Microanatomy: Cartilage and Bone

A firmly attached connective tissue that surrounds hyaline cartilage.

Perichondrium

The appearance of chondrocyte cytoplasm varies according to

chondrocyte activity

The most important proteoglycan monomer in hyaline cartilage is

Aggrecan (formed from chondroitin sulfate and keratan sulfate.)

Why do you think articular hyaline cartilage does not repair as well as hyaline cartilage in other locations?

Articular hyaline cartilage lacks a perichondrium and thus does not have a good source of new chondroblasts that could aid in repair.

Why is cartilage a key tissue in the development of the fetal skeleton and in most growing bones?

Because it maintains this property even while growing

Why do aggrecan molecules have a large affinity for water molecules?

Because of the presence of the sulfate groups, aggrecan molecules have a large negative charge with an affinity for water molecules.

Extracellular Matrix: Why is it important to the survival of chondrocytes?

Because there is no vascular network within cartilage

Fibrocartilage Location

Fibrocartilage is typically present in intervertebral discs, the symphysis pubis, articular discs of the sternoclavicular and tem- poromandibular joints, menisci of the knee joint, the triangular fibrocartilage complex of the wrist, and certain places where tendons attach to bones.

Production of hyaline cartilage

Hyaline cartilage matrix is produced by chondrocytes and contains three major classes of molecules. The macromolecules of hyaline cartilage matrix consist of collagen (predominantly type II fibrils and other cartilage-specific collagen molecules), proteoglycan aggregates containing GAGs, and multiadhesive glycoproteins (non collagenous proteins).

Each linear ______ molecule is associated with a large number of aggrecan molecules (more than 300), which are bound to the _____ by link proteins at the N terminus of the molecule to form large proteoglycan aggregates.

Hyaluronan

Fibrocartilage: chondrocyte nuclei

In a section containing fibrocartilage, a population of cells with rounded nuclei and a small amount of surrounding amorphous matrix material can typically be seen. These nuclei belong to the chondrocytes.

What does elastic cartilage matrix contain?

In addition to containing the normal components of hyaline cartilage matrix, elastic cartilage matrix also contains a dense network of branching and anastomosing elastic fibers and interconnecting sheets of elastic material

How do multiadhesive glycoproteins have clinical value?

Multiadhesive glycoproteins have clinical value as markers of cartilage turnover and degeneration.

______ influence interactions between the chondrocytes and the matrix molecules.

Multiadhesive glycoproteins, also referred to as non collagenous and non proteoglycan-linked glycoproteins

Fibrocartilage: Perichondrium

No surrounding perichondrium

What feature of fibrocartilage cells allows the matrix to respond to external environmental changes?

The cells in fibrocartilage synthesize a wide variety of extra- cellular matrix molecules not only during its development stage but also during its mature, fully differentiated state. This allows the fibrocartilage to respond to changes in the external environment (such as mechanical forces, nutritional changes, and changing levels of hormones and growth factors).

Fibrocartilage: Chondrocytes

The chondrocytes are dispersed among the collagen fibers singularly, in rows, and in isogenous groups These chondrocytes appear similar to the chondrocytes of hyaline cartilage, but they have considerably less cartilage matrix material.

What is the role of chondrocytes? How plentiful are they?

The chondrocytes are sparse but essential participants in producing and maintaining the matrix

What is responsible for the unique biomechanical properties of hyaline cartilage.

The entrapment of aggrecan-hyaluronan aggregates within the intricate matrix of collagen fibrils

Proteoglycan monomer present in fibrocartilage

The extracellular matrix of fibrocartilage contains larger amounts of versican (a proteoglycan monomer secreted by fibroblasts) than aggrecan (produced by chondrocytes). Versican can also bind hyaluronan to form highly hydrated proteoglycan aggregates

Fibrocartilage Collagen content

The extracellular matrix of fibrocartilage contains significant quantities of both type I collagen (characteristic of connective tissue matrix) and type II collagen (characteristic of hyaline cartilage). The relative proportions of these collagens can vary. The ratio between type I and type II collagen in fibrocartilage changes with age. In older individuals, there is more type II collagen because of the metabolic activity of chondrocytes, which constantly produce and discharge type II collagen fibrils into the surrounding matrix.

What structural property accounts for the diffusion of substances between blood vessels in the CT and chondrocytes in the cartilage matrix?

The large ratio of glycosaminoglycans (GAGs) to type II collagen fibers in the cartilage thus maintaining the viability of the tissue.

What are the two classes of structural molecules present in cartilage? How do they interact?

The meshwork of tension-resisting collagen fibrils and the large amounts of heavily hydrated proteoglycan aggregates, the latter being extremely weak in shear, make the cartilage well adapted to bear weight, especially at points of movement, as in synovial joints. Solid and firm but also somewhat pliable, which accounts for its resilience.

Fibrocartilage function

The presence of fibrocartilage in these sites indicates that resistance to both compression and shearing forces is required of the tissue. The cartilage serves much like a shock absorber. The degree to which such forces occur is reflected in the amount of cartilage matrix material present.

How are aggrecan molecules bound to collagen fibrils?

