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Histology of Cartilage and Bone

Cartilage and bone are specialized connective tissues. (It may be useful to review the connective tissue section of Fundamentals of Histology at this point). Like all connective tissues, cartilage and bone consist of cells and extracellular matrix. The matrix of all CTs consists of fibers (collagen, reticular, and elastic) and amorphous ground substance, which contains proteoglycans and hyaluronic acid. Different connective tissues vary in the abundance and types of cells, in the proportion of cells to matrix, and in the properties of the matrix itself, including the numbers and types of fibres and the nature of the ground substance. The matrix is secreted by some of the cells in connective tissues, fibroblasts in most CTs. In cartilage, it is chondroblasts and chondrocytes that produce the matrix, while in bone, it is osteoblasts and osteocytes.


Cartilage: Table of Contents

Cartilage is a solid connective tissue that is to a certain extent pliable, making it resilient. These characteristics of cartilage are due to the nature of its matrix. The ground substance of cartilage is rich in proteoglycans consisting of a core protein with numerous- about 100- glycosaminoglycans (GAGs) attached bottle-brush fashion around it. GAGs are made of repeating units of disaccharides, one of which is always a glycosamine (hence the name) such as glucosamine or galactosamine. (Glycosamines are also called hexosamines.) In cartilage, the GAGs attached to the core proteins are chondroitin sulfate and keratan sulfate.

The proteoglycans themselves are attached, by special linker proteins to long, rigid molecules of hyaluronic acid (HA). HA itself is a GAG, but is composed of several thousand disaccharide units, rather than several hundred or less, as are other GAGs. About eighty proteoglycans are attached to one molecule of HA.

[For those who thirst for knowledge: The repeating units of chondroitin sufate are D-glucoronic acid and N-acetylgalactosamine-(4 or 6)-sulfate. The repeating units of keratan sulfate are galactose or galactose 6-sulfate and N-acetylglucosamine 6-sulfate. The repeating units of hyaluronic acid are D-glucuronic acid and N-acetylglucosamine].

In addition to the giant HA-proteoglycan macromolecules, there are other proteoglycans as well as glycoproteins in the matrix. The matrix also has collagen fibers, but these are of a finer nature (collagen Type II vs. Collagen Type I) than the collagen fibers in most other CTs. The macromolecules are bound to the thin collagen fibres by electrostatic interactions and cross-linking glycoproteins.

Between 60 and 80 percent of the net weight of (hyaline) cartilage is water, and this large component of water accounts for the resilient nature of cartilage. Water is attracted to the negative charges in the abundant sulfate and carboxyl groups on the GAGS. This hydration permits diffusion of water-soluble molecules in the ground substance. However the movement of large molecules and bacteria is inhibited. Cartilage is poorly vascularized, and gets most of its nutrients through diffusion. In the adult, repair is poor.

The sulfate groups of the GAGs that attract water also attract molecules of basic dye. Differences in the intensity of staining within the matrix reflect differences in the abundance of proteoglycans. Staining is most intense around the cartilage cells. The collagen fibres are not visible in standard histological preparations as their refractive index is not markedly different from that of the ground substance.

Some basic dyes used to stain cartilage can undergo a color change called metachromasia. In the presence of the negatively charged groups in the GAGs, the dye molecules form aggregates of dimers or polymers which changes their absorptive properties. Rather than staining blue, the tissue will stain purple or reddish.

Their are three kinds of cartilage, hyaline cartilage, elastic cartilage and fibrocartilage.

Hyaline Cartilage:Table of Contents

Hyaline cartilage is the most abundant type of cartilage. Most of the skeleton of the fetus is laid down in cartilage before being replaced by bone. Hyaline cartilage in the adult is found in the nose, parts of the respiratory tract, at the ends of ribs and at the articular surfaces of bones.

Low power view of hyaline cartilageTable of ContentsLow power view of hyaline cartilage

Figure 1 shows a low power view of cartilage in the nasal passage of a cat. It is taken from your slide 51. In this slide, the cartilage goes on a winding path, surrounded by the glands and blood vessels of the nasal passageway. Glands and blood vessels can also be seen in the section shown. The section of cartilage visible in Figure 1 is in the shape of an upside down Y. (The two big folds in the stem of the Y are an artifact of preparation.) The things that appear as dots at this magnification are the chondrocytes, and it can be seen that the matrix stains more intensely in the middle of the cartilage (more purple) than at the periphery (more yellow). The outer part of the cartilage is surrounded by the perichondrium (red), made of dense connective tissue.

