Histology - Skeletal Connective Tissue - Cartilage and Bone

Volkmans Canals

- Bone is a well-vascularized tissue. As noted on a previous screen, the haversian canals (H) of the osteons carry the haversian blood vessels (arteries and veins) and these vessels run parallel to the long axis of the bone. Running perpendicular to the long axis of the bone are transverse canals that carry blood vessels that connect adjacent Haversian blood vessels. These transversely oriented canals are called Volkmann's canals (single arrow) and provide the channels through which the transverse oriented blood vessels can course.

Cartilage

- Cartilage is a specialized connective tissue. There are three types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage. - Hyaline cartilage is the most abundant type of cartilage; elastic cartilage and fibrocartilage have more limited distributions. - Cartilage is comprised of cells and extracellular matrix. - The cells of cartilage are called chondrocytes. - The extracellular matrix is comprised of collagenous fibers and ground substance. In addition, elastic cartilage has an abundant amount of elastic fibers present in the extracellular matrix. - Differences in the amounts and distributions of the components of the extracellular matrix determine the different types of cartilage.

Chondrocytes

- Chondrocytes are present in all three types of cartilage (hyaline cartilage, elastic cartilage, and fibrocartilage). The left hand side of the image shown above shows chondrocytes in hyaline cartilage. The arrow is pointing to one chondrocyte in an isogenous group consisting of two chondrocytes. - Numerous other chondrocytes in their isogenous groups can be seen in this image as well. - Each chondrocyte sits in its own lacuna. - The matrix immediately surrounding the lacuna tends to stain intensely and is called the territorial matrix. - The chondrocytes are responsible for producing the matrix components and help to maintain the cartilage.

Compact Bone

- Compact bone has osteons, haversian canals, Volkmann's canals and lacks the large spaces and trabeculae that are characteristics of spongy bone. - Compact bone is found in places like the shaft of the diaphyses of long bones and the tables of the flat bones of the skull.

Elastic Cartilage

- Elastic cartilage, like hyaline cartilage is comprised of cells (chondrocytes), and extracellular matrix, and is surrounded by a perichondrium. - It has a much smaller distribution than does hyaline cartilage and is found in the pinna of the ear, the epiglottis, and several other places. - It is very similar to hyaline cartilage but has an additional matrix component--elastic fibers. The collagen in the matrix, like hyaline cartilage, is Type II collagen. - Due to the large number of elastic fibers in the matrix (arrow), elastic cartilage looks less homogeneous that hyaline cartilage.

Fibro Cartilage

- Fibrocartilage, like hyaline cartilage and elastic cartilage, is comprised of chondrocytes and extracellular matrix. However, there are some important differences that distinguish fibrocartilage from the other types of cartilage. - Fibrocartilage has no perichondrium and the collagen in the matrix is Type I collagen. In addition, the amount of amorphous ground substance is reduced and this makes the collagen fibers in the matrix much more apparent--thus it looks 'fibrous'. - The chondrocytes of fibrocartilage, as in hyaline and elastic cartilage, lie in lacunae, but the formation and appearance of isogenous groups is different. - In fibrocartilage, isogenous groups very often appear as linear rows of chondrocytes rather than spherical clusters of chondrocytes.

Compact Bone (pic I)

- In the image above, which is from a decalcified section of compact bone, the larger open areas are haversian canals (you can see the remnants of blood vessels in some of them). Evidence of osteons and concentric lamellae around these haversian canals is apparent. The smaller white spaces are lacunae and in some of the lacunae it is possible to see a small, dark spot. These dark spot-like structures are the remnants of osteocytes that once resided in those lacunae.

Lacunae and Canaliculi

- In this high magnification image of a ground bone section through an osteon, a haversian canal (H) can be seen as well as several lamellae surrounding the haversian canal. The lacunae (double arrow) lie in a circumferential arrangement between the lamellae in an osteon. Radiating away from the lacunae are tiny channels or canals in the bone called canaliculi (single arrow). These tiny canaliculi are important because they permit diffusion from the haversian blood vessels of food and oxygen to the osteocytes in their lacunae, and the transport of waste products and carbon dioxide away from the osteocytes to the haversian blood vessels.

