Osteohistological features of UTCM 1557 femur and tibia.

(A) Posterior cortex of the femur of UTCM 1557 under linearly polarized light (left) and circularly polarized light (right). The cortex is composed of an inner cortex of endosteal bone, mid-cortex of CCCB, and outer cortex of avascular parallel-fibered bone. The outer cortex is separated from the mid-cortex by a resorption line. (B) Outer cortex of the antero-lateral side of the femur of UTCM 1557 under linearly polarized light. The mid-cortex is woven bone with secondary osteons and primary osteons. The outer cortex is avascular lamellar bone with a double LAG (black arrow), representing an external fundamental system. The outer cortex is separated from the mid-cortex by a resorption line. A faint line, possibly an additional LAG, is present (white arrow), though this line is not continuous beyond the region imaged. (C) Lateral corner of the femur of UTCM 1557 under linearly polarized light (top) and circularly polarized light (bottom). The mid-cortex is composed of remodeled woven tissue with primary and secondary osteons. The outer cortex at the lateral corner is avascular parallel-fibered tissue. (D) Anterior side of the tibia of UTCM 1557 under circularly polarized light. The mid-cortex on the anterior side is CCCB. The outer cortex is parallel-fibered bone of varying thickness. The outer cortex is separated from the mid-cortex by a resorption line. (E) The outer cortex of the lateral side of the tibia under linearly polarized light. Here, the outer cortex is highly organized parallel-fibered bone with a double LAG present (arrow). The outer cortex is separated from the mid-cortex by a resorption line. (F) Antero-medial corner of the tibia of UTCM 1557 under circularly polarized light (top) and linearly polarized light (bottom). The mid-cortex is composed of woven bone with primary and secondary osteons. Regions of metaplastic tissue are present posteriorly at the corner. The outer cortex is highly organized parallel-fibered bone. All sections stained with toluidine blue. Compacted coarse cancellous bone–CCCB, Endosteal bone–EB, External fundamental system–EFS, Lamellar bone–LB, Growth marks–black arrows, Medullary cavity–MC, Metaplastic tissue–MT, Parallel-fibered bone–PFB, Primary osteon–PO, Resorption line—RL, Secondary osteon–SO, Woven bone–WB.