Cell lineage in fracture healing in chimeric bone grafts

Summary

Previous studies have shown that differences in nuclear morphology are generally sufficient to determine the species origin of cells in interspecific grafts between the Japanese quail and domestic chicken. Most quail nuclei possess 1–3 large nucleolus-associated masses of heterochromatin. Chick cells, on the other hand, usually present a more diffuse, stippled distribution of nuclear heterochromatin.

Quail embryonic limb rudiments, some with and some without established marrow cavities, were explanted and grown on the chorioallantoic membrane of the chick. Three to five days post-grafting, the explants were surgically fractured and allowed to heal. Tissues were collected and histologically processed during the latter period.

The fractures healed completely within 5–6 days and no callus was established in the process. The nuclear staining pattern of the osteoblasts and osteocytes throughout the rudiments and at the fracture site indicated that they were derived from the graft. Possible sources for these cells included the periosteum, endosteum, and posthypertrophy chondrocytes. By contrast, most of the nuclei in the osteoclasts were chick-like and were apparently derived from cells originating in the host. Because the quail-like heterochromatin marker was normally present in a small number (2.5%) of chick osteoclast nuclei and was lacking in about 5% of native quail osteoclast nuclei, the precise extent of the participation of donor, i.e., quail bone and marrow stromal cells in osteoclast formation, could not be determined. However, the data suggest that in large measure the precursor cells for most osteoclasts were hematogenously derived and were carried to the grafted rudiments by the blood vascular system.