Question 6 of 6

Question 6

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Three cartilage types

• Hyaline cartilage (e.g. articular cartilage, costal cartilage, tracheal rings): a fine, randomly-woven meshwork of type II collagen fibrils embedded in a matrix extremely rich in the proteoglycan aggrecan, which binds huge amounts of water. The trapped water, resisted by the collagen mesh, behaves like a pressurised gel — giving hyaline cartilage excellent compressive resilience and a very low coefficient of friction at joint surfaces, but comparatively modest tensile strength and no fibre bundles large enough to resist tearing forces well.
• Elastic cartilage (e.g. the ear pinna, epiglottis): built on essentially the same hyaline-type matrix, but with a dense, branching network of elastin fibres running through it. This gives elastic cartilage the ability to be bent or deformed repeatedly and to spring back to its original shape, a property neither hyaline nor fibrocartilage has, at the cost of somewhat less load-bearing stiffness.
• Fibrocartilage (e.g. intervertebral discs, menisci, pubic symphysis): matrix dominated by thick, parallel bundles of type I collagen (like tendon) with comparatively little proteoglycan/water content and no perichondrium. The dense collagen bundles give fibrocartilage the highest tensile strength of the three, along its fibre direction, and let it tolerate combined compressive-plus-shear loading (e.g. torsional and axial loads on a disc), but it lacks hyaline cartilage's smooth, low-friction, purely compressive cushioning.

The structure–function pattern is the same one as bone's composite logic, applied within cartilage itself: the more a tissue's matrix is dominated by hydrated proteoglycan gel confined in a fine collagen mesh (hyaline), the better it resists pure compression and provides a low-friction bearing surface; the more elastin is added to that mesh (elastic cartilage), the more recoverable bending deformation it tolerates; and the more the matrix is replaced by thick, aligned type I collagen bundles at the expense of proteoglycan (fibrocartilage), the more tensile and shear load it can carry, at the cost of the smooth compressive cushioning hyaline cartilage provides.