Hyaline cartilage is a bluish, opalescent tissue which is widely distributed throughout the body of the vertebrates. It is found on the ventral ends of ribs, on the articular surface of bones, and in the larynx, trachea, and bronchi. They are also located at the epiphyseal plates of fetuses and growing children (epiphyseal plate is a plate of cartilage between the epiphysis and the shaft of long bones). Thus, it is the most abundant type of cartilage in the adults.
Hyaline cartilage has no blood vessels and lacks nerves. It contains three types of cells: chondrocytes, chondroblasts, and fibroblasts. There are tiny compartments inside the matrix of this tissue; these are called lacunae. Inside the lacunae, there are chondrocytes, which are the essential cells of this tissue as they build and maintain the cartilage matrix. In growing cartilages, chondrocytes divide and a partition of matrix begin to form between the daughter cells, giving rise to a nest of two or four cells. Each living chondrocyte virtually fills its own lacuna.
Chondroblasts differentiating at sites of developing cartilage secrete macromolecular constituents of cartilage matrix. Cells at the periphery of such sites produce a fibrous covering known as perichondrium. The inner portion of the covering is called chondrogenic, because it constantly gives rise to new chondroblasts, which in turn build up more matrix, adding to that already formed.
The cells in the outer portion of hyaline cartilage perichondrium develop into fibroblasts, which produce collagen. As a result, the developing cartilage becomes covered with a layer of fibrous perichondrium. This outer part of the perichondrium remains in the adult life. However, there are few instances where both layers of the perichondrium disappear. Articular cartilages, for example, lack perichondrium.
General Characteristics
In the light microscope, hyaline cartilage appears as discrete masses of tissue, which is surrounded by a dense and irregular connective tissue layer, which is called the perichondrium. The perichondrium is a layer of fibroblasts that merges with the extracellular matrix of the connective tissue immediately surrounding the cartilage.
The perichondrium has an inner layer of flattened chondroblasts, which are specially evident in growing cartilage. Chondroblasts are differentiated cells which secrete cartilaginous extracellular matrix. When the chondroblasts get surrounded by their secretion, they become chondrocytes.
Below, light micrograph of hyaline cartilage in the trachea. Several chondrocytes (CC) are present in the lacunae, which are surrounded by extracellular matrix (ECM). The perichondrium (PC) is composed of flat peripheral fibroblasts and deep chondroblasts.
