CARTILAGE
Essentials of Histology
Cartilage is
a kind of connective tissue characterized by firmness and resilience. It
is composed of cells called chondrocytes and an extracellular matrix which
is produced by these cells. Unlike other connective tissues, the cartilage
is an avascular tissue, therefore, the nutrition of cartilage occurs by
diffusion of substances between blood vessels in the surrounding connective
tissue and the chondrocytes. Most cartilages are enclosed by a well vascularized
dense connective tissue termed the perichondrium. The outermost layer of
the perichondrium has abundant collagen fibers and it is called fibrous
perichondrium while the inner layer, called chondrogenic perichondrium,
has many cells which give rise to cartilage cells.
The chondrocytes
are isolated in small cavities within the matrix, called lacunas. However,
some of the lacunas may be extremely close, separated only by a thin partition
of matrix. These arrangements of lacunas called isogenous groups, are a
distinctive feature of cartilage that aid in this recognition. The matrix
around each isogenous group stains more strongly, forming the territorial
matrix (or capsule). The areas between these matrices, which are less stained
, form the interterritorial matrix. In the light microscope, chondrocytes
appear rounded and with a basophilic cytoplasm because it contains large
amounts of endoplasmic reticulum.
The growth of
cartilage takes place by two different mechanisms: interstitial growth
and appositional growth. The interstitial growth consists in mitotic
division of the condrocytes and the secretion of new matriix between
the daughter cells lead to an expantion of the cartilage. Therefore, this
process forms new cartilage within the cartilage mass. In the appositional
growth the cartilage grow through the addition of new matrix at the
surface of preexisting cartilage. In this process the new cartilage cells
called chondroblasts are derived from the inner layer of perichondrium.
The cartilage
matrix consists of collagen fibrils (predominantly type II collagen) and
amorphous ground substance which is formed mainly of proteoglicans (proteins+glycosaminoglicans).
The specific glycosaminoglicans of cartilage are chondroitin-4 and chondroitin-6-sulfate,
keratan sulfate and hyaluronic acid. These compounds of ground substance
give to the cartilage its firm consistency and permit the nutrition of
its cells.
Classification
of cartilage is based on differences in the abundance and type of fibers
in the matrix.
There
are three types of cartilage: hyaline, elastic and fibrocartilage.
The hyaline
cartilage is the most common type and is found in the tracheal ring, larynx,
bronchi and on the joint surface of bones. This cartilage also constitues
much of the fetal skeleton and plays an important role in the growth of
many bones.The hyaline cartilage is characterized by a homogeneous amorphous
matrix. This is due in part to the small size of the collagen fibrils and
in part to the fact that the ground substance and the collagen have approximately
the same refractive index. This type of cartilage has isogenous groups
and perichondrium.
The elastic
cartilage contains elastic fibers in the matrix. These fibers can
be demonstrated only with special stains such as resorcin-fuchsin and orcein.
Like hyalin cartilage, the elastic cartilage also contain collagen fibrils(type
II collagen)in its matrix, isogenous groups and perichondrium. The elastic
cartilage is found in the external ear, auditory tube, epiglottis and some
of the laryngeal cartilage.
The fibrocartilage
or fibrous cartilage consists of large bundles of dense collagen
fibers(type I collagen) between rows of chondrocytes. Fibrocartilage is
typically present in the intervertebral discs, in the symphysis pubis,
and in tendon or ligament insertions into bones.