High power view of hyaline cartilageTable of ContentsHigher power view of the cartilage

Figure 2 shows a higher power view of the cartilage at the middle of the Y. The chondrocytes are more evident and can be seen to be clustered in groups, called isogenous groups. Such isogenous groups reflect the last mitotic division of a chondrocyte whose daughter cells have not secreted much more matrix around themselves. The matrix in the middle part of the cartilage is mottled, with areas of darker staining around the isogenous groups. This darker staining reflects the greater abundance of GAGs and is called the territorial matrix. The lighter-staining areas between clusters of cells are called the interterritorial matrix. Perichondrium can be seen around all of the cartilage shown. The border between the perichondrium and cartilage is particularly evident at the lower part of the cartilage, where the perichondrium stains quite red, while the outer part of the cartilage is more orange. The cells at the outer part of the cartilage are also smaller than those farther inside (a bit difficult to see at this magnification). The cartilage and its perichondrium lie in dense irregular CT in which blood vessels, hair follicles and glands can be seen. (You don’t have to identify glands and hair follicles.)

High power view of isogenous groupsTable of Contents

Figure 3 shows a high power view of the isogenous groups in the cartilage. The territorial and interterritorial matrices can be clearly distinguished. Within the matrix, cartilage cells sit in spaces called lacunae. High power view of isogenous groupsAn intensely staining ring is sometimes seen around the edge of a lacuna. It is called the capsule and again reflects a higher density of GAGs. In life, chondrocytes completely fill the lacunae, but during tissue preparation, the cells frequently shrink and even fall out. Thus, in cartilage tissue, you sometimes see only empty spaces where the chondrocytes once sat. Some of the cells in Figure 3 can be seen to have shrunk a bit.

Although it cannot be differentiated with the light microscope, there is an inner and an outer part of the perichondrium. The cells in the outer (or fibrous) part are regular fibroblasts that secrete the collgen type I of the perichondrium. The cells of the inner (or cellular) part, however, are capable of differentiating into chondroblasts. These chondroblasts then begin to secrete the matrix of cartilage: collagen type II fibrils and the cartilage-specific ground substance. These chondroblasts lie directly apposed to the perichondrium, when they have secreted enough matrix to be completely surrounded by it, they are referred to as chondrocytes. Thus cartilage can grow through two processes. Growth resulting from the secretion of matrix by chondroblasts of the perichondrium is called appositional growth. Growth resulting from the secretion of matrix by chondrocytes which are already embedded in matrix is called interstitial growth.

High power view of cartilage and perichondriumTable of ContentsHigh power view of cartilage and perichondrium

Figure 4 shows a high power view of the boundary between the cartilage and the perichondrium (which is torn a bit). The flattened cells lying right along the boundary of the inner perichondrium are chondroblasts. Cells completely surrounded by cartilage are chondrocytes. Chondrocytes closer to the periphery are more flattened than those deeper inside the cartilage.

Low power view of human tracheaLow power view of cartilage from human tracheaTable of Contents

Figure 5 shows a low power view of cartilage from the human trachea. The characteristics are the same as those described in the previous four figures. A few glands can be seen at the bottom right of the figure.

Elastic Cartilage:Table of Contents

The structure of elastic cartilage is very similar to that of hyaline cartilage, but in addition to the other components, its matrix has elastic fibres and interconnecting sheets of elastic material. This gives elastic cartilage an elasticity not present in hyaline cartilage. Elastic cartilage is found in the external ear, the walls of the external auditory canal, the Eustachian tube, the epiglottis and the larynx.

High power view of elastic cartilageTable of ContentsHigh power view of elastic cartilage

Figure 6 shows a high power view of elastic cartilage. The density of elastic material, especially toward the middle of the matrix, has made it appear quite red, similar in color to the perichondrium. Toward the periphery of the cartilage, wisp-like ends of the elastic fibres can be seen. With other stains, such as orcein used to stain elastin, the elastic fibres would stand out more clearly as black lines and bands.

