Interstitial versus appositional growth of cartilage
Interstitial growth expands cartilage from within, as its cells divide and make new matrix (the firm, gel-like material between the cells), while appositional growth adds layers at the surface.
Remember
Interstitial growth adds cartilage from within by cell division, whereas appositional growth adds it at the surface and, in its classical form, needs a perichondrium (the fibrous sheath around many cartilages). Articular cartilage (the cartilage covering the ends of bones in joints) has no perichondrium on its free surface, so classical appositional growth is not available there.
Memory aid
interstitial is inside-out and appositional is outside-in, like a balloon that expands from within compared with a snowball that gains layers on its outside
Interstitial growth is the balloon that swells from within, and appositional growth is the snowball that gains layers on its outside.
Cartilage can grow in two ways. In interstitial growth (the word points to what lies inside the tissue), a chondrocyte (a cartilage cell) divides inside its lacuna (the small cavity it sits in). The two daughter cells begin by sharing that space, and then each secretes new matrix, which pushes them apart. The tissue expands from the inside.
A small cluster of related chondrocytes whose daughters stay together is called an isogenous group. Isogenous means "of the same origin", so the cluster is evidence of recent division within the tissue.
In appositional growth (apposition means adding on), layers of new cartilage are added at the outside, like coats of paint. The cells doing this job are chondroblasts (young cartilage-forming cells) from the inner cellular layer of the perichondrium, the fibrous sheath that wraps many cartilages.
Where the perichondrium is absent, the surface route cannot operate in the same way. Articular cartilage, which covers the ends of bones in joints, has no perichondrium on its free surface, so the classical surface route is not available there. Fibrocartilage (a tough tissue found in discs) also generally lacks a true perichondrium. Interstitial growth does not need a perichondrium, and it is important in young, soft matrix.
In adults articular cartilage is very limited in how far it can thicken by either route: it has no perichondrium on its free surface, and with maturation the matrix stiffens and cell division falls. Neither route is ossification (the process that turns tissue into bone).
Key facts
- Interstitial growth
- cartilage expands from within as chondrocytes divide in their lacunae and deposit new matrix between the daughter cells.
- Appositional growth
- chondroblasts in the inner perichondrium add new layers of matrix from the outside.
- Isogenous group
- a cluster of related chondrocytes made when one cell divides inside its lacuna, and a sign of recent interstitial division.
- Perichondrium
- wraps many cartilages, but articular cartilage has no perichondrium on its free surface and fibrocartilage generally lacks a true one.
- Adult articular cartilage
- has very limited capacity to thicken by either mechanism.
- Ossification
- the process that turns tissue into bone, which neither growth route describes.
Exam traps
- Appositional growth of cartilage does not come from osteoblasts at a periosteum, because osteoblasts and the periosteum belong to bone. In cartilage, the chondroblasts of the perichondrium supply it.
- The synovial membrane does not take the place of a perichondrium. It makes synovial fluid (the lubricating fluid of the joint) and is not a source of chondroblasts for articular cartilage.
- Lacking a perichondrium does not leave cell enlargement (hypertrophy) as the only option, because interstitial growth, by cell division and matrix secretion, remains possible in immature cartilage.
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