Histological organization and its relationship to function in the femur of Alligator mississippiensis

Abstract

Histological analysis of a growth series of alligator femora tests the correlation between strain milieu and microstructure. From mid-diaphyseal cross-sections of these femora (n = 7), vascular canal orientation and density as well as collagen fibre organization were recorded. Throughout ontogeny, the proportion of transverse-spiral (TS) collagen in the dorsal cortex is significantly greater than it is in the ventral cortex (P = 0.008). This regional difference in the proportion of TS collagen is correlated with a regional difference in the state of peak principal strain (compressive or tensile). Nevertheless, the predominant orientation of collagen fibres is longitudinal, which is inconsistent with biomechanical hypotheses that involve peak principal or shear strains. Although the density and orientation of vascular canals do not show significant regional differences (P = 0.26 and P = 0.26, respectively), as with collagen orientation, the vascular canal orientation is predominantly longitudinal. The longitudinal organization of both the vascular canals and the collagen fibres is probably a consequence of longitudinal shifting of subperiosteal osteoid during femoral lengthening. When taken together, these data suggest that growth dynamics is the dominant influence on the histological organization of primary bony tissues in alligator femora.

Fig. 1

Location of quadrants in a stylized femoral cross-section and method of distinguishing orientation of vascular canals. Canals that are parallel to the long axis of the femur are longitudinal (Lg), and those that are more perpendicular to the long axis are transverse (Ts). Hatched regions were not analysed.

Fig. 2

Ontogenetic sampling of the bony tissue from the dorsal quadrant. Scale bars = 100 μm (A–I) and 500 μm (J–L). Superficial is up, deep is down, anterior is to the left, posterior is to the right. Specimens 1–7, general view under circularly polarized light (A,C,E,H,J,K,L). Detailed view under non-polarized light (B,D,F,G,I). Resorption cavity (RC), longitudinal canal (Lg), transverse canal (Ts).

Fig. 3

Ontogenetic sampling of the bony tissue from the ventral quadrant. Scale bars = 100 μm (A–H) and 500 μm (I–L). Superficial is down, deep is up, anterior is to the left, posterior is to the right. Specimens 1–7, general view under circularly polarized light (A,C,E,G,I,J,K). Detailed view under non-polarized light (B,D,F,H,L). Longitudinal canal (Lg), transverse canal (Ts).

Fig. 4

Ontogenetic series of collagen fibre distribution of the alligator femur. Scale bar = 1 mm. Specimen 1 (A), specimen 2 (B), specimen 3 (C), specimen 4 (D), specimen 5 (E), specimen 6 (F) and specimen 7 (G). Darker colours represent more longitudinal collagen fibres, and brighter colours represent more transverse collagen fibres. Medullary and vascular spaces are black. Dorsal is up, and anterior is left.