Articular cartilage (ar-TIK-yuh-ler KAR-tuh-lij): The layer of hyaline cartilage that covers the bone ends at a synovial joint; it is critical for reducing friction and absorbing shock and is unique because it lacks a perichondrium.

Articular surface (ar-TIK-yuh-ler SUR-fis): The smooth end of a bone that forms a synovial joint; it is covered by hyaline cartilage (articular cartilage) which lacks a perichondrium and is essential for low-friction movement.

Canaliculi (ka-nuh-LIK-yoo-ly): Extremely fine, radial channels or tunnels radiating out from the lacunae in compact bone; they house the osteocyte processes and allow for nutrient and waste exchange.

Chondrocyte (KON-droh-syt): The mature cell of cartilage, responsible for synthesizing and maintaining the matrix (cartilage); these cells reside within spaces called lacunae.

Chondrogenic perichondrium (kon-droh-JEN-ik per-ih-KON-dree-um): The inner cellular layer of the perichondrium, located adjacent to the cartilage matrix; it contains osteoprogenitor cells that can differentiate into new chondrocytes for appositional growth.

Compact bone (Cortical bone) (KOM-pakt bohn/KOR-tih-kul bohn) Dense bone tissue found on the outer surfaces of most bones; it is organized into tight, parallel cylindrical units called osteons.

Concentric lamellae (kun-SEN-trik luh-MEL-ee): The ring-like layers of bone matrix that surround the osteonic canal; the osteocytes are found between these layers.

Elastic cartilage (ih-LAS-tik KAR-tuh-lij): A type of cartilage characterized by a matrix containing abundant, dark-staining elastic fibers, which makes it highly flexible and resilient; found in the external ear and epiglottis.

Endosteal cell (en-DOS-tee-ul sell): A general term for the flattened, resting cells lining the endosteum (e.g., osteoprogenitor cells and resting osteoblasts).

Endosteum (en-DOS-tee-um): A thin, cellular membrane lining the inner surfaces of bone, including the marrow cavity and the osteonic canals; it contains osteoprogenitor cells and osteoblasts.

Epiphyseal plate (Growth plate) (ih-PIF-ih-see-ul playt): A specialized plate of hyaline cartilage located between the epiphysis and diaphysis of a growing long bone; it is the site of longitudinal bone growth through endochondral ossification.

Fibrocartilage (fy-broh-KAR-tuh-lij): A type of cartilage characterized by thick, parallel bundles of type I collagen fibers interspersed with sparse rows of chondrocytes within lacunae; it is the toughest cartilage, providing resistance to tension and compression, found in intervertebral discs and meniscus.

Fibrous perichondrium (FY-brus per-ih-KON-dree-um): The outer layer of the perichondrium, composed primarily of dense irregular connective tissue; it contains fibroblasts and blood vessels.

Hematopoietic cells (hee-muh-toh-poy-ET-ik sells): The general term for the various precursor and stem cells responsible for the production of all blood cells (red, white, and thrombocytes); they are found within the red bone marrow.

Hyaline cartilage (HY-uh-lin KAR-tuh-lij): The most common type of cartilage, characterized by a smooth, translucent, glassy matrix; it provides flexible support and reduces friction, found primarily in the nose, trachea, ribs, and on articular surfaces.

Hyaline cartilage model (HY-uh-lin KAR-tuh-lij MOD-ul): A miniature, fetal structure made of hyaline cartilage that serves as the template for most long bones; it is gradually replaced by bone tissue during endochondral ossification.

Interstitial lamellae (in-ter-STISH-ul luh-MEL-ee): Remnants of old osteons that lie in the spaces between intact osteons in compact bone; they represent bone tissue undergoing remodeling.

Intervertebral disc (in-ter-VER-tuh-brul disk): A pad of fibrocartilage that lies between adjacent vertebrae; it functions as a shock absorber and allows slight movement in the vertebral column.

Isogenous nest (eye-SAH-juh-nus nest): A cluster of two to eight chondrocytes residing within a single lacuna; this cluster is formed by the mitotic division of a single progenitor chondrocyte.

Lacuna (luh-KOO-nuh): A small, non-staining space or cavity within the rigid matrix (cartilage) or bone; it is the physical space occupied by a chondrocyte or osteocyte, respectively.

Marrow cavity (MER-oh KAV-uh-tee): The central cavity within the long bones and the spaces within spongy bone; it contains either red bone marrow (hematopoietic cells) or yellow bone marrow (adipose tissue).

Matrix (cartilage) (MAY-triks): The extensive, firm extracellular material secreted by chondrocytes; it is composed of collagen fibers, elastic fibers (in elastic cartilage), and abundant ground substance (proteoglycans) which gives cartilage its resilience.

Megakaryocyte (meg-uh-KER-ee-oh-syt): A very large, multinucleated cell found in the bone marrow; it is responsible for thrombopoiesis (shedding fragments of its cytoplasm to form thrombocytes).

Osteoblast (OS-tee-oh-blast): A cuboidal or columnar cell found on the surface of developing or remodeling bone; its function is to synthesize and secrete the unmineralized organic bone matrix (osteoid).

Osteoclast (OS-tee-oh-klast): A very large, multinucleated cell derived from fused blood monocytes; it is found in shallow depressions on the bone surface and functions to resorb (break down) old or damaged bone matrix.

