Bone Structure and Remodeling
- The skeleton is a large organ formed and degraded throughout life in air-breathing vertebrates.
- The bone matrix is the functional part of bone and is entirely extracellular.
- The organic matrix is mainly composed of collagen, which provides tensile strength.
- The matrix is mineralised by the deposition of hydroxyapatite, which provides compressive strength.
- The combination of collagen and mineral creates a composite material with excellent tensile and compressive strength.
- Bone is constantly reshaped by osteoblasts and osteoclasts.
- Osteoblasts produce and secrete matrix proteins and transport mineral into the matrix.
- Osteoclasts break down bone tissue.
- Bone remodeling is essential for maintaining bone health and integrity.
- Imbalance between bone formation and bone resorption can lead to conditions like osteoporosis.

Osteoblasts
- Osteoblasts are the major cellular component of bone.
- They arise from mesenchymal stem cells.
- Osteoblasts synthesize the organic matrix before mineralization.
- Active osteoblasts are cuboidal cells involved in bone synthesis.
- Inactive osteoblasts are flattened cells when bone formation is not actively occurring.
- Osteoblasts are bone-forming cells that play a crucial role in bone development and repair.
- They are derived from mesenchymal stem cells and are responsible for synthesizing and secreting the organic components of bone matrix.
- Osteoblasts produce collagen type I, which provides tensile strength to the bone.
- They also secrete osteocalcin and osteopontin, proteins that link the organic and mineral components of bone.
- Osteoblasts are connected to each other through tight junctions and gap junctions, allowing them to function as a unit.

Osteoclasts
- Osteoclasts are multinucleated cells derived from hematopoietic progenitors.
- They break down bone tissue.
- Osteoclasts, along with osteoblasts and osteocytes, form the structural components of bone.
- Mesenchymal stem cells and blood vessels are essential for osteoblast bone formation.
- Imbalance between bone formation and bone resorption can lead to osteoporosis.

Osteogenesis
- Bone can be formed through endochondral ossification or intramembranous ossification.
- Endochondral ossification is the process of forming bone from cartilage.
- Intramembranous ossification is the direct ossification of mesenchyme.
- Osteoblast differentiation involves regulatory transcription factors.
- Bone morphogenetic proteins (BMPs) play a crucial role in skeletal differentiation.

Osteocyte Feedback and Other Aspects
- Feedback from physical activity maintains bone mass.
- Feedback from osteocytes limits the size of the bone-forming unit.
- Secretion of sclerostin by osteocytes inhibits a pathway that maintains osteoblast activity.
- Osteon deactivates bone synthesis when it reaches a limiting size.
- Osteocyte feedback plays a crucial role in bone remodeling and maintaining bone homeostasis.
- Osteoblasts have specific morphology and can be identified through histological staining.
- Various techniques have been developed for the isolation of osteoblasts.
- Osteoblast dysfunction can lead to bone diseases, such as osteoporosis and osteogenesis imperfecta.
- Researchers study osteoblasts to better understand bone formation and remodeling.
- Osteoblasts are used in tissue engineering and have potential clinical applications.