The proliferation and differentiation of the bone-lining cell in estrogen-induced osteogenesis

The purpose of this study was to determine the capacity of the bone-lining cell, which covers most nonremodeling bone surfaces in the adult skeleton, to proliferate and contribute to the rapid endosteal osteogenic response following estrogen treatment in male Japanese quail. In control, untreated birds bone-lining cells cover most of the femoral endosteal surface. Bone-lining cells are thinly extended along bone surfaces, have flat nuclei, and account for about 77% of the cells adjacent to the bone surface. The lineal density of bone surface cells in the controls was about 21 cells/mm of bone surface. By 16-20 h after estrogen administration cells with larger, round nuclei were seen on the bone surface and some of these cells contained a 3H-thymidine (3H-TdR) label. Within the first 24 h after estrogen administration the lineal bone surface cell density had increased to about 38 cells/mm. At 20-30 h after estrogen administration, most of the cells adjacent to the bone surface were round. The peak 3H-TdR labeling of cells immediately adjacent to the bone surface occurred about 36 h after estrogen administration. By 48 h the bone surface was covered with osteogenic cells and developing medullary bone. These results suggest that the bone-lining cell in the adult appears to retain some proliferative capability and osteogenic potential. However, because a rapid increase in cell density began prior to the rapid increases in 3H-TdR labeling, as well as the appearance of mitotic figures on the bone surface, another source of cells may have also contributed to the osteogenic response induced by estrogen.