Cartilages are connective tissues that are tough, flexible, and resilient. They are found in areas of high wear and tear like bone ends, joints, and intervertebral discs. Cartilages in humans (and certain other animals) serve as a skeletal framework in the embryonic stage. With the development of humans, most of the cartilages are replaced by bones. Cartilages are essential as they provide support to the softer tissues and aid in the formation and growth of long bones.
Cartilages are made up of cells embedded inside an extracellular matrix that has a gel-like texture and rigid consistency. This matrix mainly consists of collagen and elastin fibers, proteoglycans, and water. There are three types of cartilages in our bodies namely elastic cartilage, fibrocartilage, and hyaline cartilage.
Here, we will study hyaline cartilage histology where you will learn hyaline cartilage location, hyaline cartilage structure, and hyaline cartilage functions. You can refer to the hyaline cartilage slide for a better understanding.
Hyaline cartilage (also called hyaline tissue or hyaline connective tissue) is a dominant component of the extracellular matrix and the most widespread cartilage.
The word hyaline has origins in the Greek word “hyalos” which means shiny, owing to the glossy and smooth appearance of the cartilage. Hyaline cartilage has a bluish-white, shiny, and transparent appearance.
Hyaline cartilage location in adult humans is at the bone ends of free-moving joints (as articular cartilage), nose, respiratory tract, at the end of ribs, trachea, larynx, and bronchi.
An easy way to remember hyaline cartilage location is with the term “BLANCET” which stands for:
Hyaline cartilage is extremely resilient in nature.
The matrix of hyaline cartilage is composed of collagen.
This cartilage undergoes calcification which is part of bone formation and also of the aging process.
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All cartilages are made up of chondroblasts which are also called perichondrial cells. These cells produce the ECM (or ground substance), chondrocytes, and collagen fibers.
Hyaline cartilage develops from mesenchymal cells. Mesenchymal cells are stem cells present in the bone marrow.
Hyaline cartilage structure is quite simple as it does not have any nerves or blood vessels.
The ECM (extracellular matrix) of hyaline cartilages is a gelatinous, glassy, and homogeneous substance. The ECM is rich in glycosaminoglycans (GAGs). GAGs are long polysaccharides composed of amino sugars and attract potassium and sodium ions. These ions bring along water with them which helps in regulating the water volume inside the ECM.
Hyaline cartilage is rich in:
Type II collagen.
Structural glycoproteins for example chondronectin.
Proteoglycans like aggrecan which is the most abundant proteoglycan in hyaline cartilages.
The aggrecans form noncovalent bonds with long polymers of hyaluronic acid with the help of link proteins.
Chondronectin is a multi-adhesive glycoprotein that binds to glycosaminoglycans which are collagen type II fibers. Chondronectin also binds to integrins and assists chondrocytes adhere to ECM.
Chondrocytes are present in very little amount inside hyaline cartilage mass. They are situated in matrix cavities called lacunae. Lacunae look like small white lakes when seen under a microscope. Chondrocytes are vital for the maintenance and synthesis of ECM components.
Hyaline cartilage also contains type I, IV, V, VI, IX, and XI collagen in small quantities. These collagens further help in strengthening the fibers together.
The cell of these cartilages has one or two nuclei and the protoplasm inside the cell is clear.
The cartilage is surrounded by a membrane known as perichondrium. Since hyaline cartilage has no blood vessels, the perichondrium's role is to provide nutrients to the tissue.
Hyaline cartilages serve as a temporary cartilage model during embryonic development which are important precursors to the development of the appendicular and axial skeleton.
Due to the low amount of fiber in the hyaline tissue, it provides a cushion that acts as a shock absorber at the junction of joints.
Hyaline cartilage provides a smooth surface for joint movements. As the articular cartilage, it can withstand pressure and friction which occur during weight-bearing activities.
Hyaline cartilage trachea allows them to maintain an open position by providing support to softer tissues in that region.
The most important function of hyaline cartilage is providing respiratory support.
Hyaline cartilages are translucent cartilages that are found in places where the bone ends meet joints, nose, larynx, respiratory tract, ribs, and trachea. The main function of hyaline cartilages is to provide cushions and minimize friction between joints during heavy activities.
1. What are the other two types of cartilages in our system apart from hyaline cartilage?
Apart from hyaline cartilage, our body has two more types of cartilages:
This has a high concentration of elastic fibers and is present in ECM. Elastic cartilage has a yellowish appearance and is the most pliable cartilage amongst all cartilages in our body. These cartilages do not calcify. The chondrocytes in elastic cartilage are present in thread-like networks which are made up of elastic fibers. The primary function of elastic cartilage is to provide strength and support. It also helps in maintaining the shapes of certain structures like external ears. Elastic fiber is also present in the epiglottis and middle ear’s auditory tube (eustachian tube).
These cartilages are found mainly in the intervertebral discs and also where the tendon connects to the bone. Fibrocartilages contain extremely large bundles of collagen and are very tough and strong tissues. They are similar to other fibrous tissues but have chondrocytes and cartilage ground substance. These cartilages can resist shear forces and compression. The strength of fibrocartilage comes from having thick and dense collagen layers and alternating layers of hyaline cartilage. Fibrocartilage does not contain a perichondrium and they usually form a transitional layer between hyaline cartilage and a ligament or tendon.
2. What is osteoarthritis and its connection with hyaline cartilage?
Osteoarthritis is a condition that generally occurs as we age. With age, the hyaline cartilage reduces in quality as the number of chondroitin in the articular cartilage diminishes. On the other hand, the number of chondrocytes increases in the deeper layers. The extracellular matrix also loses proteoglycans with aging. Hyaline cartilage suffers wear and tear easily since it is responsible for absorbing shock and is used heavily in daily activities. With wear and tear, degeneration of hyaline cartilage occurs, and the articular cartilage gets fragmented and releases matrix metalloproteinases. This is a source of pain in joints which becomes a chronic condition leading to osteoarthritis.