Cartilage
Objectives
Students should aim to recognize and differentiate the 3 types of cartilage,
understand the composition of the extracellular matrix and the role of chondrocytes
in producing and maintaining it.
Cartilage is a specialized form of connective tissue containing chondrocytes
which secrete, and are surrounded by, an extensive intercellular matrix. Chondrocytes
occur singly or in isogenous groups, composed of 2-8 cells derived by
mitosis from a single chondrocyte. The cells are in the lacunae (cavities)
within the matrix. Matrix stains more intensely immediately adjacent to the
lacunae and the dark staining zone is called the capsule. The strength
and durability of cartilage are properties of the matrix, which is an interlaced
network of collagenous and/or elastic fibers in a ground substance, a gel of
complex proteoglycans. The collagen is mostly Type II. How does this differ
from dense C.T.?
There are three types of cartilage characterized by the composition of the
intercellular matrix.
(1) hyaline, (2) elastic and (3) fibrocartilage.
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Hyaline cartilage is found lining articular surfaces, and in
the nasal septum, tracheal rings, costal cartilages, and the epiphyseal
cartilage of growing bone. Study the trachea on slide #72 (trachea).
Even numbered boxes usually contain slides stained for elastic tissues,
odd numbered boxes have slides stained with hematoxylin and eosin. The
former will have a brown coloration, the latter, the pink and blue you
are familiar with. In this slide considerable fading has occurred, and
the normal basophilia of freshly stained cartilage is almost absent. The
trachea is a large open tube, with cartilage constituting the principal
component of its wall. |
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Using the scanning lens, locate the cartilage rings. At higher power,
identify single chondrocytes and locate isogenous groups.
There are clear areas between many of the chondrocytes and the walls of
their lacunae because of shrinkage of the cells brought about by fixation,
and because some chondrocytes had lipid droplets which dissolved during
histological preparation. |
Identify the perichondrium. Note the gradation of cell shape changes
from fibroblast-like in the outer layer to the round cell shape within the cartilage.
The perichondrium is acidophilic due to the preponderance of collagen fibers.
At the interface between perichondrium and cartilage, transitional cells can
be seen which are still elongated but are beginning to be surrounded by matrix.
The cartilage matrix contains collagen fibers, but these are very difficult
or impossible to see. Some areas may show some dark-staining material which
could be indicative of elastic fibers, but these are best seen in a later slide.
Note that the cartilage matrix should be slightly basophilic, especially around
the chondrocytes, due to anionic properties of proteoglycans.


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Elastic cartilage is found in the ear and epiglottis, where
it provides a rigid but elastic, framework. Its principal components are
elastic fibers but type II collagen is also present. Some elastic fibers
may be present in the tracheal cartilage. Examine a section through infant
ear, slide #11 (even boxes only), which has been stained for elastin
with resorcin fuchsin. The purple staining interlaced fibers run in all
directions, and appear as dots when cut transversely. The nuclei and cytoplasm
of the cells are not stained well. The cartilage in this section is immature
and therefore richer in cells than adult cartilage. Compare this arrangement
of cells and fibers of elastic cartilage with that of hyaline cartilage
of the trachea. |
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Fibrocartilage is found in intervertebral discs, the pubic symphysis,
in menisci of joints, and often occurs where tendon and ligament are joined
to bones. It resembles a dense C.T. which contains islands of chondrocytes
embedded in cartilage matrix. Its appearance varies with its location.
The major and characteristic intercellular component of this tissue consists
of thick, interlaced collagenous fibers. The ground substance is much
less abundant than in hyaline cartilage due to the preponderance of fibers.
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Examine a section of human intervertebral disc, slide #12. Most
of the tissue on this slide is fibrocartilage, with some dense C.T., and possibly
fat, on the periphery. Examine the general structure of this tissue. It is difficult
to distinguish from dense C.T. Careful examination will reveal that some cells
are within lacunae. You may not be able to discern the difference between chondrocyte
and fibroblasts by nuclear shape, but if a lacunus surrounds the cell, it is
a chondrocyte; if not, it is a fibroblast. Interspersed among the deeper staining
fibrous areas are less acidophilic areas where there are chondrocytes, accumulations
of ground substance, and fewer coarse fibers. There may be wrinkles in the sections
which stain deeply (artifacts). Compare the appearance of the cells of the cartilage
islands in the fibrocartilage with those of hyaline cartilage and of dense connective
tissue.