Background: The mammalian mandible develops around Meckel's cartilage and other secondary cartilages, including the dentary. There have already been many studies of the development of the rat mandible that have employed histological serial sections. However, no previous investigators have captured the three-dimensional features of the developmental process.Methods: In this study, the technique of double staining with alizarin red S and alcian blue was employed directly on whole body specimens to investigate the three-dimensional development of the rat mandible.Results The mandible is the largest and strongest bone of the face, and it is essential for the function of mastication. In mammals, the mandible consists mainly of a dentary bone, but Meckel's cartilage appears transiently during the early stages of development, a period of great interest to anatomists.Following the critical description of Meckel's cartilage (Meckel, 1820), some researchers claimed that it did not ossify or play any role in the development of the mandible (Brock, 1876). Low (1909) reported that Meckel's cartilage ossified partly but did not contribute to mandibular development or growth, whereas Bhaskar et al. (1953) and Frommer and Margolies (1971) stated that the cartilage ossified enchondrally and contributed to the growth of the mandible. The problem of the role of Meckel's cartilage now seems to have been solved for the rat and mouse because it appears to ossify and also plays a part in the development of the mandible.A few studies have been reported on the development of the dentary or second cartilages in the rat (Bhaskar, 1953;Youssef, 1969), but they were based on crosssectional examination of the tissues and the construction of models based on serial sections. These models were not exact or detailed. Because it takes considerable time and effort to make models, it is not possible to observe many specimens or stages. To date, no study has captured the three-dimensional features of the developmental process of the rat mandible.Thus, the aim of this paper was to describe the development of the dentary and second cartilages and of Meckel's cartilage in the rat mandible by using a technique of direct double-staining of whole body specimens. A second goal was to clarify the role of the cartilages in the developmental process. MATERIALS AND METHODSThis study was based on direct observation of doublestained whole body specimens from 2 male and 12 female Wistar rats. The rats were maintained at Kyushu Dental College under the care of the Animal Research Center. All animals were housed singly in a 12-hr light-dark cycle and were given laboratory chow and water. The male rats and the female rats, which were older than 8 weeks and were already sexually mature, were kept together for 12 hr overnight. The next morning, we observed vaginal smears. The day on which sperm was found in the vagina was considered to be day 0. The pregnant rats were anesthetized with ether and then killed by cervical dislocation. The embryos were removed by...

We found that the molar socket obstructs the developing mandible, causing it to emerge medial to Meckel's cartilage. Our results also indicate that the secondary mandibular cartilages may contribute to supplementary growth in response to local factors.