Human_Histology

Elastic cartilage possesses greater flexibility than hya­ line cartilage and readily recovers its shape after being deformed.

Distribution of Elastic Cartilage

It forms the “skeletal” basis of the auricle (or pinna) and of the lateral part of the external acoustic meatus.

The wall of the medial part of the auditory tube is made of elastic cartilage.

The epiglottis and two small laryngeal cartilages

(corniculate and cuneiform) consist of elastic cartilage. The apical part of the arytenoid cartilage contains elastic fibers but the major portion of it is hyaline.

Note that all the sites mentioned above are concerned

either with the production or reception of sound.

Fibrocartilage

On superficial examination this type of cartilage (also called white fibrocartilage) looks very much like dense

fibrous tissue (Plate 6.3). However, in sections it is seen to be cartilage because it contains typical cartilage cells surrounded by capsules. The matrix is pervaded by nume­ rous collagen bundles amongst which there are some fibroblasts. The fibers merge with those of surrounding connective tissue.

There is no perichondrium over the cartilage. This kind of cartilage has great tensile strength combined with considerable elasticity. The collagen in fibrocartilage is different from that in hyaline cartilage in that it is type I collagen (identical to that in connective tissue), and not type II. The fibrocartilage in contrast to hyaline cartilage does not undergo calcification.

Distribution of Fibrocartilage

Fibrocartilage is most conspicuous in secondary cartila­ ginous joints or symphyses. These include the joints between bodies of vertebrae (where the cartilage forms intervertebral discs); the pubic symphysis; and the manubriosternal joint.

Plate 6.3: Fibrocartilage

A

B

Fibrocartilage. (A) As seen in drawing; (B) Photomicrograph

Fibrocartilage is characterized by:

Presence of prominent collagen fibers arranged in bundles with rows of chondrocytes intervening between the bundles

Perichondrium is absent

This kind of cartilage can be confused with the appearance of a tendon. However the chondrocytes in fibrocartilage are rounded but in a tendon, fibrocytes are flattended and elongated.

Key

1.Chondrocytes

2.Ground tissue with collagen fibers

In some synovial joints the joint cavity is partially or completely subdivided by an articular disc. These discs are made up of fibrocartilage. (Examples are discs of the temporo­mandibular and sternoclavicular joints, and menisci of the knee joint).

The glenoidal labrum of the shoulder joint and the acetabular labrum of the hip joint are made of fibro­ cartilage.

In some situations where tendons run in deep grooves on bone, the grooves are lined by fibrocartilage. Fibro­ cartilage is often present where tendons are inserted into bone.

Pathological Correlation

Cartilage-forming (Chondroblastic) Tumors

The tumors which are composed of frank cartilage or derived from cartilage-forming cells are included in this group. This group

Contd...

Contd...

comprises benign lesions like osteocartilaginous exostoses (osteochondromas), enchondroma, chondroblastoma and chondromyxoid fibroma, and a malignant counterpart, chondrosarcoma.

Osteocartilaginous Exostoses or Osteochondromas: These are the commonest of benign cartilage-forming lesions. Exostoses arisefrommetaphysesoflongbonesasexophyticlesions,most commonly lower femur and upper tibia and upper humerus.

Enchondroma: Enchondroma is the term used for the benign cartilage-forming tumor that develops centrally withn the interior of the affected bone, while chondroma refers to the peripheral development of lesion.

Chondroblastoma: Chondroblastoma is a relatively rare benign tumor arising from the epiphysis of long bones adjacent to the epiphyseal cartilage plate. Most commonly affected bones are upper tibia and lower femur and upper humerus.

Chondrosarcoma: Chondrosarcoma is a malignant tumor of chondroblasts.

Bone is a rigid form of connective tissue in which the extracelluar matrix is impregnated with inorganic salts, mainly calcium phosphate and carbonate, that provide hardness.

GENERAL FEATURES

If we examine a longitudinal section across a bone (such as the humerus), we see that the wall of the shaft is tubular and encloses a large marrow cavity. The wall of the tube is made up of a hard dense material that appears, on naked eye examination, to have a uniform smooth texture with no obvious spaces in it. This kind of bone is called compact bone.

When we examine the end of long bone and diploë of flat bones we find that the marrow cavity does not extend into them. They are filled by a meshwork of tiny rods or plates of bone and contain numerous spaces, the whole

Fig. 7.1: Some features of bone structure as seen in a longitudinal section through one end of a long bone (Schematic representation)

appearance resembling that of a sponge. This kind of bone is called spongy or cancellous bone (cancel = cavity). The spongy bone at the bone ends is covered by a thin layer of compact bone, thus providing the bone ends with smooth surfaces (Fig. 7.1). Small bits of spongy bone are also present over the wall of the marrow cavity.

Where the bone ends take part in forming joints they are covered by a layer of articular cartilage. With the exception of the areas covered by articular cartilage, the entire outer surface of bone is covered by a membrane called the periosteum. The wall of the marrow cavity is lined by a membrane called the endosteum.