Osteocyte (OS-tee-oh-syt): The mature bone cell, derived from an osteoblast that has become completely surrounded by matrix (bone); it resides in a lacuna and maintains the surrounding bone tissue via its cellular processes.

Osteocyte process (OS-tee-oh-syt PRAH-ses): Long, slender cytoplasmic extensions of an osteocyte that extend into canaliculi; they allow neighboring osteocytes to communicate and exchange nutrients via gap junctions.

Osteoid (OS-tee-oyd): The unmineralized organic component of bone matrix secreted by osteoblasts; it is composed primarily of type I collagen and ground substance.

Osteon (Haversian System) (OS-tee-on/hah-VER-zhun sis-TEM): The fundamental, cylindrical structural unit of compact bone; it consists of a central osteonic canal surrounded by concentric layers of bone matrix (concentric lamellae).

Osteonic canal (Haversian canal) (os-tee-ON-ik kuh-NAL/hah-VER-zhun kuh-NAL): The central canal of an osteon; it contains blood vessels, nerves, and loose connective tissue.

Osteoprogenitor cell (os-tee-oh-proh-JEN-uh-ter sell): A undifferentiated stem cell found in the periosteum and endosteum of bone; its function is to divide and differentiate into osteoblasts.

Perforating canal (Volkmann’s canal) (PUR-fuh-ray-ting kuh-NAL/VOLK-muhns kuh-NAL): A transverse or oblique channel that runs perpendicular to the osteonic canals; it connects adjacent osteons and transmits blood vessels and nerves from the periosteum or marrow cavity.

Periosteal bone collar (per-ee-OS-tee-ul bohn KOL-er): A thin layer of bone deposited by osteoblasts from the periosteum that encircles the diaphysis of the hyaline cartilage model; its formation precedes the breakdown of cartilage and the development of the primary ossification center.

Periosteum (per-ee-OS-tee-um): A dense layer of vascular connective tissue that covers the outer surface of bone, except where articular cartilage is present; it contains an outer fibrous layer and an inner osteoprogenitor cell layer and is essential for bone growth and repair.

Primary ossification center (PRY-mer-ee os-ih-fih-KAY-shun SEN-ter): The first site where bone matrix is deposited and mineralized; it forms in the center of the hyaline cartilage model’s diaphysis (shaft) during bone development.

Red bone marrow (red bohn MER-oh): Hematopoietically active tissue found in the marrow cavity of flat bones and the epiphyses of long bones; it contains hematopoietic cells and reticular tissue.

Ruffled border (RUF-uld BOR-der): The highly folded and invaginated plasma membrane on the side of the osteoclast facing the bone matrix; this region secretes acid and lysosomal enzymes to dissolve mineralized bone.

Secondary ossification center (SEK-un-der-ee os-ih-fih-KAY-shun SEN-ter): An area of bone formation that develops later than the primary center; it forms in the epiphyses (ends) of long bones and is responsible for the ossification of the bone ends.

Spongy bone (Cancellous/Trabecular bone) (SPUN-jee bohn/KAN-suh-lus/truh-BEK-yuh-lur bohn): Porous bone tissue found in the interior of most bones; it consists of a latticework of interconnected bone struts (trabeculae) that enclose spaces filled with red bone marrow.

Synovial joint (sih-NOH-vee-ul joynt): A highly mobile joint where the bone ends are covered by articular cartilage and separated by a joint cavity containing synovial fluid; it is encapsulated by a fibrous layer and a lining synovial membrane.

Tide mark (tyd mark): A distinct, basophilic (blue-staining) line located in the deepest layer of articular cartilage; it separates the vascularized subchondral bone from the overlying avascular cartilage.

Trabecula (pl. Trabeculae) (truh-BEK-yuh-luh/truh-BEK-yuh-lee): The thin, interconnected struts or plates of bone that make up spongy bone; they are lined by endosteum.

Yellow bone marrow (YEL-oh bohn MER-oh): Bone marrow that is predominantly composed of adipose (fat) cells; it is inactive in hematopoiesis and replaces red bone marrow in the diaphysis of adult long bones.

Zone of calcification (growth plate) (zohn uv kal-suh-fih-KAY-shun): The thin region of the epiphyseal plate where the enlarged chondrocytes die, and the surrounding matrix becomes mineralized (calcified), forming rigid struts.

Zone of hypertrophy (growth plate) (zohn uv hy-PER-truh-fee): The region of the epiphyseal plate where the chondrocytes have stopped dividing, become greatly enlarged, and begin to produce alkaline phosphatase, preparing the matrix for calcification.

Zone of ossification (growth plate) (zohn uv os-ih-fih-KAY-shun): The region of the epiphyseal plate closest to the diaphysis; here, osteoblasts deposit osteoid onto the calcified cartilage struts, and osteoclasts resorb the dead cartilage and bone to remodel the primary bone.

Zone of proliferation (growth plate) (zohn uv proh-lif-uh-RAY-shun): The region of the epiphyseal plate where chondrocytes are rapidly dividing, forming characteristic columns of flat cells; this division pushes the epiphysis away from the diaphysis, causing longitudinal growth.

Zone of reserve cartilage (growth plate) (zohn uv ri-ZERV KAR-tuh-lij): The region of the epiphyseal plate closest to the epiphysis; it contains resting chondrocytes that anchor the plate to the bone.