Fibrocartilage:Table of Contents

Fibrocartilage has characteristics intermediate between those of hyaline cartilage and dense connective tissue. Its presence indicates the need for resistance to compression and shear forces. It is found in the intervertebral disks, the symphysis pubis, the articular discs of the sternoclavicular and temperomandibular joints, the menisci of the knee joint and some places where ligaments or tendons attach to bones. The amount of cartilage in fibrocartilage is variable, it generally occupies a smaller amount of the tissue. There is no perichondrium.

High power view of fibrocartilageHigh power view of fibrocartilageTable of Contents

Figure 7 shows a high power view of fibrocartilage from a ligament. The isogenous groups of cartilage cells are in a line, surrounded with a bit of purple matrix, the rest of the tissue is dense connective tissue (stains pink).

Low power view of cruciate ligamentLow power view of cruciate ligamentTable of Contents

Figure 8 shows fibrocartilage from the cruciate ligament of the knee. Many rows of isogenous groups are seen. There is a relatively greater amount of matrix in the fibrocartilage shown here than was present in the previous figure.

Embryology of Cartilage:Table of Contents

Cartilage arises from mesenchyme. Some mesenchyme cells aggregate to form a blastema. The cells of the blastema begin to secrete cartilage matrix and are then called chondroblasts. They move apart as they deposit matrix, and when they are completely surrounded by matrix they are called chondrocytes. The mesenchymal tissue surrounding the blastema gives rise to the perichondrium.

Bone:Table of Contents

Bone is a connective tissue distinguished by the fact that its matrix is mineralized by calcium phosphate in the form of crystals very similar to hydroxyapatite. The minerals are both in the (Type I) collagen fibers which constitute about 95% of the matrix, and in the ground substance. Bone is both resilient and hard. Its resilience is due to the organic matter (collagen), its hardness due to the inorganic minerals. Bone serves as a storage site for calcium and phosphate. Blood calcium levels are regulated by the hormones parathormone (parathyroid hormone), which raises blood calcium levels by stimulating bone resorption, and calcitonin, which reduces blood calcium by suppressing bone resorption and increasing osteoid calcification. (Osteoid is the matrix secreted by osteoblasts and osteocytes prior to mineralization).

There are two kinds of mature bone: compact bone and spongy bone. Compact bone is also called dense bone and cortical bone. Spongy bone is also called cancellous bone, trabecular bone and medullary bone. [Another term, lamellar bone, is sometimes used to mean only compact bone, and sometime used to denote mature bone, whether spongy or compact, as opposed to immature bone (see below). Therefore, the term lamellar bone is a bit ambiguous].

Compact bone and spongy bone are found in specific locations. In long bones, most of the thickness of the diaphysis is made of compact bone, with a small amount of spongy bone facing the marrow cavity. The ends (epiphyses) of long bones, however, consist mostly of spongy bone covered with a shell of compact bone. The flat bones of the skull have a middle layer of spongy bone sandwiched between two relatively thick layes of compact bone.

Compact Bone:Table of Contents

Compact bone is composed of cylindrical structures called osteons or Haversian systems. An osteon consists of concentric lamellae of bone matrix, mainly collagen fibres, surrounding a central canal called the Haversian canal, which contains small blood vessels and nerves. The long axis of osteons is usually parallel to the long axis of the bone. The collagen fibres within any one lamella are generally parallel with one another, but the collagen fibres in the different lamellae of an osteon are oriented at different angles. This increases the strength of the osteon. Canals called Volkmann’s canals link the Haversian canals of different osteons with one another and with the marrow cavity. They provide the major route for blood vessels from the marrow cavity to the Haversian canals of osteons.

Between the lamellae of an osteon are lacunae containing bone cells called osteocytes. Canaliculi connect the lacunae with one another and with the Haversian canal. The canaliculi contain the processes of the osteocytes, which communicate with one another via gap junctions. Thus nutrients and other substances, such as hormones, can pass from blood vessels in the Haversian canal to distant osteocytes via a sort of bucket brigade. Osteocytes can be involved in both bone deposition and bone resorption.

In addition to the lamellae of osteons, lamellae not belonging to any osteon can be seen in compact bone. These are called interstitial lamellae and are the remnants of previous osteons. They reflect the fact that bone is not static but is constantly being remodelled. In addition, the inner and outer surfaces of long bone have lamellae that run the length of the shaft. They are called, respectively, the inner and outer circumferential lamellae.

