Primary cilia exist in a small fraction of cells in trabecular bone and marrow.

Primary cilia are potent mechanical and chemical sensory organelles in cells of bone lineage in tissue culture. Cell culture experiments suggest that primary cilia sense fluid flow and this stimulus is translated through biochemical signaling into an osteogenic response in bone cells. Moreover, in vivo, primary cilia knockout in bone cells attenuates bone formation in response to loading. However, understanding the role of the primary cilium in bone mechanotransduction requires knowledge of its incidence and location in vivo. We used immunohistochemistry to quantify the number of cells with primary cilia within the trabecular bone tissue and the enclosed marrow of ovine cervical vertebrae. Primary cilia were identified in osteocytes, bone lining cells, and in cells within the marrow, but were present in only a small fraction of cells. Approximately 4% of osteocytes and 4.6% of bone lining cells expressed primary cilia. Within the marrow space, only approximately 1% of cells presented primary cilia. The low incidence of primary cilia may indicate that cilia either function as mechanosensors in a selected number of cells, function in concert with other mechanosensing mechanisms, or that the role of primary cilia in mechanosensing is secondary to its role in chemosensing or cellular attachment.