Connective Tissue Structures and Functions

Synovial Membrane

• Synovial membrane consists of fibroblast-like and macrophage-like cells
• Synovial membrane does not have a basement membrane or junctional complexes, despite resembling epithelium
• Synovial membrane blood capillaries are fenestrated, allowing exchange of materials between the blood and synovial fluid

Tendons and Ligaments

• Dense regular connective tissue
• Tendons contain long, parallel bundles of collagen fibers, separating elongated nuclei of fibrocytes
• Thin cytoplasm of fibrocytes is divided into numerous processes extending between adjacent collagen fibers

Fibroblasts

• Have large, active nuclei and eosinophilic cytoplasm
• Cytoplasm tapers off in both directions along the axis of the nucleus
• Nuclei are clearly visible, but the eosinophilic cytoplasmic processes resemble collagen bundles in H&E-stained sections

Assembly of Type I Collagen

• Rod-like triple-helix collagen molecules (300 nm long) self-assemble in a highly organized lengthwise arrangement with overlapping regions
• Collagen fibrils are assembled by the regular overlapping arrangement of subunits
• Fibrils have characteristic cross striations (alternating dark and light bands) when observed in the EM
• Fibrils assemble into larger collagen fibers visible by light microscopy
• Fibers often form into even larger bundles and linked together by other collagens

Articular Cartilage

• Collagen fibers run perpendicular to the tissue surface and then bend gradually to form a broad arch parallel to the surface
• Proteoglycan aggregates bound to hyaluronic acid and collagen fill the space among collagen fibers, binding large amounts of water
• Acts as a biomechanical spring in articular cartilage
• When pressure is applied, water is forced out of the cartilage matrix into the synovial fluid
• When pressure is released, water is attracted back into the interstices of the matrix, facilitating nutrient exchange and waste removal

Hyaline Cartilage

• Found in articular surfaces of movable joints, walls of large respiratory passages (nose, larynx, trachea, bronchi), ribs and sternum, and epiphyseal plates of long bones
• The dry weight of hyaline cartilage is 40% collagen embedded in a firm, hydrated gel of proteoglycans and structural glycoproteins
• Most of the collagen is type II, though minor types are present
• Aggrecan (250 kD) is the most abundant proteoglycan, with 150 GAG side chains of chondroitin sulfate and karatan sulfate
• Cartilage cells and matrix often shrink during preparation, resulting in irregular shapes and retracted chondrocytes from the matrix.
• Chondrocytes respire under low-oxygen tension
• Glucose is metabolized by anaerobic glycolysis

Perichondrium

• Dense connective tissue covering hyaline cartilage
• Contains collagen type I and fibroblasts
• Inner layer contains progenitor cells for chondroblasts, which divide to differentiate into chondrocytes

### Elastic Cartilage

• Possesses a network of elastic fibers in addition to collagen type II, giving it a yellowish color in the fresh state
• Found in the auricle of the ear, walls of external auditory canals, auditory tubes, epiglottis, and cuneiform cartilage.
• Elastic fibers are usually demonstrated with orcein or resorcin fuchsin staining
• Perichondrium is similar to that found in hyaline cartilage

Fibrocartilage

• A combination of hyaline cartilage and dense connective tissue, with gradual transitions between the tissues
• Found in intervertebral discs, attachments of certain ligaments, and the pubic symphysis
• Chondrocytes produce a matrix containing type II collagen
• Fibrocartilage often contains a sparse matrix around chondrocytes
• Regions with chondrocytes and hyaline matrix are separated by type I collagen bundles and scattered fibroblasts
• Proteoglycans are less abundant in the matrix, making it more acidophilic than hyaline or elastic cartilage.

This quiz covers key aspects of the synovial membrane, tendons, ligaments, and fibroblasts. Learn about their cellular composition, structural characteristics, and the unique properties of type I collagen assembly. Test your understanding of connective tissue in this focused assessment.