Development of the Jaws and Teeth in Mouse Lemurs.

Rodent models have provided a detailed body of knowledge on craniodental development. Yet, it is unknown how well altricial mammals such as mice parallel primate and specifically human craniodental development. To address this, here we document the ontogeny of jaw-related structures in a cross-sectional prenatal sample of mouse lemurs (Microcebus spp.). Mouse lemurs follow the same developmental sequence as humans and mice with respect to the earliest appearance of jaw structures. Meckel's cartilage forms before any membranous bone of the mandibular body; mandibular basal bone begins to ossify before teeth enter the bud stage. Lower jaw structures commence morphogenesis before the corresponding maxillary elements. The basal bone of both jaws becomes more complex in larger fetuses. As tooth germs mature, the follicle-a supporting connective tissue outside the papilla and enamel organ-matures into a cellular, loosely fibrous tissue. Surrounding the follicle is a sheet-like connective tissue, the epifollicular membrane. In tooth germs reaching the bell stage, alveolar bone forms within this membrane. One subtle distinction of mouse lemurs to prior descriptions of fetal anthropoids is the lemur's faster maturation of the deciduous canine compared to incisors, the reverse of the anthropoid pattern. However, broadly it appears that all primates studied thus far have a very prolonged duration of dental maturation, which continues long after other facial elements (e.g., hard palate) are fully formed. This scenario contrasts with altricial rodents, in which palate formation occurs near parturition, and only a day before the first permanent molar reaches the bell stage.