Interrelationship Between Microstructure & Macrostructure in the Growing Hard Palate
Authors: Miller Courtney A. , Steele Ashley , Organ Jason , Menegaz Rachel 
Published: May 2022

Bone strength is the result of microstructure (material properties) and macrostructure (size and shape), and deficiencies in either can produce skeletal fragilities with increased likelihood of injury. The micro‐ and macro‐architecture work together during re/modeling, ensuring that bone is resistant to repetitive loading and fracture. Bone is particularly responsive to loading during the rapid growth occurring in early life. Within the craniofacial complex, the hard palate is a unique structure that undergoes continuous loading due to resting/active tongue pressure and bite force. Because of this, palatal growth is theorized to drive midfacial growth. The aim of this study is to investigate longitudinal changes in palatal micro‐ and macro‐structure in the context of a perturbation to type I collagen (Col1). Col1 is the structural protein that forms osteoid (the precursor to bone), forms a scaffold for bone mineralization, and determines bone toughness. Here we use the OIM mouse (B6C3FE a/a‐Col1a2OIM/J), a strain with a Col1 mutation. We hypothesize that OIM and unaffected wild‐type (WT) mice will be most similar in bone micro‐ and macro‐structure in regions of the palate that experience high loading and therefore necessitate high rates of bone modeling or remodeling to form a structure which can withstand the stress.

Mice were micro‐CT scanned at two timepoints: week 4 (juvenile) and week 16 (adult). BMD was collected at 3 regions (anterior, mid, and posterior palate). Mann‐Whitney U tests were used to compare BMD between genotypes. 28 fixed and 10 sliding landmarks were placed across the palate. Geometric morphologic analyses were conducted to determine variation between genotypes in overall shape and curvature throughout growth.

Average BMD decreased along an anteroposterior gradient for all mice. While juvenile OIM mice had lower palate BMD than WT mice at all regions, adult OIM mice had significantly lower BMD at only the anterior and posterior regions but not the midpalate. Procrustes ANOVA revealed significant differences in palatal shape between the genotypes at both the juvenile (p=0.001) and adult (p=0.0015) stage. Principal component analyses revealed that juvenile OIM mice had a shorter anterior palate and broader palate compared to WT mice, while adult OIM mice had a shorter posterior palate than WT mice. OI mice had flatter palatal arches in the coronal plane as juveniles, and in the midsagittal plane as juveniles and adults.

Results show significant differences in palatal BMD and morphology between genotypes throughout growth, with fewer significant differences at the adult stage. Increased loading of the anterior palate during incisive gnawing may result in the convergence of bone macrostructure among adult mice, while increased strain at the midpalatal suture may necessitate increased BMD even in the presence of a biomineralization defect. Further research is needed to understand the functional significance of the anteroposterior palatal gradient and its relation to soft tissue attachments, particularly given the transition in oral behaviors between juveniles and adults.