Osteoblasts are specialized bone cells, that play a role in bone formation, hence, these cells also called as bone builder cells.
Osteoblasts express parathyroid hormone (PTH)-receptors on their membrane during bone remodeling.
The main important roles in bone remodeling including expression and production of osteoclastogenic factors (for Osteoclastogenesis), synthesis of bone matrix proteins and mineralisation of bone.
Osteoblastic cells include various population of cells, they are immature osteoblast lineage cells, differentiating osteoblasts and matrix-producing mature osteoblasts.
The various differentiation stages of osteoblast also influence the efficient contribution of these cells to bone remodeling.
The absence of osteoblasts production and differentiation is related to lack of functional osteoclasts in organisms. However, conditional reduction in mature osteoblast number is only associated with lass of bone formation but there is no reduction in osteoclast resorption. Thus, proliferation and differentiation of osteoblasts are very important activities to maintain the bone strength and integrity.
Osteocytes are other important bone cells derived from osteoblast cells. Osteocyte initiates and directs the subsequent bone remodeling process, thus repairs damaged or injured bone.
During bone remodeling (or formation), a sub-cellular population of osteoblasts undergoes terminal differentiation and becomes engulfed by unmineralized bone matrix (osteoid), at that time they are represented as osteoid-osteocytes.
During mineralization of the bone matrix, these cells are embedded with in matrix, at this time, these cells are called osteocytes.
Osteocytes form a complex network that extended throughout mineralized bone. The cell body of the osteocyte contains fluid-filled cavities (ellipsoidal space) called lacunae. While the dendritic processes embedded in tiny cylindrical channels called canaliculi. These cells highly abundant in mineralized bone. Around 90–95% of bone contains these cells in skeletally mature adults.
These long dendrite-like processes spread throughout canaliculi (tunnels) within the mineralized bone matrix. These dendrite-like processes connect with other osteocytes within the mineralized bone and nearby blood vessels and also interact with osteoblast cells on the bone surface.
Osteocytes respond to mechanical strains, and this network is thought to be crucial part in the detection of mechanical load and related bone micro-damages (microscopic cracks or fractures on the mineralized bone).
Osteocytes regulate deposition of minerals and interactions at the bone matrix level, and they also behave as endocrine cells producing factors that target various organs such as the kidney to regulate phosphate transport during mineralization. Hence, osteocyte seems to be a major local specialized factor for many of bone’s functions.
In in-vivo, mechanical strain on bone tissue is a key regulatory factor that fecilitate bone modeling and remodeling through osteocyte network.
Conclusion: Specifically, the main role of osteocytes signaling in skeletal metabolism as follows
1) Sensing mechanical loads on bone.
2) Regulation of systemic mineralization.
3) Regulation of local mineralization.
4) Bone formation by osteoblasts.
5) Bone resorption by osteoclasts.