Cartilage

CARTILAGE

Key points:

Cartilage is connective tissue with a semi-solid extracellular matrix that comprises collagen fibers and ground substance, which provides both support and protection for other body tissues.
Three types covered here: hyaline, fibrous, and elastic.

Hyaline cartilage

Most abundant type of cartilage in the body.
Hyaline cartilage forms most of the fetal skeleton and is important in endochondral bone growth until the end of adolescence.
In the adult, it persists in the nose, trachea, and larynx, thorax, and also covers the articular surfaces of long bones (where it has no perichondrium).
Degeneration and calcification of hyaline cartilage may either be physiologic or pathologic. It is physiologic in the case of endochondral bone formation but it is pathologic in the case of osteoarthritis, which leads to pain and restricted joint movement. There are other forms of arthritis, such as rheumatoid arthritis, which is an inflammatory form of arthritis in which the immune system aggressively attacks the cartilage, bone, and synovial membranes of the joints
Calcification (the processes by which cartilage is replaced by osseous tissue) is common in hyaline cartilage, but very rare in elastic or fibrous cartilages.

Hyaline Cartilage Layers:

Perichondrium, outer surface
Matrix

Perichondrium

Comprises inner and outer layers (although in slow-growing or inactive perichondrium, it is not always possible to visually distinguish two separate layers).
Inner layer is the chondrogenic (aka cellular) layer; it comprises chondrogenic cells.
Outer layer it he fibrous layer; it comprises Type I collagen fibers, blood vessels, which supply nutrients to the cartilage below, and, fibroblasts, which are thought to produce collagen fibers and/or chondroblasts (although intertextual variation exists regarding this point).

Matrix

Appears glass-like under the microscope.
Is basophilic and stains purple on H & E section
Contains territorial matrixes that comprise the proteoglycan-rich, dark staining area around the lacunae.
Ground substance of the matrix, which is gel-like, comprises the following components:

Proteoglycan aggregates, which are bound to collagen fibrils, and which provide a semi-solid structure to cartilage, Chondronectin, which adheres collagen fibers to chondrocytes, and, Itercellular Water, which is 60-80% of net weight of the cartilage (varies by location).

Matrix comprises type II collagen fibers, which are invisible in standard histologic preparations.
Houses lacunae, which are spaces in the matrix.
Inside the lacunae are the chondrocytes, which are mature cartilage cells that generate and maintain the matrix.
Chondroblasts are derived from chondrogenic cells, and eventually mature into chondrocytes.
Clusters of chondrocytes are called isogenous groups.
Interterritorial matrix outside of the isogenic groups; it stains lightly.

Elastic Cartilage

Also known as yellow fibrocartilage.
Shares many similarities to hyaline cartilage (for instance, it also has perichondrium and contains invisible type II collagen fibers within the matrix).
Helpful distinguishing feature of elastic cartilage are its interwoven elastic fibers, which provide flexibility.
Elastic cartilage provides flexibility and resistance to permanent deformation.
Present in the external ear, the auditory (Eustachian) tube, and the epiglottis.

Fibrous Cartilage

Also shares many similarities with hyaline cartilage
Two helpful distinguishing features are its lack of perichondrium and its rows of chondrocytes that have type I and type II fibers in between them.
The bundles of type 1 and type II collagen fibers provide strength and durability for shock absorption.
The matrix is dense and contains relatively little ground substance.
The type of joint and person's age determine the relative proportions of these two collagen fiber types within the fibrous cartilage.
NO perichondrium is present in fibrous cartilage.
Fibrous cartilage is found, most notably, in intervertebral discs, pubic symphysis, menisci of the knee, and tendons where properties of stress resistance are particularly important.

Formation and Growth of Cartilage

Two types of growth:

Interstitial and appositional growth; both types occur simultaneously during early development; the relative rate of each type of growth determines the shape and structure of the cartilage.

Interstitial growth:

Interstitial growth occurs during early stages of cartilage formation, and in the growth plates and articular cartilages of growing long bones.
Cells divide within matrix to produce daughter cells that secrete their own matrix.

Appositional growth

New matrix forms at the periphery of the cartilage.
Chondrogenic cells in perichondrium differentiate to become chondroblasts
Chondroblasts secrete new matrix, eventually become chondrocytes.
Older chondrocytes remain active and maintain matrix.

Clincal Correlation:

Hyaline cartilage that has been only nominally damaged can slowly repair itself via appositional growth, but severely damaged cartilage is often replaced by tougher, more fibrous connective tissue.

Hormones that impact the growth of hyaline cartilage:

Stimulators: thyroxine, testosterone, and somatotropin.
Inhibitors: cortisone, hydrocortisone, and estradiol.

Nutritional states affect bone growth:

Hypovitaminosis A diminishes the thickness of epiphyseal plates; decrease in growth rate.
Hypervitaminosis A accelerates ossification of epiphyseal plates; short stature (dwarfism).
Hypovitaminosis C inhibits matrix production and distorts cartilage columns in epiphyseal plates (bones are weak, repair of fractures obstructed; scurvy develops)
Hypovitaminosis D inhibits calcification of matrix, causing softening of the bones (osteomalacia); in children, growing bones become bowed (rickets)

Cartilage

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