The function of bone lining cells is not well clear, but these cells seem to play an important role in coupling bone resorption to bone formation [ 9].
Bone lining cell are the compressed and elongated inactive osteoblasts and cover the resting bone surface.
At the end of the bone formation period, osteoblasts become flattened lining cells, are embedded into the bone matrix as osteocytes or die by apoptosis.
Particularly, flattened bone-lining cells are thought to be quiescent osteoblasts that form the endosteum on trabecular and endosteal surfaces and underlie the periosteum on the mineralized surface.
Bone lining cells functions are not completely understood, but it has been shown that these cells prevent the direct interaction between osteoclasts and bone matrix, when bone resorption should not occur, and also participate in osteoclast differentiation, producing osteoprotegerin (OPG) and the receptor activator of nuclear factor kappa-B ligand (RANKL) [ 14, 53].
Bone-lining cells were arranged along the surface of the bone (Fig. 2a).
Moreover, the bone lining cells, together with other bone cells, are an important component of the BMU, an anatomical structure that is present during the bone remodeling cycle [ 9].
In healthy joints, RANKL expression has been described in bone lining cells of osteoblast lineage, synovial T cells, and chondrocytes [ 9, 15- 19], whereas in inflamed arthritic joints, it is detected in synovial fibroblasts, T and B cells, osteoclasts, and chondrocytes [ 6, 20- 23].
The cycle is completed by coordinated actions of osteocytes and bone lining cells [ 10, 11].
