Bone is a specialised form of connective tissue consisting of cells, fibres and extracellular matrix. It differs from other connective tissues in that its extracellular elements are calcified, thus making it a hard, yet elastic substance with high tensile and compression strength. Its protective and supportive functions are often emphasized at the expense of its role in calcium and phosphate homeostasis.
Most bones have a dense outer cortex consisting of compact bone and an inner trabecular region consisting of spongy bone. The cut surface of compact bone appears solid, while the cut surface of spongy or cancellous bone has the appearance of a sponge.
Coetzee et al p91-97
Stevens and Lowe p234-248
Bone cells are osteoprogenitor cells, osteoblasts, osteocytes and osteoclasts. Each cell has a specific function in bone physiology.
Osteoprogenitor cells are derived from mesenchyme. They are the stem cells of bone and form osteoblasts.
Osteoblasts synthesize and secrete the organic component of bone matrix. The abundant rough ER, typical of active protein secreting cells, makes the cytoplasm distinctly basophilic.
Osteocytes are osteoblasts surrounded by mineralized bone and are situated in lacunae. The cells are flattened and spindle shaped and communicate with each other via long cytoplasmic processes. These are found in the canaliculi that connect the lacunae to each other. It is thought that the osteocytes maintain the osteoid.
Osteoclasts are large multinucleated cells derived from monocytes. They play a major role in bone remodeling by enzymatically eroding mineralized bone and releasing the minerals. Osteoclast activity is stimulated by parathormone in response to serum calcium levels and inhibited by calcitonin.
Bone contains a specialised extracellular matrix called osteoid. Osteoid is a Type I collagen embedded in a supporting glycosaminoglycan gel that contains proteins like osteocalcin with a high affinity for calcium. It is synthesized and secreted by osteoblasts and is normally deposited in regular parallel sheets as lamellar bone. After a maturation phase, the osteoblasts release accumulated calcium and phosphate, which forms hydroxyapatite crystals that grow by accretion.
Bone is continually remodeled by new bone deposition and mineralization by active osteoblasts and selective resorption by osteoclasts.
Sheets of mesenchymal cells condense as bone-forming membranes. This process is known as intramembranous ossification and results in the formation of flat bones.
Endochondral ossification involves the transformation of previously formed cartilage into long bones and permits elongation and thickening of the bone during fetal development and childhood growth. Hyaline cartilage develops from immature mesenchymal tissue and assumes the approximate shape of the bone. The cartilage model is subsequently transformed into bone by osteoid deposition and mineralization.
Osteomalacia is the failure of newly developed osteoid to mineralize with an increased tendency of bone fractures. Osteomalacia in growing children leads to a disease called rickets and produces permanent skeletal deformities.
Osteoporosis involves the thinning of bones and especially prevalent among the elderly. The bones are more fragile and prone to fracture. Fracture of the femur neck is common in osteoporotic patients.