Cartilage is an avascular, smooth, and elastic tissue with various forms that allow it to function in a variety of ways.
It can act as a smooth surface for bone articulation within joints, play a role in bone formation, and also provide structural support, such as the cartilage rings in the trachea.
The majority of the tissue consists of extracellular matrix or ECM, and only a relatively small number of chondrocyte cells.
Based on the appearance and specific function, cartilage is separated into three main types: hyaline cartilage, elastic cartilage, and fibrocartilage.
Hyaline cartilage has a “glassy” appearance and can be found on many articular surfaces as well as cartilage that provides structural support.
Elastic cartilage contains a dense network of branching elastic fibers that allow it to be more flexible, which is why it’s often found in structures that need to be firm, yet still flexible.
Some examples include the epiglottis, larynx, and the pinna of the external ear.
And finally, fibrocartilage contains dense connective tissue with collagen fibers, as well as bands of extracellular matrix.
This type of cartilage is found in structures such as the meniscus and intervertebral discs.
Let’s first focus on hyaline cartilage, which is the most common type of cartilage.
This image is an example of hyaline cartilage on the articulating surface of a long bone, with the articulating surface located at the top of the image.
The image on the right is a longitudinal section of the trachea, which is an example of hyaline cartilage providing structural support.
Both images were stained with hematoxylin and eosin or H&E for short. With this stain, the cartilage stains a dark purple due to its strong affinity for hematoxylin.
If we take a closer look at the tracheal hyaline cartilage, it has an outer layer of dense irregular connective tissue surrounding the cartilage called the perichondrium.
The perichondrium is also further separated into an outer fibrous layer and an inner chondrogenic layer.
Within the chondrogenic layer, there are spindle-shaped cells that differentiate into small cells called chondroblasts as they migrate deeper into the cartilage matrix.
These cells will start producing cartilage matrix and eventually mature into chondrocytes.
The chondrocytes are found within small cavities called lacunae that are often arranged in clusters of 2 to 4 cells.
The extracellular matrix, with its “glassy” appearance, is composed of mostly type II collagen and proteoglycan, which is an amorphous gelatinous substance, which is often referred to as ground substance.
Although articular hyaline cartilage is very similar to tracheal hyaline cartilage, a key difference between them is that articular cartilage does not have a perichondrium of connective tissue surrounding it.
The lack of a perichondrium ends up limiting the articular cartilage’s ability to grow.