Low power view of ground compact bone. Low power view of ground compact boneTable of Contents

Figure 9 is a low magnification of ground bone. Ground bone is bone that has been dried, cut (with a saw, not a microtome) and ground to a sufficient thinness to be viewed with a microscope. Ground bone is used to observe the architecture of bone. This section is not stained.

Note the numerous osteons, of variable sizes and shapes. The size of their Haversian canals is also quite variable. Many interstitial lamellae are seen among the osteons. A Volkmann’s canal is seen arising from one Haversian canal. (Sometimes you will see a Volkmann’s canal making a "textbook" connection between two Haversian canals. This one is less perfect.) The lacunae can be seen as black spots lying between the lamellae of osteons. This section does not show inner or outer circumferential lamellae.

High power view of ground compact boneHigh power view of ground compact boneTable of Contents

Figure 10 shows a higher power view of compact bone. A complete osteon is shown at the centre of the figure with parts of several osteons surrounding it. The lacunae can be clearly seen, and the canaliculi are discernible as fine black lines.

 Stained, demineralized compact boneTable of Contents

Figure 11 shows compact bone that has been demineralized and stained (taken from slide 88, of the dog jaw). Stained, demineralized compact boneThe two lines at the right are an artifact of the tissue folding. The osteons appear as red circles with the Haversian canal (blue) in the centre. Lacunae appear as dots. Many interstitial lamellae are visible. Several Volkmann’s canals are evident, even though the section did not go right through the canals themselves. Even at higher magnifications, the canaliculi would not be distinguishable.

Spongy Bone:Table of Contents

Spongy bone is composed of bone spicules, also called trabeculae, of varying shapes and sizes. The spaces between the spicules are filled with marrow. The composition of spongy bone (cells and matrix) is the same as that of compact bone. In spongy bone, however, the lamellae of collagen are not arranged concentrically around a central canal, but run parallel to one another. Osteocytes sit in lacunae between lamellae.

Low power view of spongy boneLow power view of spongy boneTable of Contents

Figure 12 shows a low power view of spongy bone from the dog jaw (slide 88). The variability of the bone spicules is evident. In the spaces between the spicules, the myeloid elements of bone marrow can be seen as bluish patches.

Junction of compact and spongy boneJunction of compact and spongy boneTable of Contents

Figure 13 shows the meeting of compact bone (above) with spongy bone (below) in the jaw (slide 88).

Bone Cells:Table of Contents

Osteoprogenitor cells (periosteal and endosteal), osteoblasts, osteocytes, osteoclasts

Just as cartilage is surrounded by a perichondrium, bone is surrounded by a periosteum of dense connective tissue. As in the perichondrium, the periosteum has two layers: an outer fibrous layer with typical fibroblasts, and an inner cellular layer, which contains osteoprogenitor cells. The osteoprogenitor cells in this location are called periosteal cells. They are capable of giving rise to osteoblasts, which secrete the extracellular matrix of bone.

The marrow surface of compact bone, and the spicules of spongy bone, are lined by an (often single) layer of cells called the endosteum (endosteal cells). Like the periosteal cells, these endosteal cells are also osteoprogenitor cells, capable of becoming osteoblasts. (The two names periosteal cells and endosteal cells refer to their different locations, both function as osteoprogenitor cells).

Osteoblasts secrete the collagen fibres and ground substance (matrix) of bone and are responsible for the calcification of the matrix. They retain the ability to divide and communicate by thin cytoplasmic processes which form gap junctions.

When an osteoblast is completely surrounded by matrix, it is called an osteocyte. Osteocytes are responsible for maintaining the matrix, and can both secrete and resorb matrix. Thus osteoprogenitor cells, osteoblasts and osteocytes are all part of a series derived from mesenchyme cells. There is evidence that osteocytes and osteoblasts can revert to earlier stages.

Another type of cell called the osteoclast is responsible for the resorption of bone. These large, multinucleated cells arise from monocytes. They release lysosomes, organic acids and hydorlytic enzymes to break down bone matrix.

Sharpey’s fibres in jaw bone of dogSharpey’s FibresTable of Contents

Collagen fibres from tendons and ligaments extend into bone tissue at an angle, and become continuous with the collagen fibres of the bone matrix. Figure 14 shows Sharpey’s fibres in the jaw bone, extending downward from the connective tissue above.

