Cartilaginous models of prenatal bones are covered by a membrane known as the perichondrium. Inner perichondrial cells differentiate into chondrocytes so that, in addition to interstitial growth, new cells and matrix may be added superficially in a process known as appositional growth. Cartilage may acquire numerous elastic or collagen fibres to become elastic cartilage or fibrocartilage, respectively. But cartilage also contains some unusual minor types of collagen, such as Type M collagen, which is much shorter than other collagen molecules, and has a helical molecular structure that is stabilized by disulphide bonds. Under the microscope, there are a number of features of cartilage that indicate an animal's physiological age.
Appositional growth is the dominant growth process in older animals.
Young chondrocytes are flattened or elliptical with their long axis parallel to the surface of the cartilage. In older animals, chondrocytes form nests or isogenous groups within single lacunae. Unfortunately these things are invisible to the butcher, and seldom examined even by scientists, otherwise they would be very useful in judging a carcass. Bones under the microscope Section of very young bone, with marrow cavity towards the top, and calcified tissue stained red. Oxygen, nutrients and waste products may travel to and from the chondrocytes in cartilage by diffusion through the surrounding matrix but, when the matrix becomes ossified by the deposition of submicroscopic hydroxyapatite crystals, diffusion is greatly reduced.