Bone: Cell Types Composing the Tissue

Osteoblasts

The main function of the osteoblasts is to secrete the organic components of bone, collectively called osteoid. Osteoid consists partially of type 1 collagen fibers, which are deposited by the osteoblasts in different orientations, giving bone its tensile strength. Also secreted by the osteoblasts is chondroitin sulfate, a type of glycosaminoglycan (GAG) and the primary component of the ground substance of bone. GAGs are sugars that attach to proteins as part of a proteoglycan. In the case of bone, chondroitin sulfate strengthens the protein matrix, giving bone its resistance to compressive stimuli. In addition, osteoblasts secrete hydroxyapatite, a hard calcium-based salt that mineralizes the previously secreted organic matrix. This mineralization gives bone further resistance to compression. Osteoblasts also secrete alkaline phosphatase to create sites for calcium and phosphate deposition, which allows for bone mineral crystals to grow at the site, become mineralized, and form bone. Alkaline phosphatase is present on the osteoblast surface until they either differentiate into osteocytes or bone lining cells. Bone lining cells are inactive osteoblasts that line the surface of all the bony spicules of spongy bone. Once activated, they become osteoblasts and deposit new bone. Their retraction and exposure of the surface of the bone matrix initiates resorption of bone in the dynamic process of bone remodeling.

Osteoblasts communicate using their cytoplasmic processes in a cellular organization called a syncytium. As osteoblasts are continually secreting osteoid around them, they often become surrounded by their own secretion, at which point they are known as osteocytes. Alternatively, osteoblasts can cover the surface of the bone as bone lining cells. Bone lining cells are essentially inactivated osteoblasts and can be reactivated to continue their secretory function. They also play a role in osteoclast recruitment as they cover and uncover the surface of the bone.

Osteoclasts

Osteoclasts are large cells that perform a complementary function to osteoblasts. Derived from a fusion of multiple precursor monocytes (uninucleated cells that can differentiate into various phagocytic cells), osteoclasts are multinucleated and phagocytotic (they possess the ability to ingest and degrade extracellular substances).

Each osteocyte has two surfaces: a smooth surface facing away from the bone and a highly evaginated/ruffled or active surface adjacent to the bone surface that increases the surface area for the secretion of hydrolytic enzymes for digestion of amorphous/inorganic/mineralized bone, leaving exposed the organic collagen fibers. Due to the many secretory granules, osteoclasts must maintain many vesicles (a cell storage organelle), which they secrete through their ruffled border.

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