Bone Structure, Growth, and Remodeling

Flashcards covering bone composition, bone cell types, anatomical regions of long bones, microscopic osteon structures, ossification modes, and regulatory factors in bone remodeling.

Osteoprogenitor Cells

Mesenchymal stem cells located in the endosteum and inner cellular layer of the periosteum that divide to produce osteoblasts.

Osteoblasts

Immature bone-building cells that synthesize and secrete organic bone matrix compound (osteoid) and promote mineralization during osteogenesis.

Osteoid

Uncalcified organic bone matrix secreted by osteoblasts, composed mostly of Type I collagen.

Osteocytes

Mature, non-dividing bone cells trapped inside lacunae that maintain the protein and mineral content of the surrounding bone matrix.

Osteoclasts

Large, multinucleated giant cells derived from monocyte and macrophage stem cell lineages that resorb and break down bone matrix.

Hydroxyapatite

The chief inorganic calcium phosphate mineral of bone with the chemical formula $\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$ or $\text{Ca}_5(\text{PO}_4)_3\text{OH}$, comprising $50\text{--}70\%$ of bone matrix to provide structural strength.

Osteogenesis Imperfecta (OI)

A genetic disorder (brittle bone disease) caused in $90\%$ of cases by dominant mutations in the COL1A1 or COL1A2 genes, leading to a deficiency in Type I collagen.

Epiphysis

The expanded end region of a long bone composed of spongy (cancellous) bone containing red bone marrow responsible for blood cell production.

Metaphysis

The region of a long bone between the epiphysis and diaphysis that contains the epiphyseal growth plate.

Diaphysis

The long, central shaft of a long bone containing compact (cortical) bone surrounding a central medullary cavity filled with yellow bone marrow.

Epiphyseal Plate

A layer of hyaline cartilage in the metaphysis responsible for longitudinal bone growth, which closes and converts into the epiphyseal line between ages $18\text{--}25$.

Periosteum

The outer membrane covering bone that serves a protective function, acts as an attachment point for muscles, and contains osteoblasts for bone growth and healing.

Endosteum

The inner cellular membrane lining the medullary cavity of bone that contains osteoprogenitor cells.

Osteon

The microscopic structural unit of compact bone (also called a Haversian system) consisting of concentric rings of matrix surrounding a central canal.

Lamellae

Concentric rings of calcified bone matrix that form the functional structure of an osteon.

Lacunae

Small cavities arranged along lamellae that contain osteocytes.

Haversian Canal

The central longitudinal canal of an osteon housing blood vessels, lymphatic vessels, and nerves.

Canaliculi

Microscopic canals in bone matrix that contain osteocyte cytoplasmic extensions, enabling nutrient exchange and inter-cellular communication.

Osteosclerosis

A disorder characterized by abnormal hardening and density of bone resulting from an excessive increase in bone matrix.

Endochondral Ossification

The process of bone formation in which a hyaline cartilage model is converted into bone, beginning at $2\text{ months}$ of fetal life in long bones.

Intramembranous Ossification

The direct formation of bone from mesenchymal stem cells without a cartilage model intermediate, forming flat bones and clavicles starting at $8\text{ weeks}$ of intrauterine life.

Appositional Growth

Growth in bone diameter/thickness occurring when new bone matrix is deposited beneath the periosteum while old bone is resorbed along the medullary cavity.

RANKL

Receptor Activator of Nuclear Factor Kappa-B Ligand; a signaling molecule secreted by osteoblasts that binds to RANK on osteoclasts to drive osteoclast differentiation and activation.

Osteoprotegerin (OPG)

A decoy receptor secreted by osteoblasts that binds and sequesters RANKL, thereby inhibiting osteoclast differentiation and bone resorption.

Sclerostin

A protein encoded by the SOST gene and produced by osteocytes that inhibits Wnt signaling, thereby halting osteoblast proliferation and bone formation.

V-ATPase

A proton pump in osteoclasts that utilizes energy from ATP hydrolysis to pump $\text{H}^+$ ions into the resorption space, producing an acid environment for matrix breakdown.

Osteoporosis

A skeletal disorder characterized by reduced bone mass where bone resorption exceeds bone formation ($\text{Resorption} > \text{Formation}$).

Osteopetrosis

A disorder characterized by abnormally dense bones resulting when bone formation exceeds bone resorption ($\text{Formation} > \text{Resorption}$).