Cartilage structure and growth

Cartilage is a strong, flexible type of connective tissue that makes up part of your nose, your ear, and provides cushioning between your joints.

Its job is to support and connect various parts of your body, like the costal cartilage that connects your ribs to your sternum or breastbone.

Usually, there’s a layer of connective tissue that wraps around cartilage called the perichondrium.

The perichondrium has an outer layer that contains fibrous connective tissue and blood vessels, and it has an inner layer that contains chondroblasts.

Chondroblasts secrete the proteins that make up the extracellular matrix of the cartilage - which has a gel-like consistency.

Eventually, these chondroblasts get trapped inside the very matrix that they create, in small holes called lacunae. When that happens chondroblasts turn into chondrocytes.

Chondrocytes don’t make much extracellular matrix, instead they maintain and repair the extracellular matrix.

The extracellular matrix is composed of protein fibers like collagen which gives it strength and elastin which gives it flexibility.

And these protein fibers are embedded in a viscous gel, made of water and proteoglycan aggregates which are large molecules that look a bit like a centipede.

A long chain of hyaluronic acid molecules called a hyaluronan makes up the body of this proteoglycan aggregate, and hundreds of proteoglycans make up the legs.

These proteoglycan legs are basically proteins attached to long chains of sugars called glycosaminoglycan or GAGs.

Now, cartilage has two patterns of growth, appositional growth and interstitial growth.

Appositional growth occurs when chondroblasts secrete new matrix along existing surfaces and this causes the cartilage to expand and widen.

In interstitial growth, chondrocytes secrete new matrix within the cartilage and this causes it to grow in length.

Both types of growth can be seen in the growing bones of children and teenagers before they reach adulthood.

In long bones like the femur, there is a region called the metaphysis, which lies between the diaphysis, the shaft of the bone, and the epiphyses, the two ends of the bone.

The metaphysis contains the epiphyseal plate, also known as the growth plate, which has chondrocytes that divided and produce cartilage. This is an example of interstitial growth.

As more and more cartilage is made, the bones lengthen and when the older chondrocytes die off, osteoblasts, or bone making cells, move into the cartilage and start to ossify it, or turn it into bone - by depositing minerals like calcium and phosphate.

While the bones are growing in size, the articular cartilage that covers the tips of the bones also expands by secreting more matrix to cover the expanding bone. This is a mix of appositional and interstitial growth.

By the end of adolescence, the growth plate gets completely ossified and all the cartilage is replaced by bony tissue, and at that point a person can no longer grow taller. The articular cartilage stops expanding but the chondrocytes continue to secrete matrix to repair any damage.