Mechanisms that underlie co‐variation of the brain and face

Abstract

The effect of the brain on the morphology of the face has long been recognized in both evolutionary biology and clinical medicine. In this paper we describe factors that are active between development of the brain and face and how these might impact craniofacial variation. First, there is the physical influence of the brain, which contributes to overall growth and morphology of the face through direct structural interactions. Second, there is the molecular influence of the brain, which signals to facial tissues to establish signaling centers that regulate patterned growth. Importantly, subtle alterations to these physical or molecular interactions may contribute to both normal and abnormal variation. These interactions are therefore critical to our understanding of how a diversity of facial morphologies can be generated both within species and across evolutionary time.

Larger mammals have longer faces because of size-related constraints on skull form

“…Shortening of the face is characteristic of domestication in mammals, which often selects for smaller size, and has been reported in pigs, cattle, dogs and cats 21 . In mice mutants, relatively small brains co-occur with prognathic faces 19,22 . This covariation Figure 3 | Procrustes form space trajectories.…”

“…Wireframe shape diagrams, with the face emphasized using lighter colours, help to visualize 3D cranial landmark configurations (in side view) at of the group-specific form trajectories. among anatomical parts can result from gene pleiotropy and linkage, from developmental interactions between pathways or tissues, from the cross-tissue action of hormones or the epigenetic action of muscles or environmental factors on bones [22][23][24] .…”

“…Regardless of the fine details, the shortening of the face in such a large animal, as us humans, is an exception to the allometry between size and facial proportions that generally holds for mammals. One of the proximate reasons why our face is unusually short for our body size is that we need to accommodate a large, rapidly growing brain that forces the facial prominences to be spread apart as they begin to elongate 22 . The enlargement of the brain, the progressive reduction in canines and molars and an increasing reliance on preprocessed food, which reduces the need of powerful masticatory muscles, may have overcome in our lineage the basic allometric facial constraint that we have identified, allowing humans to evolve an unusual phenotype in terms of cranial proportions.…”

Apaf1 apoptotic function critically limits Sonic hedgehog signaling during craniofacial development

“…In addition, the morphology of the neural tube, composed of neuroectoderm, regulates the positioning of both the overlying cranial mesoderm that is responsible for the musculature and vasculature, as well as the positioning of adjacent endodermal tissue that develops into structures such as the pharyngeal pouches. 24 Seven prominences contribute to the face: the frontonasal (FNP), paired maxillary, paired mandibular and paired nasal prominences. These prominences consist of an outer ectodermal cell layer and an internal neural crest-derived mesodermal core.…”

“…Signaling interactions between adjacent prominences and from underlying mesenchyme and neuroectoderm direct the morphogenetic events that pattern the developing face. 24 For example, upper jaw morphogenesis results from reciprocal signaling between the forebrain, the neural crest and the surface ectoderm. Precise cellular regions of Sonic hedgehog (Shh) expression in the ventral forebrain and fibroblast growth factor 8 (Fgf8) expression in the cephalic ectoderm are termed 'signaling centers' as they convey the signal to the adjacent tissue.…”