Cartilage structure and growth

Definitions & Key takeaways

Cartilage is a type of connective tissue that is found in the body. It has a smooth, rubbery texture made of chondrocytes embedded in the extracellular matrix, all of which are enveloped by a connective tissue known as the perichondrium. Cartilage is found in different body parts, such as joints, the external ear, and the nose. It helps to support and protect the body's joints and assists with growth. Cartilage major function is to provide structural support to the parts where it's found. The cartilage grows in two major distinct patterns: Appositional and interstitial growth patterns. Appositional cartilage growth occurs when cartilage cells grow from the perichondrium. On the other hand, if the cartilage cells grow from the chondrocytes within the cartilage, it is called interstitial growth.

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Introduction0:00–0:22

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.

Cartilage structure0:22–1:40

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.

Cartilage growth patterns1:40–3:23

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. There are three main types of cartilage - elastic cartilage, hyaline cartilage, and fibrocartilage.

Elastic cartilage3:23–4:06

Elastic cartilage is the rarest type of cartilage in the body. Elastic cartilage has the highest density of chondrocytes and the lowest density of protein fibers.
The protein fibers are mostly loosely organized elastin fibers as well as some type II collagen fibers. Elastic cartilage is the softest and most flexible type of cartilage.
It’s only found in tissue that can be bent or folded repeatedly without damage like the pinnae, which is this external part of your ear, and the epiglottis which is the flap that covers the opening of the larynx when you eat.
Alright, next is hyaline cartilage which is the most common type of cartilage in the body. Hyaline cartilage has a medium density of cells and medium density of protein fiber.

Hyaline Cartilage4:06–5:18

The main protein fibers are type II collagen fibers along with some loosely organized elastin fibers. This makes hyaline cartilage stronger but less flexible than elastic cartilage.
Hyaline cartilage makes up structures like your nose which needs to be stiff enough to let the nostrils stay open, but also flexible enough to let you squeeze them shut as you approach a skunk.
Hyaline cartilage also make up the walls of the larynx, and the half ring shaped tracheal cartilages that hold the trachea open.
It also makes up the articular cartilages which covers the tips of bones that forms a joint. The hyaline cartilage provides a low friction surface so that the bones don’t rub against each other every time you move.
This is also the type of cartilage makes up the costal cartilages that attaches your ribs to the sternum, allowing the chest to expand and recoil during respiration.
Finally the cartilage found in the growth plates are also made up of hyaline cartilage. The third type of cartilage is the fibrocartilage, which has the lowest density of cells and the highest density of protein fiber giving it the greatest tensile strength.

Fibrocartilage5:18–6:00

Unlike elastic and hyaline cartilage which have type II collagen, fibrocartilage chondroblasts secrete type I collagen which is much tougher and also found in bones.
Type I collagen is densely packed and organized in parallel rows, making it resistant to compression and stretching, but also a lot less flexible.
This is why fibrocartilage is found in weight-bearing structures, like the meniscus between the femur and tibia in the knees, and the intervertebral discs between the vertebrae in the spine.

Review6:00–6:32

Alright, as a quick recap. When cartilage grows from its edges or the perichondrium we call it the appositional growth, and if it grows from chondrocytes within the cartilage, it is called the interstitial growth.
Hyaline cartilage is the most common and it’s found mainly between joints, in the nose, larynx, and trachea. Elastic cartilage is the most flexible and it’s found in the external ear and the epiglottis.
Fibrocartilage is the most tough and it’s found in the meniscus and intervertebral discs between the vertebrae in the spine.