Embryology and Formation of Bone:Table of Contents

Intramembranous ossification, endochondral ossification

In the embryo, bone tissue arises through two processes, intramembranous ossification and endochondral ossification. In intramembranous ossification, bone is formed directly from mesenchymal tissue. The flat bones of the skull and face, the mandible and the clavicle develop in this manner. In endochondral ossification, a cartilage model of the bone is formed first, and is later replaced by bone. The weight-bearing bones of the axial skeleton and the bones of the extremities (= most of the skeleton) develop in this manner.

The first bone to arise, whether from mesenchyme or from cartilage (or in fracture repair postnatally), is in the form of spicules. These first spicules are made of immature bone, also called woven bone. In immature bone, the collagenous lamellae are not arranged in parallel or concentric arrays (as in mature spongy and compact bone, respectively), but are randomly oriented and loosely intertwined (hence woven). Immature bone also has more ground substance than mature bone. Consequently, immature and mature bone show different staining characteristics, immature bone stains more with hematoxylin and mature bone more with eosin.

The spicules of immature bone are remodelled. The remodelling process can eventually give rise to more spongy bone or to compact bone. The remodelling of bone continues throughout life (remember those interstitial lamellae in compact bone represent former osteons). Immature bone is the predominant bone in the developing fetus. In the adult, most immature bone is replaced by mature bone, but immature bone is seen where bone is being remodelled or repaired, and in certain specific areas, such as the alveolar sockets of the oral cavity. Because most of the spongy bone (= in form of spicules) you look at in the lab is from the embryo, you may erroneously come to think of spongy bone as synonymous with immature bone.

When bone matrix is first secreted, it is not yet mineralized and is called osteoid. As mentioned above, osteoblasts also bring about the mineralization of bone.

Intramembranous Ossification:Table of Contents

The first step in intramembranous ossification is the aggregation of mesenchyme cells in the area where bone is to be formed. The tissue in this area becomes more vascularized, and the mesenchyme cells begin to differentiate into osteoblasts, which secrete the collagen and ground substance (proteoglycans) of bone matrix (collectively called osteoid). The osteoblasts maintain contact with one another via cell process. The osteoid becomes calcified with time, and the processes of the cells (called osteocytes when they are surrounded with matrix) become enclosed in canaliculi. Some of the mesenchymal cells surrounding the developing bone spicules proliferate and differentiate into osteoprogenitor cells. Osteoprogenitor cells in contact with the bone spicule become osteoblasts, and secrete matrix, resulting in appositional growth of the spicule. Intramembranous ossification begins at about the eighth week in the human embryo.

Low power view of intramembranous ossificationTable of ContentsLow power view of intramembranous ossification

Figure 15 shows a low power view of intramembranous ossification in the head of the pig embryo. A number of bony spicules are seen lying in the mesenchyme from which they developed. A band of cartilage (not involved with these spicules) is seen lying below them. Osteocytes can be seen within the spicules, and each spicule is surrounded by osteoblasts.

High power view of bone spicule.High power view of bone spiculeTable of Contents

A high power view of one of the spicules from Figure 15 (the seahorse-shaped one at the far right) is shown in Figure 16. Numerous osteocytes are seen enclosed within the spicule, and most of the periphery is lined by osteoblasts. (A characteristic feature of immature bone is a greater abundance of cells than in mature bone). Several developing blood vessels are seen in the mesenchyme.

 High power view of bone spicules with osteoclastsHigh power view of bone spicules with osteoclastsTable of Contents

Figure 17 shows a high power view of a bone spicule with three osteoclasts, only one of which (the top one) is obvious. Although this osteoclast will be multinucleate, only one nuceus, toward the left of the cell, can be clearly seen. No osteoblasts are evident around this spicule, but osteocytes can be seen within it. The structure of the mesenchyme can also be seen quite well: small spindle-shaped cells with processes. These processes contact the processes of neighboring cells via gap junctions. In life, the extracellular space is occupied by a viscous ground substance. It is lost during routine histological tissue preparation.

Endochondral Ossification:

Endochondral ossification also begins with the aggregation of mesemchyme cells, but these differentiate into chondroblasts which secrete hyaline cartilage matrix. The cartilage is secreted in the general shape of the bone that it will become, and grows by both interstitial (mostly in length) and appositional (mostly in width) growth.

