LEE, A.H.*; ESPINOZA, N.; BLOB, R.W.; Univ. of California, Berkeley; Erskine College; Clemson Univ.: Locomotor bone strain and ontogenetic histology of the humerus in Alligator mississippiensis

Recent studies suggest the state and magnitude of strain strongly correlate with the microstructural organization of a limb bone. That correlation has been well tested in bones of animals that grow rapidly but not well in animals that grow more slowly. This study reports measurements of in vivo locomotor bone strain (n = 3) and ontogenetic histology (n = 11) from the mid-diaphyseal humerus of Alligator mississippiensis. We test whether the state of peak principal strain correlates with collagen fiber and vascular canal orientation across humeral cross-sections. In addition, we test for a correlation between shear strain magnitude and collagen fiber and vascular canal orientation. Tensile strains occur on the ventral cortex of the humerus and shift to compressive at more dorsal positions on the cortex. Principal strain orientations and high shear strain magnitudes suggest that the humerus experiences substantial torsion during locomotion. Histology of the ontogenetic series of humeri reveals a dominance of longitudinally oriented collagen fibers and vascular canals. Transversely oriented collagen fibers and canals become more abundant in cortical regions that experience rapid growth. These data suggest that local variations in rate of tissue growth have a dominant influence on the histological organization of primary bony tissues in alligator humeri.