Osteocytes are the most abundant cell type residing within mature bone tissue, representing approximately 90-95% of all bone cells. These living cells are intricately embedded within the hardened bone matrix. Osteocytes are long-lived and play an important role in bone maintenance. Their presence helps the skeleton adapt and respond to various demands placed upon it.
Formation and Structure of Osteocytes
Osteocytes originate from osteoblasts, the cells responsible for forming new bone tissue. As osteoblasts secrete the bone matrix around themselves, they become enveloped and trapped within this mineralized substance, transforming into mature osteocytes. This embedding process gives osteocytes their distinctive star-like shape, with a relatively small cell body that ranges from 5 to 20 micrometers in diameter.
Extending from the cell body are numerous long, slender, branching cytoplasmic extensions, numbering between 40 and 60 per cell. These processes extend into a network of tiny channels within the bone matrix called canaliculi. The cell body resides in a small, fluid-filled space known as a lacuna. Together, the lacunae and canaliculi form the lacuno-canalicular system. This system allows osteocytes to connect with each other and with osteoblasts on the bone surface, allowing communication and the exchange of nutrients and waste products throughout the dense bone tissue.
Mechanosensing and Bone Remodeling
A primary function of osteocytes is their role as mechanosensors, detecting mechanical forces applied to the skeleton. When physical loads, such as those experienced during exercise or weight-bearing activities, are placed on bone, they create subtle deformations that drive interstitial fluid to flow through the canaliculi. Osteocytes detect this fluid movement, translating the physical stimulus into biochemical signals.
Detecting fluid flow prompts osteocytes to initiate signaling events, orchestrating bone remodeling. They communicate with other bone cells, specifically osteoblasts, which build new bone, and osteoclasts, which break down old or damaged bone. Osteocytes can release signaling molecules like nitric oxide, prostaglandins, and ATP in response to mechanical stimulation, influencing the activity of these bone-forming and bone-resorbing cells. This communication network ensures bone adapts its structure and strength in response to mechanical demands, becoming stronger where more support is needed and remodeling areas of less stress.
Regulation of Mineral Homeostasis
Beyond their role in bone remodeling, osteocytes also contribute to the body’s mineral balance, particularly in regulating calcium and phosphate levels in the bloodstream. While bone serves as the body’s main mineral reservoir, osteocytes play an active part in managing this supply. They can directly influence the release of minerals from the bone matrix into the circulation, especially when blood calcium levels drop.
This process, known as osteocytic osteolysis, involves osteocytes modifying the bone immediately surrounding their lacunae. They facilitate the removal of calcium and phosphate ions from the bone surface, making these minerals available for other physiological needs, such as nerve function and muscle contraction. Osteocytes also produce fibroblast growth factor 23 (FGF23).
Osteocytes as Endocrine Cells
Osteocytes extend their influence beyond the local bone environment by functioning as endocrine cells, meaning they produce and release hormones that travel through the bloodstream to affect distant organs and tissues. This systemic communication highlights their integrated role in the body’s broader physiological systems.
One hormone produced by osteocytes is sclerostin, encoded by the SOST gene. Sclerostin inhibits bone formation by interfering with the Wnt/β-catenin signaling pathway, which is important for osteoblast activity. Another hormone is FGF23, which osteocytes produce to regulate phosphate metabolism, mainly by increasing phosphate excretion and decreasing active vitamin D production in the kidneys. The ability of osteocytes to secrete these hormones demonstrates their role in maintaining whole-body mineral balance and bone mass.
Role in Bone Disease and Aging
The functions of osteocytes are evident when compromised, as seen in bone diseases and during aging. With advancing age, there is a natural decline in osteocyte number and connectivity, coupled with an increase in osteocyte death through processes like apoptosis. This reduction in functional osteocytes can impair the bone’s ability to sense mechanical loads and coordinate remodeling.
When osteocytes cannot effectively detect mechanical stress, the signals to build new bone may be diminished, even under normal loading conditions. This dysfunction contributes to an imbalance in bone remodeling, where bone breakdown may outpace bone formation, leading to conditions such as osteoporosis. The loss of osteocyte signaling pathways, including those involving sclerostin and FGF23, also contributes to skeletal fragility and reduced bone mass seen in the aging skeleton.