Cranial Fibrous Connective Tissue Joints and Experiments in their Growth

Cranial sutures and periodontal ligaments (PDL) are fibrous connective tissue articulations found between intramembranous bones and teeth of the vertebrate cranium. Growth and remodeling of these tissues is believed to be partially regulated by biomechanical loading patterns that include stresses relating to chewing. Masticatory advances in the structure and function of the cranium that enabled mammalian‐style chewing is commonly tied to the origins and evolution of this group. Such oral processing is an advantageous feeding behavior that often involves an individual handling a food item in the mouth while it is probed, tested, and reduced. To what degree masticatory overuse or underuse shapes the complexity and ossification around these articulations can be predicted based on prior experimental and comparative work. Here I report on a mouse model system that has been used to experimentally manipulate dietary material properties, label bone formation, and investigate central nervous system proprioceptive tissues of the cranial fibrous joints. All animal use has been conducted in conformance with the FASEB Statement of Principles for the use of animals in research and education. Treatment groups were fed diets of contrasting material properties, such as dry and powderized (with and without pumice added), gelatin, and enhanced pelleted rodent chow. Animals were raised from weaning to adulthood and calvarial suture morphology as well as sites of bone formation within the PDL were compared between groups. Predicted intergroup variation is observed in body mass, masticatory muscle mass, and mandibular corpus size suggesting that masticatory overuse is associated with muscle and bone growth. Cranial suture complexity is not as obviously impacted in this experiment because the masticatory underuse category also had more complex sutures. Regions of bone formation around the molar PDL in the mandibular corpus did not show any clear pattern of contrast between groups. The counterintuitive results from the cranial sutures indicate a confounding factor in this study. One such factor likely includes masticatory stress that results from other types of non‐dietary strains, like chewing on cage surroundings (e.g., metal wires). Nevertheless, treatment groups were different in many morphological aspects and this probably results from the contrasting patterns (either magnitudes, frequency, or both) of chewing behavior that occurs during diets with diverse and enriching material properties. To what degree the mammalian central nervous system receives sensory input from these ligaments that help to coordinate chewing kinematics is unknown. A possible method for answering this question is presented.