Key Terms

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articular cartilage: thin layer of cartilage covering an epiphysis; reduces friction and acts as a shock absorber

articulation: where two bone surfaces meet

bone: hard, dense connective tissue that forms the structural elements of the skeleton

canaliculi: (singular = canaliculus) channels within the bone matrix that house one of an osteocyte’s many cytoplasmic extensions that it uses to communicate and receive nutrients

cartilage: semi-rigid connective tissue found on the skeleton in areas where flexibility and smooth surfaces support movement

central canal: longitudinal channel in the center of each osteon; contains blood vessels, nerves, and lymphatic vessels; also known as the Haversian canal

closed reduction: manual manipulation of a broken bone to set it into its natural position without surgery

compact bone: dense osseous tissue that can withstand compressive forces

diaphysis: tubular shaft that runs between the proximal and distal ends of a long bone

diploë: layer of spongy bone, that is sandwiched between two the layers of compact bone found in flat bones

endochondral ossification: process in which bone forms by replacing hyaline cartilage

endosteum: delicate membranous lining of a bone’s medullary cavity

epiphyseal line: completely ossified remnant of the epiphyseal plate

epiphyseal plate: (also, growth plate) sheet of hyaline cartilage in the metaphysis of an immature bone; replaced by bone tissue as the organ grows in length

epiphysis: wide section at each end of a long bone; filled with spongy bone and red marrow

external callus: collar of hyaline cartilage and bone that forms around the outside of a fracture

flat bone: thin and curved bone; serves as a point of attachment for muscles and protects internal organs

fracture: broken bone

fracture hematoma: blood clot that forms at the site of a broken bone

hematopoiesis: production of blood cells, which occurs in the red marrow of the bones

hole: opening or depression in a bone

hypercalcemia: condition characterized by abnormally high levels of calcium

hypocalcemia: condition characterized by abnormally low levels of calcium

internal callus: fibrocartilaginous matrix, in the endosteal region, between the two ends of a broken bone

intramembranous ossification: process by which bone forms directly from mesenchymal tissue

irregular bone: bone of complex shape; protects internal organs from compressive forces

lacunae: (singular = lacuna) spaces in a bone that house an osteocyte

long bone: cylinder-shaped bone that is longer than it is wide; functions as a lever

medullary cavity: hollow region of the diaphysis; filled with yellow marrow

modeling: process, during bone growth, by which bone is resorbed on one surface of a bone and deposited on another

nutrient foramen: small opening in the middle of the external surface of the diaphysis, through which an artery enters the bone to provide nourishment

open reduction: surgical exposure of a bone to reset a fracture

orthopedist: doctor who specializes in diagnosing and treating musculoskeletal disorders and injuries

osseous tissue: bone tissue; a hard, dense connective tissue that forms the structural elements of the skeleton

ossification: (also, osteogenesis) bone formation

ossification center: cluster of osteoblasts found in the early stages of intramembranous ossification

osteoblast: cell responsible for forming new bone

osteoclast: cell responsible for resorbing bone

osteocyte: primary cell in mature bone; responsible for maintaining the matrix

osteogenic cell: undifferentiated cell with high mitotic activity; the only bone cells that divide; they differentiate and develop into osteoblasts

osteoid: uncalcified bone matrix secreted by osteoblasts

osteon: (also, Haversian system) basic structural unit of compact bone; made of concentric layers of calcified matrix

osteoporosis: disease characterized by a decrease in bone mass; occurs when the rate of bone resorption exceeds the rate of bone formation, a common occurrence as the body ages

perforating canal: (also, Volkmann’s canal) channel that branches off from the central canal and houses vessels and nerves that extend to the periosteum and endosteum

perichondrium: membrane that covers cartilage

periosteum: fibrous membrane covering the outer surface of bone and continuous with ligaments

primary ossification center: region, deep in the periosteal collar, where bone development starts during endochondral ossification

projection: bone markings where part of the surface sticks out above the rest of the surface, where tendons and ligaments attach

proliferative zone: region of the epiphyseal plate that makes new chondrocytes to replace those that die at the diaphyseal end of the plate and contributes to longitudinal growth of the epiphyseal plate

red marrow: connective tissue in the interior cavity of a bone where hematopoiesis takes place

remodeling: process by which osteoclasts resorb old or damaged bone at the same time as and on the same surface where osteoblasts form new bone to replace that which is resorbed

reserve zone: region of the epiphyseal plate that anchors the plate to the osseous tissue of the epiphysis

secondary ossification center: region of bone development in the epiphyses

sesamoid bone: small, round bone embedded in a tendon; protects the tendon from compressive forces

short bone: cube-shaped bone that is approximately equal in length, width, and thickness; provides limited motion

skeletal system: organ system composed of bones and cartilage that provides for movement, support, and protection

spongy bone: (also, cancellous bone) trabeculated osseous tissue that supports shifts in weight distribution

trabeculae: (singular = trabecula) spikes or sections of the lattice-like matrix in spongy bone

yellow marrow: connective tissue in the interior cavity of a bone where fat is stored

zone of calcified matrix: region of the epiphyseal plate closest to the diaphyseal end; functions to connect the epiphyseal plate to the diaphysis

zone of maturation and hypertrophy: region of the epiphyseal plate where chondrocytes from the proliferative zone grow and mature and contribute to the longitudinal growth of the epiphyseal plate