Type I collagen is the most abundant protein in the human body. Which feature distinguishes it ultrastructurally from Type II collagen on electron microscopy?

• A Type I has a 67 nm (D-period) banding pattern with a 40 nm gap zone; Type II has no banding and is amorphous
• B Type I forms a network meshwork (non-fibrillar); Type II forms parallel fibers
• C Both types show identical D-period banding but differ only in hydroxylysine glycosylation patterns
• D Type I forms thick, parallel fibrils (larger diameter) in dermis and bone; Type II forms thin fibrils in an amorphous matrix (cartilage) ✓

Explanation

Type I collagen (found in dermis, bone, tendon, ligament, sclera) forms large-diameter, tightly packed, parallel fibrils visible on EM with a characteristic 67 nm D-period banding (due to the quarter-stagger arrangement of tropocollagen molecules). Type II collagen (hyaline and elastic cartilage, vitreous humor) forms much thinner, randomly oriented fibrils embedded in a rich proteoglycan matrix, making them difficult to resolve individually on EM — they appear fine and amorphous. This difference underlies the tensile strength of dermis/bone versus the compressive resistance of cartilage.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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