Mandibular growth rates in human fetal development

A morphometric analysis of changing proportions in the developing mandible was undertaken in 18 human embryos and fetuses of both sexes (developmental age from 8 to 14 weeks, crown-rump length, CRL, from 34 to 110 mm), previously cleared and stained with a specific method for bone (alizarin red S). Reference points were located on the mandible, i.e. condylar process (Pcl), coronoid process (Pco), gnathion (GN), gonion (GO), superior symphyseal point (SSP), for measuring linear dimensions, i.e. Pcl-GN, Pcl-Pco, Pco-GN, GO-GN, SSP-GN. The gonial (Pcl-GO-GN) and the (Pcl-GN-Pcl) angles were also measured. All linear dimensions were correlated with the CRL by bivariate allometry (ln y = ln a + b ln x): they all grew with positive allometry, except GO-GN with isometry. The mandibular ramus grew relatively faster than the body, both in length and height, and the greatest growth rate was found for ramus height. The relation between mandibular shape and the craniofacial structures was investigated using scale drawings obtained from photographs of fetal skulls in lateral view. In the youngest fetuses the mandible was prognathic, then became retrognathic. During the period investigated the zygomatic process and squama of the temporal bone were in a lower and more inclined position in relation to the transverse plane passing through the zygomatic arch than in the newborn and adult. This study identifies parameters fitting changing trends in height, length and shape of the human mandible during the prenatal period (8–14 weeks); moreover, it emphasizes that the mandibular growth patterns differ significantly from those of successive developmental periods.