Biology Quiz on Fibronectin Structures

Fibronectin

• Highly hydrated, observed as empty spaces upon dessication in mounted slides.
• Molecular Weight between 235 - 270 kDa; exists in dimeric form.
• Synthesized by fibroblasts and epithelial cells.
• Contains binding sites for cells, collagen, and glycosaminoglycans.
• Facilitates normal cell adhesion and migration.

Glycosaminoglycans (GAGs)

• Also termed mucopolysaccharides; consist of linear polysaccharides with repeating disaccharide units.
• Comprised of hexosamine (glucosamine or galactosamine) and uronic acids (glucuronic or iduronic acid).
• Heparan sulfate is notable for its high molecular weight (hundreds to thousands of kDa).
• GAGs are intensely hydrophilic, bind cations, and are crucial in maintaining tissue hydration.

Laminin

• Trimeric glycoprotein with a molecular weight of 200 - 400 kDa; possesses a cross-shaped structure.
• Primarily found in the basal laminae, aiding the adhesion of epithelial cells.

Integrin

• Functions as matrix receptors that recognize specific sequences on laminin, fibronectin, and various extracellular matrix proteins.
• Composed of heterodimers with α and β chains; acts as a crucial link between cells and the ECM.

Types of Collagen

• Type IV collagen forms sheet-like structures in basal laminae; does not form fibrils or fibers but exists as unpolymerized procollagen.
• Fibrillar collagens (Types I, II, III) are significant structural proteins that form fibrils.
• Type VII collagen serves as linking/anchoring collagen, connecting fibrillar collagens and ECM components; prevalent across numerous cells.

Collagen Composition

• Major amino acids include glycine (33.5%), proline (12%), and hydroxyproline (10%).
• Tissue collagen content can be quantified via hydroxyproline measurement.
• Unique characteristics include hydroxylation of proline and lysine, yielding hydroxyproline and hydroxylysine.

Elastic Fibers

• Composed of thin, branching fibers that form irregular networks, characterized by their yellow color in fresh samples.
• Capable of stretching up to 150% of their length while readily returning to the original shape (recoil).
• Composed of an amorphous region with elastin and a sheath of 14nm microfibrils.

Elastin

• Secreted as proelastin (molecular weight of 70,000), polymerizes to form elastin.
• Highly resistant to boiling and chemical extraction; not digested by trypsin.
• High in valine and alanine, and contains unique amino acids desmosine and isodesmosine.

Cells of Connective Tissue

• Connective tissue cells support epithelial structures and fill spaces between fibers and muscle sheaths.
• Most notable cell types include fibroblasts and macrophages, responsible for tissue maintenance and immune responses.

Mast Cells

• Oval to round cells (20-30μm), filled with basophilic granules; central, small nucleus.
• Metachromatic properties allow for color change of basic dyes from blue to purple/red.
• Key players in local inflammatory responses, releasing leukotrienes and various bioactive substances (e.g., histamine, heparin).
• Two subtypes: connective tissue mast cells (primarily heparin) and mucosal mast cells (primarily chondroitin).

Dense Connective Tissue

• Similar components to loose connective tissue but dominated by collagen fibers.
• Features fewer cells, with fibroblasts being predominant.
• Types include dense irregular connective tissue (fibers orient randomly) and dense regular connective tissue (provides multi-directional stress resistance).

This quiz covers the properties and structures related to fibronectin, including its hydration status and molecular weight. Participants will explore how fibronectin functions in the body, particularly in relation to fibroblasts and epithelial cells. Test your understanding of this essential protein!