These highly charged proteoglycan aggregates are bound to the collagen matrix fibrils by electrostatic interactions and multiadhesive glycoproteins

What are the three types of cartilage, how are they characterized?

Three types of cartilage that differ in appearance and mechanical properties are distinguished on the basis of characteristics of their matrix: Hyaline cartilage is characterized by matrix containing type II collagen fibers, GAGs, proteoglycans, and multiadhesive glycoproteins. Elastic cartilage is characterized by elastic fibers and elastic lamellae in addition to the matrix material of hyaline cartilage. Fibrocartilage is characterized by abundant type I collagen fibers as well as the matrix material of hyaline cartilage.

Hyaline cartilage: Collagen

Type II collagen constitutes the bulk of the fibrils. type IX collagen facilitates fibril interaction with the matrix proteoglycan molecules; type XI collagen regulates the fibril size; and type X collagen organizes the collagen fibrils into a three-dimensional hexagonal lattice that is crucial to its successful mechanical function. In addition, type VI collagen is also found in the matrix, mainly at the periphery of the chondrocytes where it helps to attach these cells to the matrix framework. Because types II, VI, IX, X, and XI are found in significant amounts only in the cartilage matrix, they are referred to as cartilage-specific collagen molecules.

Elastic cartilage matrix: calcification

Unlike hyaline cartilage, which calcifies with aging, the matrix of elastic cartilage does not calcify during the aging process.

Fibrocartilage: fibroblast nuclei

Within the fibrous areas are nuclei that are flattened or elongated. These are fibroblast nuclei.

Is elastic cartilage surrounded by a perichondrium

Yes

Importance of water binding loosely in hyaline cartilage

allow diffusion of small metabolites to and from the chondrocytes

Chondrocytes that are active in matrix production display

areas of cytoplasmic basophilia, which are indicative of protein synthesis, and clear areas, which indicate their large Golgi apparatus

Hyaline cartilage: Articular Joint surfaces

articular cartilage. the free, or articular, surface has no perichondrium. Also, on the opposite surface, the cartilage contacts the bone, and there is no perichondrium. Articular cartilage is a remnant of the original hyaline cartilage template of the developing bone, and it persists throughout adult life.

Cartilage

avascular connective tissue that consists of chondrocytes and an extensive extracellular matrix

Hyaline cartilage: Capacity for repair

capacity for repair is limited, under normal circumstances it shows no evidence of abrasive wear over a lifetime. An exception is articular cartilage, which, in many individuals, breaks down with age

Distribution of chondrocytes in hyaline cartilage

chondrocytes are distributed either singularly or in clusters called isogenous groups

Fibrocartilage

consists of chondrocytes and their matrix material is a combination of hyaline cartilage with dense connective tissue.

Elastic cartilage is distinguished by the presence of ____ in the cartilage matrix.

elastin

location of elastic cartilage

external ear, the walls of the external acoustic meatus, the auditory (Eustachian) tube, and the epiglottis of the larynx.

hyaline cartilage location

forms most of the embryonic skeleton; covers the ends of long bones in joint cavities; forms costal cartilages of the ribs; cartilages of the nose, trachea, and larynx

The ground substance of hyaline cartilage contains three kinds of glycosaminoglycans:

hyaluronan, chondroitin sulfate, and keratan sulfate.

Importance of water binding tightly to aggrecan-hyaluronan aggregates

imparts resilience to the cartilage.

Chondrocytes also secrete _____ [enzyme]

metalloproteinases (matrixins), enzymes that degrade cartilage matrix, allowing the cells to expand and reposition themselves within the growing isogenous group.

Hyaline Cartilage function

provides a low-friction surface, participates in lubricating synovial joints, and distributes applied forces to the underlying bone.

chondrocytes in isogenous groups

represent cells that have recently divided.

Stains to view elastic fibers

resorcin- fuchsin, silver and orcein

Function of Perichondrium

serves as the source of new cartilage cells. When actively growing, the perichondrium appears divided into an inner cellular layer, which gives rise to new cartilage cells, and an outer fibrous layer. This division is not always evident, especially in perichondrium that is not actively producing new cartilage or in very slow-growing cartilage.

Chondrocytes

specialized cells that produce and maintain the extracellular matrix.

Chondrocytes that are less active/inactive in matrix production display

the Golgi apparatus is smaller; clear areas of cytoplasm, when evident, usually indicate sites of extracted lipid droplets and glycogen stores. In such specimens, chondrocytes also display considerable distortion resulting from shrinkage after the glycogen and lipid are lost during preparation of the tissue.

Normal matrix turnover depends on

the ability of the chondrocytes to detect changes in matrix composition.

Cartilage Internal Remodeling: role of the matrix

the matrix acts as a signal transducer for the embedded chondrocytes. Thus, pressure loads applied to the cartilage, as in synovial joints, create mechanical, electrical, and chemical signals that help direct the synthetic activity of the chondrocytes. As the body ages, however, the composition of the matrix changes, and the chondrocytes lose their ability to respond to these stimuli.

Why aren't the collagen fibers visible in hyaline cartilage

their refractive index is the same as that of the matrix