Skeleton intervertebral discs. It is not as hard and rigid as bone but is stiffer and less flexible than muscle. Cartilage is composed of specialized cells called chondrocytes that produce a large amount of extracellular matrix composed of Type II collagen (except Fibrocartilage which also contains type I collagen) fibers, abundant ground substance rich in proteoglycan, and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in the relative amounts of these three main components. Unlike other connective tissues, cartilage does not contain blood vessels. The chondrocytes are supplied by diffusion, helped by the
Cartilage and SoxE, analogs of the vertebrate Sox5/6 and Sox9, in the developing cartilage. The cartilage growth pattern is the same than in vertebrate cartilage. In gastropods, the interest lies on the odontophore, a cartilaginous structure that supports the radula. The most studied species regarding to this particular tissue is "Busycon canaliculatum". The odontophore is a vesicular cell rich cartilage, consisting on vacuolated cells containing myoglobin, surrounded by a low amount of extra cellular matrix containing collagen. The odontophore contains muscle cells along with the chondrocytes in the case of "Lymnaea" and other mollusks that graze vegetation. Sabellid polychaetes The Sabellid