A 3D analysis of growth trajectory and integration during early human prenatal facial growth

Abstract

Significant shape changes in the human facial skeleton occur in the early prenatal period, and understanding this process is critical for studying a myriad of congenital facial anomalies. However, quantifying and visualizing human fetal facial growth has been challenging. Here, we applied quantitative geometric morphometrics (GM) to high-resolution magnetic resonance images of human embryo and fetuses, to comprehensively analyze facial growth. We utilized non-linear growth estimation and GM methods to assess integrated epigenetic growth between masticatory muscles and associated bones. Our results show that the growth trajectory of the human face in the early prenatal period follows a curved line with three flexion points. Significant antero-posterior development occurs early, resulting in a shift from a mandibular prognathic to relatively orthognathic appearance, followed by expansion in the lateral direction. Furthermore, during this time, the development of the zygoma and the mandibular ramus is closely integrated with the masseter muscle.

Figure 1

Measurement of muscle cross-sectional areas. The cross-sectional area (CSA) of the temporalis and masseter muscles were measured from MRI slices obtained at the level of a plane placed parallel and just superior to the zygomatic arch (1) and a plane parallel to the mandibular plane (2), respectively.

Figure 2

Standardization of open-mouth position of mandible. Figure depicts the series of steps performed to standardize the open-mouth position of the mandible (see Materials and Methods). Briefly, a generalized Procrustes superimposition was performed to optimally register all specimen at the centroid (A, grey dot). The complete set of mandibular coordinates only, was translated such that the mid-condylar point for each specimen coincided with the centroid of the overall superimposition (B). Mandibular coordinates of each specimen were then rotated around its condylar axis, to find the best rotation degree which minimized the Procrustes distance between its configuration and that of the mean mandibular configuration (C). Finally, the rotated set of the mandibular coordinates of each specimen were translated back so that the mid-condylar points returned to their original positions, while maintaining the rotation (D). These new set of mandibular coordinates were recombined with the Procrustes coordinates of the midface.

Figure 3

Facial skeletal morphogenesis along the growth trajectory. The PC plots in the top row depicts the distribution of specimens along PCs 1, 2 and 3. Colored dots represent individual specimen (blue = youngest and yellow = oldest) and the 3 dimensional red line indicates the growth trajectory from youngest to oldest. Columns 1–5 depict the skull shape changes from an infero-lateral, inferior and frontal views (2nd, 3rd and 4th rows respectively). The “red star” in each PC plot corresponds to the estimated gestation week during which significant changes in facial shape can be noted, as represented by the bend in the red line. Red arrows indicate relative outward and forward growth, while blue arrows indicate relative inward in backward growth in specific areas of the fetal skull.

Figure 4

Masseter muscle—facial bone integration. Plots of singular warp (SW1) of the CSAi of the masseter muscle (x-axis) against SW1 of the shape of the zygoma (a, y-axis) and the ramus of the mandible (b, y-axis). Scores on these axes are significantly correlated (r = 0.737; RV = 0.506; P = 0.0001; 95% confidence interval = 0.625, 0.819 and r = 0.725; RV = 0.408; P = 0.0001; 95% confidence interval = 0.609, 0.81, respectively).

Figure 5

Change in shape in the zygoma and mandibular ramal areas correlated to masseter muscle growth. The shape changes seen in the zygoma (a–d) and the ramus of the mandible (e–h) along the first singular warp (SW1), on the cross-sectional areas (CSAi) of the masseter muscles from − 2 (a,c,e,g) and + 2 (b,d,f,h) standard deviations (SD). The body of the zygoma appears to expand in an antero-lateral direction with increasing masseter CSAi (a,b Inferior view; c,d Lateral-inferior view). The width of the ramus and the coronoid process appears to expand, and the mandibular body shifts laterally with increasing masseter CSAi (e,f inferior view; g,h lateral view).