Bone cells

Bone tissue consists of five major cell types: Bone lining cells, osteoblasts, osteocytes, osteoclasts and osteogenic cells.

• Bone lining cells are flat, resting inactive osteoblasts or surface osteocytes and helps to maintain the matrix. They cover >80% ⁶ of the cancellous bone surface as a thin continuous layer. Boning lining cells control the ion movement between bone and the body. The outer surface called the periosteal cells and the internal surface is called the endosteal cells. The bone lining cells are in contact with osteoblasts and osteocytes via gap junction.^1,7,8
• Osteoblast form bone tissue by secretion of bone matrix, comprising 4-6% ⁹ of the total bone cells. Osteoblasts have different functions. The main function of osteoblasts is secretion including collagen and calcium binding proteins of unmineralized bone, osteoid. Secondly osteoblasts regulate the activity of osteoclasts. Thirdly, osteoblasts play an important role in calcium homeostasis. After differentiation, an osteoblast becomes a quiescent bone lining cell or becomes an osteocyte when they are surrounded by mineralized matrix or apoptosis.^1,8,10
• Osteocytes are the most common cells in bone, 90-95% ¹¹ of the whole bone. They are embedded in the bone matrix with an osteocyte space called the lacunae. The osteocytes network, called canalicular network, connect them to osteoblasts, other osteocytes and bone surface lining cells. Osteocytes also act as mechanosensors. They respond on mechanical load through canalicular fluid flow and transform this into chemical signals, which cause bone resorption by osteoclasts or bone formation by osteoblasts.^2,11
• Osteoclasts are multinucleated cell and derived from mononuclear cells of the hematopoietic stem cell. Osteoclasts resorb damaged or superfluous bone tissue and contribute crucially to bone shape.⁹ Resorbing the bone matrix involves proteolytic cleavage of the organic matrix and dissolution of crystalline hydroxyapatite. Osteoclasts are found into the resorption lacuna. This is an extracellular space between the bone matrix and the ruffled border membrane.^1,8,12
• Osteogenic cells or osteoprogenitor cells found in the membranous endosteum and periosteum. They differentiate into bone lining cells or osteoblasts when stimulated. Osteogenic cells are mitotically active stem cells.¹