Sometime during the growth of this cartilage model (starting at about week 12 in the human fetus), some of the inner perichondrial cells begin to give rise to osteoblasts instead of chondroblasts. (As a result, the former perichondrium is now called the periosteum.) In long bones, this process begins at the mid-region of the bone. The newly formed osteoblasts secrete osteoid, forming a bone collar around the cartilage model. Therefore the very first bone that is formed during endochondral ossification is considered to arise by intramembranous ossification.

With the formation of the bone collar, the cartilage cells in the underlying cartilage begin to hypertrophy and secrete alkaline phosphatase. The surrounding cartilage matrix becomes calcified. The bone collar and the calcified matrix inhibit diffusion of nutrients, and the chondrocytes begin to die. The matrix breaks down, and the lacunae of the dying chondrocytes become confluent making a large cavity. At the same time, blood vessels grow through the bone collar, bringing osteoprogenitor (periosteal) cells with them. These differentiate into osteoblasts when they come in contact with the calcifying cartilage matrix. Osteoid is deposited on the calcifying matrix which is eventually removed. (This is the endochondral part of endochondral ossification.) The bone spicules are remodelled by osteoclasts and osteoblasts. The invading blood vessels also bring with them cells which dfferentiate into the hematopoietic cells of the bone marrrow.

In long bones, the midregion where bone formation is initiated is called the primary ossification centre. Secondary ossification centres will develop after birth at the ends (epiphyses) of the bone. During the years of growth, the primary and secondary ossification centres are separated by a cartilage plate called an epiphyseal (growth) plate. This plate allows the bone to grow in length.

Cartilaginous rib of pig embryoCartilaginous rib of pig embryoTable of Contents

Figure 18 shows part of a rib of the pig embryo that has not yet begun the process of ossification. The cartilaginous rib is surrounded by a perichondrium.

Cartilage cells undergoing hypertrophy under bone collar in ribTable of ContentsCartilage cells undergoing hypertrophy

Figure 19 shows the other end of the rib, in which the first step of endochondral ossification has begun (you will recall this is actually a form of intramembranous ossification). A bone collar lined with osteoblasts (which arose from what used to be the perichondrium and is now the periosteum) can be distinguished, but is most easily seen toward the right of the figure. The cartilage cells are very obviously undergoing hypertrophy, but the matrix is not yet calcified, and no osteoid has yet been deposited.

Bone collar and hypertrophic cartilage cellsHigh power view of bone collar and
hypertrophic cartilage cells
Table of Contents

A high power view of the bone collar and the hypertrophic cartilage cells is shown in Figure 20. The lacunae are enlarged and becoming confluent and the matrix is reduced. The bone collar is lined with osteoblasts.

High power view of blood vessels in hypertrophic cartilage cellsblood vessels in hypertrophic cartilage cellsTable of Contents

Figure 21 (taken from slide 97 of the vertebral column of a young cat) shows a high power view of some blood vessels that have invaded the hypertrophic cartilage during endochondral ossification. The lacunae can be seen to be hypertrophic, the chondrocytes are shrunken due to tissue preparation. Red blood cells can be seen in the vessels but the osteoprogenitor cells cannot be distinguished. The vertebrae ossify postnatally, in humans the process is not completed until adulthood.

Endochondral ossification in a long boneEndochondral ossification in a long boneTable of Contents

Figure 22 is taken from slide 21 (tibia and knee joint of rat). It shows endochondral ossification at a later stage, in the primary ossification centre of a long bone. Remnants of purple cartilage matrix are surrounded by bone, staining pink. In between the bone spicules are the myeloid elements of bone marrow. The bone collar of the long bone is not visible in the field of view.

Epiphyseal plate of a long boneTable of ContentsEpiphyseal plate of a long bone

Figure 23 shows a low power view of the epiphyseal plate. The primary ossification centre that you saw in Figure 22 is toward the right of Figure 23. The epiphyseal plate itself is the vertical purple band, and a secondary ossification centre (at the epiphysis of the bone) is seen at the left of the figure. The layers of the epiphyseal plate extend from left to right. The rat from which this slide was made was reaching maturity, and hence the layers of the plate are not as large or as well-defined as you might see in textbooks. Read the lecture notes or text for a description of events in each zone.

