Bone Structure, Function and Dynamics

• Bone is hard due to calcification occurring shortly after the bone matrix is created
• Bone acts as a mineral reservoir, especially for calcium
• Compact and cancellous (spongy) bone are the two types of bone
• Bone contains hollow spaces called the marrow cavity, which houses bone marrow
• Bone marrow is a hemopoietic organ
• Bone cells include osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts which derive from bone marrow

Bone Functions

• Bones support and protect organs like the brain, spinal cord, lungs, and heart
• Bones contain the marrow cavity, which houses bone marrow that's a hemopoietic organ
• Bones act as levers for muscles, multiplying the force for movement
• Bones store minerals; for example, bones store about 99% of the body’s calcium

Bone Dynamics

• Bone is a dynamic tissue that constantly changes shape based on stress
• Pressures lead to bone resorption
• Tension results in new bone development

Periosteum

• The external surface is covered by periosteum, except at synovial articulations
• The periosteum consists of an outer dense fibrous connective tissue layer made of collagen fibers known as Sharpey fibers
• Periosteum consists of an inner cellular layer containing osteoprogenitor (osteogenic) cells and occasional osteoblasts

Endosteum

• The central cavity of a bone is lined with endosteum
• The endosteum is a specialized thin connective tissue composed of a monolayer of osteoprogenitor cells and osteoblasts

Cells of Bone

• Osteoprogenitor cells differentiate into other types of bone cells
• Osteoblasts secrete and calcify bone matrix; they become quiescent and known as osteocytes when surrounded by matrix
• Osteocytes are housed in spaces known as lacunae
• Osteoclasts are multinucleated giant cells that resorb and remodel bone

Bone Preparation

• Bone is a hard tissue so methods are employed to prepare it for study
• Decalcified sections involve decalcifying bone in an acid solution, retaining its shape but making it flexible
• Ground sections involve sawing undecalcified bone into thin slices
• Ground sections are followed by grinding the sections with abrasives between glass plates

Osteoprogenitor Cells

• Osteoprogenitor cells are derived from embryonic mesenchymal cells and retain the ability to undergo mitosis
• Osteoprogenitor cells are located in the inner cellular layer of periosteum, haversian canals, and the endosteum
• Osteoprogenitor cells can differentiate into osteoblasts, and may differentiate into chondrogenic cells under low oxygen tension
• Osteoprogenitor cells have a spindle shape and a pale-staining oval nucleus, with scant pale-staining cytoplasm
• Osteoprogenitor cells are most active during intense bone growth

Osteoblasts

• Osteoblasts synthesize the organic matrix of bone and have receptors for parathyroid hormone
• Osteoblasts are located on the bone surface in a sheet-like arrangement of cuboidal to columnar cells
• When actively secreting matrix, osteoblasts exhibit a basophilic cytoplasm

Osteocytes and Lacunae

• Osteoblasts exocytose secretory products, each cell surrounded by bone matrix
• The resulting cell is called an osteocyte
• The space occupied by an osteocyte is a lacuna
• Most bone matrix calcifies, but osteoblasts and osteocytes are separated from the calcified substance by a noncalcified layer: the osteoid
• Surface osteoblasts that cease to form matrix become bone-lining cells reverting to a flattened-shaped quiescent state
• Bone-lining cells are similar to osteoprogenitor cells, but are likely incapable of cell division; they can be reactivated with proper stimulus

Osteocytes

• Mature bone cells originate from osteoblasts that became trapped in their lacunae
• Osteoblasts transform into osteocytes becoming flat, lenticular-shaped cells trapped in their lacunae within the calcified bony matrix
• Canaliculi are narrow, tunnel-like spaces radiating from the lacuna that house cytoplasmic processes of the osteocyte
• Canaliculi contain extracellular fluid with nutrients and metabolites sustaining the osteocytes
• The space between the osteocyte and the walls of the lacuna and canaliculi is the periosteocytic space, occupied by extracellular fluid
• Osteocytes can release osteoprogenitor cell recruiting factors to assist in bone remodeling

Osteoclasts

• Multinucleated cells originate from granulocyte-macrophage progenitors (preosteoclasts)
• Osteoclasts are responsible for resorbing bone and undergo apoptosis after finishing the process
• Osteoclasts are large, multinucleated cells with as many as 50 nuclei and an acidophilic cytoplasm
• Osteoclasts occupy shallow depressions on bone surface called Howship lacunae (resorption bays)

Bone Matrix Composition

• Bone matrix has inorganic and organic constituents

Inorganic Component

• The inorganic component constitutes about 65% of the bone's dry weight
• Composed of calcium and phosphorus, along with bicarbonate, citrate, magnesium, sodium, and potassium

Organic Component

• The organic component constitutes approximately 35% of the bone's dry weight
• The predominant organic component is type I collagen, with small amounts of type V, VII, XI, and XII collagen
• Collagen accounts for about 80% to 90% of the organic component
• Collagen is formed in large (50-70 nm in diameter) highly cross-linked bundles, and causes the matrix to be acidophilic

Bone structure

• Bones are classified by anatomical shape, includes long, short, flat, irregular, sesamoid
• Long bones have a shaft between two heads (e.g. tibia)
• Short bones have similar width and length (e.g. carpal bones)
• Flat bones are thin and plate-like (e.g. skull bones)
• Irregular bones have shapes that don't fit listed categories (e.g. sphenoid and ethmoid of skull)
• Sesamoid bones, shaped like sesame seeds, which develop within tendons, where they increase the mechanical advantage for the muscle (e.g. patella)

Bone Examination

• The femur can be examined to observe cut longitudinal section revealing two different types of bone structure
• Compact bone appears very dense on the surface
• Cancellous bone is also known as spongy or trabecular bone, that's on the inside, that lines the marrow cavity

Details of Trabeculae

• Trabeculae/spicules are composed of bone lamellae arranged randomly
• Trabeculae form the internal surface of compact bone, protruding into the marrow cavity, and form cancellous bone
• Bone lamellae contain lacunae housing osteocytes, which are nourished by marrow diffusion

Haversian and Volkmann Canals

• Haversian canals are microscopic tubes or tunnels in cortical bone that house nerve fibers and capillaries
• Volkmann's canals are small channels in bone that transmit blood vessels from the periosteum into the bone to communicate with the haversian canals

Bone Lamellae

• Compact bone consists of closely packed osteons or haversian systems
• Osteon consists of a central osteonic (haversian) canal surrounded by concentric rings (lamellae) of matrix
• Bone cells (osteocytes) are located in lacunae between the rings of matrix

Bone anatomy of immature bone

• The shaft of a long bone is called the diaphysis, the articular ends are called epiphyses
• In a growing individual, the diaphysis is separated from the epiphysis by the cartilaginous epiphyseal plate that's resorbed when growth is finished
• The epiphyseal articulating surface is covered with a thin compact bone layer over spongy bone with an articular hyaline cartilage polished covering
• The epiphysis-diaphysis transition is the metaphysis, where spongy bone columns are located whose epiphyseal plate results in bone length growth