Hyaline Cartilage

- Is comprised of cells (chondrocytes) and the extracellular matrix. - The chondrocytes tend to be found singly (near the connective tissue covering called the perichondrium), or in small groups consisting of 2, 3, or 4 chondrocytes. These small groups of chondrocytes are referred to as isogenous groups or nests and are readily visible in this image. Each chondrocyte sits in a little space (lacuna) within the cartilage matrix. The matrix immediately adjacent to these isogenous groups (arrow) tends to stain more intensely than the rest of the matrix and has been called the territorial matrix. The rest of the matrix tends to stain less intensely and has been referred to as the interterritorial matrix (indicated by the asterisk). The perichondrium is the connective tissue covering that surrounds most hyaline cartilage (it is not present on the hyaline cartilage that forms the articular surfaces of bones). The type of collagen found in the matrix of hyaline cartilage is Type II collagen.

Compact Bone - Osteons and Interstitial Lamellae

- It can be seen in this ground bone section that the osteon (bracket) is comprised of concentrically arranged bone lamellae that surround a central (haversian) canal. - Osteons are found primarily in the compact bone of the diaphyses of long bones. They are roughly cylindrical in shape and run parallel to the long axis of the bone. The lamellae (single arrow) form the mineralized intercellular matrix of bone and contain collagen and calcium hydroxyapatite embedded in an amorphous ground substance. - Between adjacent osteons are incomplete interstitial lamellae that fill in the spaces between the complete lamellae associated with the osteons. - Between the lamellae are lacunae (double arrows) that are spaces in the bone where osteocytes reside in the living tissue. In this image, it can be seen that within an osteon the lacunae are arranged in a circumferential manner, alternating with the circumferentially disposed lamellae.

Epiphyseal Plate

- The epiphyseal plate is comprised of hyaline cartilage and is formed during long bone development in a process referred to as endochondral bone formation. - It is found between the epiphysis and diaphysis of a developing long bone and has a distinct orientation or polarity to it.

Osteoblasts (pic I)

In the example above, there are some osteocytes trapped in their lacuna within the bony spicule. The mesenchymal-like connective tissue (M) is richly vascularized with blood vessels.

Fibrocartilage (Pic I)

In this composite image of two examples of fibrocartilage, the image on the left has been stained with H & E. In the example on the right, the section of fibrocartilage was stained with a trichrome stain that stains collagen blue.

Cells of Bone

The composite image seen on the screen above indicates three types of bone cells: osteoblasts, osteocytes, and osteoclasts. The osteoblast is the bone cell responsible for bone formation, the osteocyte is the bone cell found in the lacunae of mature bone, and the osteoclast is the bone cell responsible for bone resorption.

Chondrocyte (Pic I)

The left hand side of the image shown above shows chondrocytes in hyaline cartilage. The arrow is pointing to one chondrocyte in an isogenous group consisting of two chondrocytes.

Osteoblasts

The osteoblast is responsible for the formation of bone extracellular matrix. As this matrix accumulates and becomes mineralized, the osteoblast becomes trapped in a space (lacuna). At that point the osteoblast becomes an osteocyte. Osteoblasts are derived from osteoprogenitor cells. Osteoblasts are basophilic cells and are seen lying along bony surfaces (arrows) where they frequently take on an epithelioid appearance.

Osteoclasts

The osteoclast (labeled by the arrow in the image seen above) is responsible for bone resorption and is derived from a fusion of blood monocytes. The osteoclast typically appears as a large, multi-nucleated cell adjacent to bony surfaces actively being resorbed. Frequently, there is evidence of the resorbtive activities of the osteoclast by the creation of a depression in the bony surface adjacent to the osteoclast. This depression, when apparent, is referred to as Howship's lacuna (labeled 'H' in the image above).

Osteocytes

The osteocyte (labeled by the arrows on the image seen above) is derived from the osteoblast as the osteoblast becomes trapped within the bone that is being formed. The osteocytes thus come to reside in little spaces within the bone called lacunae (white spaces in the image above) and receive their nutritional supplies via tiny canaliculi that interconnect adjacent lacunae and eventually the haversian canal.