Bone Histology

Vocabulary flashcards covering the histology of bone tissue, including its cellular components, matrix structure, types of ossification, skeletal remodeling, and joint classifications based on Dr. Ramez Zaid's lecture notes.

Bone (osseous) tissue

A specialized C.T. that form the main constituent of the adult skeleton, providing support, protection for vital organs, and harboring the bone marrow.

Inorganic matter

Consists of approximately $50\%$ of the dry weight of bone matrix, primarily composed of Calcium hydroxyapatite crystals and non-crystalline calcium and phosphate.

Hydration shell

A layer of water and ions surrounding hydroxyapatite crystals that facilitates the exchange of ions between minerals and body fluids.

Organic matter

Components of the bone matrix including $\text{Type I collagen}$ (which appears pink on H&E slides) and ground substance containing proteoglycans and glycoproteins like osteonectin and osteocalcin.

Osteoblasts

Cells that synthesize and secret the organic components of the bone matrix (osteoid); they are located at external and internal surfaces and can be active (cuboidal/columnar) or inactive (flat).

Osteoid

A layer of new uncalcified matrix secreted by active osteoblasts at the cell surface in contact with existing bone matrix.

Osteocytes

Flat, almond-shaped cells derived from osteoblasts, enclosed within lacunae, and possessing long dendritic processes that communicate via gap junctions through canaliculi.

Osteoclasts

Very large, motile, multinucleated cells formed by the fusion of bone marrow-derived cells that play a major role in bone resorption.

Howship lacunae

Depressions or cavities in the bone matrix, also known as resorption cavities, where osteoclasts reside during bone resorption.

Ruffled border

Folded irregular projections on the surface-facing bone matrix of active osteoclasts, surrounded by a clear cytoplasmic zone rich in actin filaments.

Periosteum

An external covering layer of bone consisting of an outer dense connective tissue layer and an inner cellular layer of osteoblasts and mesenchymal stem cells.

Sharpey's fibers

Bundles of periosteal collagen fibers (perforating fibers) that penetrate the bone matrix to bind the periosteum to the bone.

Endosteum

A very thin layer composed of osteoblasts and osteoprogenitor cells that lines the internal trabeculae projecting into marrow cavities.

Compact (cortical) bone

Dense bone areas without cavities that constitute $80\%$ of the total bone mass.

Cancellous (trabecular or spongy) bone

Bone composed of numerous interconnecting cavities, constituting $20\%$ of the total bone mass.

Diploë

The thicker layer of spongy bone situated between two layers of compact bone (plates) in flat bones.

Lamellar bone

The bone tissue of adults characterized by multiple layers (lamellae) of calcified matrix organized parallel or concentrically around a central canal.

Osteon (Haversian system)

A complex of concentric lamellae surrounding a small canal containing blood vessels, nerves, and endosteum, forming a cylinder parallel to the long axis of the diaphysis.

Volkmann's canals

Transverse perforating canals that allow Haversian canals to communicate with the marrow cavity, the periosteum, and each other.

Woven bone

The first bone tissue to appear in embryonic development and fracture repair, characterized by random disposition of $\text{Type I collagen}$ fibers and lower mineral content.

Intramembranous ossification

A process where osteoblasts differentiate directly from mesenchyme and begin secreting osteoid, forming most flat bones.

Endochondral ossification

A process where a pre-existing hyaline cartilage model is eroded and invaded by osteoblasts to form bone.

Epiphyseal plate

The region of cartilage connecting the epiphyses to the diaphysis responsible for the growth in length of long bones.

Parathyroid hormone (PTH)

A hormone stimulated by low blood $Ca^{2+}$ that acts indirectly on osteoclasts by binding to osteoblasts to promote bone resorption.

Calcitonin

A hormone stimulated by elevated blood $Ca^{2+}$ that acts directly on osteoclasts to slow matrix resorption.

Osteoporosis

A condition characterized by an imbalance in bone turnover where resorption is much greater than formation, frequently seen in the elderly and postmenopausal women.

Rickets

A condition in children caused by calcium deficiency resulting in slow growth and weak, bowed legs due to incomplete calcification of the matrix.

Synarthroses

Joints where very limited or no movement occurs, including synostoses, syndesmosis, and symphyses.

Diarthroses

Joints that allow great mobility, featuring a capsule enclosing a joint cavity containing lubricating synovial fluid.

Type A synoviocytes

Macrophage-like cells found in the synovial membrane that are important in phagocytosis.

Type B synoviocytes

Fibroblast-like cells in the synovial membrane responsible for the production of hyaluronic acid.