Skeletal System & Bone Development

Functions of the skeletal system

Support, protection, movement, mineral storage, blood cell production, metabolism, and endocrine regulation.

Bone matrix organic components

35% composition consisting mainly of collagen fibers and proteoglycans.

Bone matrix inorganic components

65% composition consisting of hydroxyapatite crystals (calcium phosphate).

Osteogenesis imperfecta

Genetic disorder caused by collagen mutations, such as glycine substitution, leading to fragile bones.

Osteoclasts

Multinucleated cells derived from monocytes that perform bone resorption using a ruffled border and acid.

Osteoblasts

Bone-forming cells that synthesize organic matrix and matrix vesicles for mineralization.

Osteocytes

Mature bone cells housed in lacunae that maintain matrix and sense mechanical strain.

Canaliculi

Tiny tunnels in the bone matrix containing osteocyte cell processes for intercellular communication.

Woven bone

Immature bone with randomly oriented collagen fibers, formed during development and fracture repair.

Lamellar bone

Mature bone tissue characterized by an organized, layered orientation of collagen fibers.

Cancellous (spongy) bone

Porous bone tissue consisting of trabeculae filled with marrow, found inside compact bone.

Cortical (compact) bone

Dense, hard bone tissue forming the protective exterior of bones and containing osteons.

Periosteum

Outer double-layered membrane covering bones, featuring a fibrous outer layer and cellular inner layer.

Sharpey's fibers

Collagen fibers penetrating from the periosteum deep into bone to strengthen tendon attachment.

Endosteum

A cellular membrane lining all internal bone cavities, including trabecular spaces and canals.

Osteon (Haversian system)

The structural unit of compact bone, consisting of concentric lamellae surrounding a central canal.

Volkmann's (perforating) canals

Canals running perpendicular to the long axis of a bone that transmit blood vessels.

Diaphysis

The tubular shaft of a long bone, composed primarily of compact bone.

Epiphysis

The expanded end of a long bone, containing spongy bone and articular cartilage.

Epiphyseal plate vs. line

Plate is hyaline cartilage for growth in length; line is its ossified remnant after growth ceases.

Intramembranous ossification

Bone formation within connective tissue membranes, forming flat bones like the skull and clavicles.

Endochondral ossification

Bone formation that replaces a hyaline cartilage model, forming most bones of the skeleton.

Appositional bone growth

Growth in bone width where osteoblasts lay down bone under the periosteum.

Vitamin D role in bone

Necessary for intestinal calcium absorption; deficiency causes rickets or osteomalacia.

Vitamin C role in bone

Required for collagen synthesis by osteoblasts; deficiency leads to scurvy.

Sesamoid bones

Small bones embedded in tendons, like the patella, that alter muscle pull direction and reduce friction.

Paranasal sinuses

Air-filled spaces in skull bones that decrease skull weight and act as resonating chambers.

Annulus fibrosus

The tough, outer fibrocartilage ring of an intervertebral disk.

Nucleus pulposus

The soft, gelatinous inner core of an intervertebral disk that distributes pressure.

Greenstick fracture

An incomplete fracture where the bone bends and partially breaks, common in children.