Histology Lecture Notes: Cartilage and Bone

Vocabulary practice cards covering the structural features, cell types, growth processes, and joint classifications associated with cartilage and bone tissue as described in Chapters 7 and 8.

Cartilage

A tough and durable form of supporting connective tissue with an extracellular matrix high in GAGs and proteoglycans interacting with collagen and elastic fibers.

Chondrocytes

Mature cartilage cells that occupy matrix cavities called lacunae and maintain the extracellular matrix longterm.

Chondroblasts

Cartilage cells that undergo mitosis and secrete extracellular matrix components; they eventually mature into nondividing chondrocytes.

Perichondrium

A sheath of dense connective tissue surrounding most cartilaginous structures that harbors the blood supply and neural components serving the cartilage.

Hyaline Cartilage

The most common type of cartilage characterized by a homogeneous matrix with type II collagen and aggrecan; found in articular surfaces, respiratory passages, and epiphyseal plates.

Aggrecan

The most abundant proteoglycan of hyaline cartilage, consisting of a core protein of approximately $250\,kDa$ with about 150 GAG side chains of chondroitin sulfate and keratan sulfate.

Chondronectin

A structural multiadhesive glycoprotein in cartilage that binds GAGs, collagen, and integrins, mediating adherence of chondrocytes to the ECM.

Isogenous Aggregates

Small groups of rounded chondrocytes originating from a single chondroblast by mitotic divisions.

Territorial Matrix

The ECM immediately surrounding each lacuna, which is richer in GAGs than collagen and often stains more darkly (basophilic).

Interstitial Growth

Cartilage growth involving the mitotic division of preexisting chondroblasts within the tissue.

Appositional Growth

Cartilage growth involving the differentiation of chondroblasts from progenitor cells in the perichondrium.

Elastic Cartilage

Cartilage characterized by an abundant network of elastic fibers in addition to type II collagen, providing increased flexibility; found in the external ear and epiglottis.

Fibrocartilage

A mixture of hyaline cartilage and dense connective tissue with bundles of type I collagen; it lacks a perichondrium and provides high tensile strength in intervertebral discs.

Osteocytes

Mature bone cells localized in lacunae between lamellae that communicate via canaliculi and serve as mechanosensors.

Osteoblasts

Bone-growing cells synthesized from mesenchymal stem cells that secrete the organic components of the matrix (osteoid) and facilitate calcification.

Osteoclasts

Giant, multinucleated, motile cells derived from fused monocytes that are involved in removing calcified bone matrix and remodeling bone tissue.

Howship Lacunae

Enzymatically etched depressions or resorption cavities on the bone matrix surface where osteoclasts reside during resorption.

Osteoid

A layer of collagen-rich organic material secreted by osteoblasts between the cell layer and the preexisting bone surface before it is mineralized.

Hydroxyapatite

The inorganic mineral of bone matrix with the chemical formula $Ca_{10}(PO_4)_6(OH)_2$ that provides hardness and resistance.

Bone Lining Cells

Flattened cells that cover matrix surfaces after osteoblasts complete synthetic activity, if they do not undergo apoptosis or become osteocytes.

Canaliculi

Small channels (cylindrical spaces) in the bone matrix that house osteocyte dendritic processes and allow the exchange of metabolites with blood capillaries.

Osteocalcin

A vitamin K-dependent polypeptide secreted by osteoblasts that binds $Ca^{2+}$ ions to concentrate the mineral locally during calcification.

Periosteum

An external connective tissue layer of bone with an outer fibrous layer and an inner cellular layer containing osteoprogenitor cells.

Endosteum

A thin layer of connective tissue lining the internal marrow cavities of bone, containing osteoprogenitor cells and osteoblasts.

Compact Bone

The dense area near the surface of bones, also known as cortical bone, representing approximately 80% of total bone mass.

Cancellous Bone

The deeper bone region with interconnecting cavities and spicules (trabeculae), also known as spongy or trabecular bone.

Woven Bone

Newly formed primary bone characterized by randomly arranged collagen fibers and lower mineral content; the first bone tissue to appear in development and fracture repair.

Lamellar Bone

Mature secondary bone organized into discrete sheets of calcified matrix (lamellae) with highly ordered, parallel collagen fibers.

Osteon

The basic functional unit of compact bone comprising concentric lamellae surrounding a central canal (Haversian system).

Perforating Canals

Transverse channels, also known as Volkmann canals, that connect central osteon canals to each other and to the marrow cavity and periosteum.

Intramembranous Ossification

The process by which flat bones form directly within condensed sheets of embryonic mesenchymal tissue.

Endochondral Ossification

The process by which bone forms within a preexisting model of hyaline cartilage, typical of long bone development.

Epiphyseal Plate

A plate of hyaline cartilage between the epiphysis and diaphysis that allows for longitudinal bone growth through a sequence of five distinct zones.

Parathyroid Hormone (PTH)

A hormone that raises low blood calcium levels by indirectly stimulating osteoclasts to resorb bone matrix and release $Ca^{2+}$.

Calcitonin

A hormone produced by the thyroid gland that reduces elevated blood calcium levels by directly targeting osteoclasts to slow matrix resorption.

Synarthroses

Joints that allow very limited or no movement, subdivided into synostoses, syndesmoses, and symphyses.

Diarthroses

Freely mobile joints that unite long bones and feature a joint cavity filled with synovial fluid.

Synovial Membrane

Specialized connective tissue lining the joint capsule of diarthroses that produces synovial fluid through type A and type B cells.

Type A Synovial Cells

Macrophage-like cells in the synovial membrane that remove wear-and-tear debris from the synovial fluid.

Type B Synovial Cells

Fibroblastic cells in the synovial membrane that produce abundant hyaluronan and proteoglycans for the synovial fluid.

Nucleus Pulposus

The central gel-like body of an intervertebral disc that acts as a shock absorber; it is composed of hyaluronan, type II collagen, and residual notochord cells.