Short answers, pulled from the story.
Cartilage is composed of specialized cells called chondrocytes that produce a collagenous extracellular matrix and an abundant ground substance rich in proteoglycan and elastin fibers. Its matrix contains glycosaminoglycans, proteoglycans, collagen fibers, and sometimes elastin. The main proteoglycan is aggrecan and the main collagen is collagen type II.
Cartilage is classified into three types: elastic cartilage, hyaline cartilage, and fibrocartilage. They differ in their relative amounts of collagen and proteoglycan. The meniscus of the knee is a fibrocartilage structure, while articular cartilage is a subset of hyaline cartilage.
Cartilage heals slowly because chondrocytes are bound in lacunae and cannot migrate to damaged areas. Hyaline cartilage also has no blood supply, so new matrix is deposited slowly, and cartilage-specific inflammation involving M1 and M2 macrophages and mast cells further hinders complete healing.
Cartilage does not contain blood vessels or nerves, which makes it insensitive. Nutrition reaches the chondrocytes by diffusion, aided by fluid flow generated when articular cartilage is compressed or elastic cartilage is flexed. Some fibrocartilage, such as the meniscus of the knee, has partial blood supply.
Osteoarthritis is a disease of the whole joint in which articular cartilage thins and eventually wears away completely, resulting in bone-against-bone contact, reduced motion, and pain. It affects joints exposed to high stress and is treated by arthroplasty, replacing the joint with a synthetic one made of a cobalt chromoly stainless steel alloy and ultra-high-molecular-weight polyethylene.
Cartilaginous fish, the Chondrichthyes, which include sharks, rays, and chimaeras, have a skeleton composed entirely of cartilage. Cartilage tissue is also found in some arthropods such as horseshoe crabs, some mollusks such as marine snails and cephalopods, and some annelids like the sabellid polychaetes.
Articular cartilage has an aggregate modulus typically in the range of 0.5 to 0.9 MPa and a Young's modulus typically between 0.45 and 0.80 MPa. These properties are measured using a confined compression test run in creep or relaxation mode, where reaching equilibrium can take hours.