Short-faced mice and developmental interactions between the brain and the face

Abstract

The length of the face represents an important axis of variation in mammals and especially in primates. Mice with mutations that produce variation along this axis present an opportunity to study the developmental factors that may underlie evolutionary change in facial length. The Crf4 mutant, obtained from the C57BL/6J (wt/wt) background by chemical mutagenesis by the Baylor Mouse Mutagenesis Resource, is reported to have a short-faced phenotype. As an initial step towards developing this model, we performed 3D geometric morphometric comparisons of Crf4 mice to C57BL/6J wild-type mice focusing on three stages of face development and morphology--embryonic (GD 9.5-12), neonatal, and adult. Morphometric analysis of adult Crf4 mutants revealed that in addition to a shortened face, these mice exhibit a significant reduction in brain size and basicranial length. These same features also differ at the neonatal stage. During embryonic face formation, only dimensions related to brain growth were smaller, whereas the Crf4 face actually appeared advanced relative to the wild-type at the same somite stage. These results show that aspects of the Crf4 phenotype are evident as early as embryonic face formation. Based on our anatomical findings we hypothesize that the reduction in facial growth in Crf4 mice is a secondary consequence of reduction in the growth of the brain. If correct, the Crf4 mutant will be a useful model for studying the role of epigenetic interactions between the brain and face in the evolutionary developmental biology of the mammalian craniofacial complex as well as human craniofacial dysmorphology.

Fig. 1

Micro-CT scan of mouse embryo, gestational day 10, right side. Six paired epithelium-covered mesenchymal tissue buds surround the primitive oropharynx, three to a side (Mx, MNP, LNP) and, after merging and fusing together, give rise to the midface. e, eye; FNM, frontonasal mass; LNP, lateral nasal prominence; Md, mandibular arch; MNP, medial nasal prominence; Mx, maxillary prominence.

Fig. 2

Landmarks on mouse embryo head (A–D) in para-frontal (A), frontal (B), lateral (C) and occipital (D) views. Landmark positions on neonate (E–H) and adult (I–L) mouse skulls, in dorsal (E,I), ventral (F,J), lateral (G,K), occipital (H) and internal (L) views.

Fig. 4

Mean shapes of adult C57BL/6J wildtype (A,B), Crf4 mutant (C,D) and ghrhr-mutant (E,F) skulls in lateral and superior views.

Fig. 5

(A) Scatterplot of principal component scores (PC1 and PC2) for adult Crf4, C57Bl/6J and ghrhr null mice along with wireframe deformations showing the shape variation along PC1 and PC2. (B) Scatterplot of CVA scores. In both graphs, the wireframe deformations are to scale (not magnified), showing the shape that corresponds to the extreme values of the graph.

Fig. 6

(A) EDMA analysis results showing inter-landmark distances that differed by more than 3% between Crf4 and C57BL/6J mice. (B) Comparisons of key measurements showing those that differ most dramatically between the two strains.

Fig. 7

(A) Scatterplot of PC1 and cranial centroid size (scale) for Crf4, C57Bl/6J and ghrhr null mice. (B) PCA of the residuals of multivariate regression of the Procrustes coordinates on log centroid size. Following Mitteroecker et al. (2004), we use the notation RSC (residual shape components). (C) Form analysis (PCA of Procrustes Coordinate data as well as centroid size) for all three groups. showing principal component scores for the first three components. Following Mitteroecker et al. (2004), we use the notation SSPC (Size–Shape principal components).

Fig. 8

(A) Plot of PC1 against PC2 for size-standardized Procrustes superimposed landmark data for the pooled neonatal sample. Below are wireframes showing shape change across PC1 within this sample. (B) EDMA results showing all inter-landmark distances that differ by more than 2%. Yellow lines mark distances in Crf4 neonates that are longer than C57 neonates. Red lines mark distances in Crf4 neonates that are shorter than C57 neonates.

Fig. 9

Multivariate pooled within-group multivariate regression of shape on tail somite stage for the embryonic sample. The wireframes show the variation that corresponds to the regression scores on the y-axis.

Fig. 10

Comparison of Crf4 and C57BL/6J embryos by canonical variate analysis. The samples are standardized to tail somite stage 16 by pooled within-group multivariate regression. The distribution of the canonical variate scores is shown below. The wireframes depict the variation along the canonical variate and are to scale with the histogram below.

Fig. 3

Craniofacial development of C57Bl/6J embryos showing examples of ontogenetic subsets: (A) TS 4–10, (B) TS 11–18, (C) TS 19–25. Frontal view.

Fig. 11

Canonical variates comparisons of Crf4 and C57BL/6J embryos for the three ontogenetic subgroups (Fig. 3). Within each ontogenetic group, shape is standardized to the median tail somite stage. The wireframes depict variation along the canonical variates and are to scale with the histograms to the right.