ontogenesis of the rat mandible.

Female Sprague-Dawley rats were hypophysectomised (HX) at 30 days of age. A "basal control group" (BC) was sacrificed on the same day surgery was performed. An "age-matched intact control group" (CON) was also included. HX and CON rats were sacrificed when aged 6 months. Body weight was monitored weekly. Mandibular growth was estimated directly on the right hemimandible by taking measurements between stable anatomical points. Its mechanical properties were determined using a three-point bending mechanical test. Load was applied transversely to the bone axis at a point immediately posterior to the posterior surface of the third molar. The left hemimandibles were ashed in a muffle furnace at 600 degrees C for 18h and the ash weight obtained. Calcium content in the ashes was determined by atomic absorption spectrophotometry. It was taken as the mandibular calcium mass. Histomorphometric studies were performed on decalcified hemimandibles: total interradicular bone, bone volume, and height of the periodontal ligament were measured.

Morphometric studies indicated that hypophysectomy in juvenile rats induced mandibular growth cessation, which was limited to the posterior part of the bone. Thus, the mandible maintained its juvenile proportions and showed an important deformation relative to age. In spite of the reduced bone size, both the mandibular weight and the calcium bone mass increased more than two times in ontogenia. Histomorphometric studies revealed that the interradicular bone volume was markedly increased. These findings strongly suggest that the bone that forms the mandible of the hypophysectomised rat under the conditions of the present study showed a higher than normal density. As evidenced from biomechanical studies, these bone properties, plus the significant stiffening of bone material tissue, were presumably responsible for the unnecessary and marked increment in the "load capacity" suffered by the mandible of the hypophysectomised rat during ontogenesis.