Low power view of vertebrae and diskLow power view of vertebrae and diskTable of Contents

Figure 24 is a low power view of parts of two vertebrae separated by the intervertebral disk. Toward the disk side, both vertebrae are still cartilaginous. (The cartilage stains more red than in the previous pictures you have seen.) Toward the middle of each vertebra, ossification is proceeding and bone spicules separated by marrow spaces can be seen. The intervertebral disk consists of a puffy-looking nucleus pulposus (a remnant of the notochord) and a ring of fibrocartilage.

High power view of intervertebral diskHigh power view of intervertebral diskTable of Contents

Figure 25 shows a higher power view of the central part of Figure 24. The fibrocartilage and nucleus pulposus of the disk can be seen between the cartilage of two vertebrae. Because of the staining on this slide, the fibrocartilage may be harder to identify than that which you saw in the section on cartilage (Figures 7 and 8).

Cartilage ossification boundary in a vertebraCartilage–ossification boundary in a vertebraTable of Contents

Figure 26 shows a high power view of the boundary between the cartilage and the ossification centre in a vertebra. In the spicules (left), the bone is slightly more eosinophilic than the cartilage matrix on which it is being deposited. The tear in the cartilage (top right) could represent the start of a new ossification centre.

Tendon and muscleTendon and muscleTable of Contents

Figure 27 shows a tendon joining a muscle. The striations in the muscle are barely visible. Tendon is made of regularly arranged bundles of collagen secreted by fibroblasts (tendon cells). In this section, the tendon appears a bit wavy. This is not the case if the tissue is completely straight when sectioned. A few strands of the tendon can be seen to penetrate into the muscle.

The Synovium Table of Contents

The synovium is the joint cavity between two movable bones. The bone surfaces in the synovium are covered in cartilage, called the articular cartilage, which lacks a perichondrium. The ends of the two bones are enclosed and united by an articular capsule, which has two parts. The outer part is a sleeve of fibrous tissue (fibrous capsule) which extends well beyond the articular cartilage of each bone. The inner part is called the synovial membrane, it lines the fibrous capsule and is reflected onto the bone, which it covers right up to the articular cartilage. Thus the joint cavity is lined everywhere with either articular cartilage or synovial membrane.

The synovial membrane is a thin sheet of connective tissue, with abundant blood vessels and lymphatics. The surface facing the joint cavity is lined by epithelioid cells which secrete hyaluronic acid and phagocytize debris. (They do not sit on a basement membrane so they cannot be called an epithelium.) The hyaluronic acid and a dialysate of plasma from the blood vessels of the membrane constitute the synovial fluid, a viscous substance which lubricates the joints.

Synovial membranes can be quite variable in appearance, they may be closely apposed to the fibrous capsule or thrown into many folds (sometimes called villi), they may rest directly on periosteum or on loose connective, dense fibrous or adipose tissue. If extensive folds in the membrane are present, islets of cartilage may develop inside them, by metaplasia of the synovial fibroblasts.

Inflammation of the synovial membrane is called rheumatoid arthritis, and results in the stimulation of the pain ending of nerves. The formation of uric acid crystals in the joints is called gout, and damages the articular cartilage. Various kinds of trauma to the joint can damage the cartilage to the point that it begins to calcify and be replaced by bone. This can lead to a bony fusion called ankylosis which results in a loss of mobility.

Low power view of synovial jointLow power view of synovial jointTable of Contents

Figure 28 shows a low power view of part of a synovial joint (from slide 21 of the rat tibia). The articular cartilage at the end of one of the bones can be seen covering the bony spicules and marrow spaces of its epiphysis. The fibrous capsule and the synovial membrane lining it can be seen at the left. (These tissues are very difficult to scan and in this computer image are barely distinguishable from background, they will be a bit easier to see through a real microscope.)

High power view of synovial membraneHigh power view of synovial membraneTable of Contents

Figure 29 shows a higher power view of the synovial membrane (again slightly hard to see). The membrane is made of loose connective tissue with some areas of adipose tissue, and is lined with epithelioid cells. No blood vessels can be unequivocally identified in the field of view.

High power view of the articular cartilageHigh power view of the articular cartilageTable of Contents

Figure 30 shows a high power view of the articular cartilage covering the epiphysis. Note the absence of a perichondrium. The synovial membrane can be seen at the left.



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