The marrow cavity and the spaces of spongy bone (present at the bone ends) are filled by a highly vascular tissue called bone marrow. At the bone ends, the marrow is red in color. Apart from blood vessels this red marrow contains numerous masses of blood forming cells (hemopoietic tissue). In the shaft of the bone of an adult, the marrow is yellow. This yellow marrow is made up predominantly of fat cells. Some islands of hemopoietic tissue may be seen here also. In bones of a fetus or of a young child, the entire bone marrow is red. The marrow in the shaft is gradually replaced by yellow marrow with increasing age.

THE PERIOSTEUM

The external surface of any bone is, as a rule, covered by a membrane called periosteum (Fig. 7.1).

Note: The only parts of the bone surface devoid of periosteum are those that are covered with articular cartilage.

The periosteum consists of two layers, outer and inner. The outer layer is a fibrous membrane. The inner layer is cellular. In young bones, the inner layer contains numerous osteoblasts, and is called the osteogenetic layer. (This layer is sometimes described as being distinct from periosteum). In the periosteum covering, the bones of an adult osteoblasts are not conspicuous, but osteoprogenitor cells present here can form osteoblasts when need arises, e.g.

Fig. 7.2: Longitudinal section of compact bone

in the event of a fracture. Periosteum is richly supplied with blood. Many vessels from the periosteum enter the bone and help to supply it.

Functions of Periosteum

The periosteum provides a medium through which muscles, tendons, and ligaments are attached to bone. In situations where very firm attachment of a tendon to bone is necessary, the fibers of the tendon continue into the outer layers of bone as the perforating fibers of Sharpey (Fig. 7.2). The parts of the fibers that lie within the bone are ossified; they have been compared to “nails” that keep the lamellae in place.

Because of the blood vessels passing from periosteum into bone, the periosteum performs a nutritive function.

Because of the presence of osteoprogenitor cells in its deeper layer, the periosteum can form bone when required. This role is very important during development. It is also important in later life for repair of bone after fracture.

The fibrous layer of periosteum is sometimes described as a limiting membrane that prevents bone tissue from “spilling out” into neighboring tissues. This is based on the observation that if periosteum is torn, osteogenic cells may extend into surrounding tissue forming bony projections (exostoses). Such projections are frequently seen on the bones of old persons. The concept of the periosteum as a limiting membrane helps to explain how ridges and tubercles are formed

Chapter 7 Bone 61

A B C

Figs. 7.3A to C: Bone cells. (A) Osteocyte; (B) Osteoblast;

(C) Osteoclast (Schematic representation)

on the surface of a bone. At sites where a tendon pulls upon periosteum, the latter tends to be lifted off from bone. The “gap” is filled by proliferation of bone leading to the formation of a tubercle (Such views are, however, hypothetical).

ELEMENTS COMPRISING BONE TISSUE

Like cartilage, bone is a modified connective tissue. It consists of bone cells or osteocytes (Figs. 7.3A to C) that are widely separated from one another by a considerable amount of intercellular substance. The latter consists of a homogeneous ground substance or matrix in which collagen fibers and mineral salts (mainly calcium and phosphorus) are deposited.

In addition to mature bone cells (osteocytes) two additional types of cells are seen in developing bone. These are bone producing cells or osteoblasts, and bone removing cells or osteoclasts (Figs. 7.3 and 7.4). Other cells present include osteoprogenitor cells from which osteoblasts and osteocytes are derived; cells lining the surfaces of bone; cells belonging to periosteum; and cells of blood vessels and nerves which invade bone from outside.

Osteoprogenitor cells

These are stem cells of mesenchymal origin that can proliferate and convert themselves into osteoblasts whenever there is need for bone formation. They resemble fibroblasts in appearance. In the fetus such cells are numerous at sites where bone formation is to take place. In the adult, osteoprogenitor cells are present over bone surfaces (on both the periosteal and endosteal aspects).

Osteoblasts

These are bone forming cells derived from osteoprogenitor cells. They are found lining growing surfaces of bone, sometimes giving an epithelium-like appearance. However, on closer examination it is seen that the cells are of varied shapes (oval, triangular, cuboidal, etc.) and that there are numerous gaps between adjacent cells.

The nucleus of an osteoblast is ovoid and euchromatic. The cytoplasm is basophilic because of the presence of

62 Textbook of Human Histology

Fig. 7.4: Photomicrograph of bone cells showing osteoblast rimming the suface of the bone; multinucleated osteoclast and osteocyte

Ob: osteoblast (short arrow); Oc: osteoclast (long arrow); Os: osteocyte (arrow head)

abundant rough endoplasmic reticulum. This, and the presence of a well developed Golgi complex, signifies that the cell is engaged in considerable synthetic activity. Numerous slender cytoplasmic processes radiate from each cell and come into contact with similar processes of neighboring cells.

Osteoblasts are responsible for laying down the organic matrix of bone including the collagen fibers. They are also responsible for calcification of the matrix. Alkaline phosphatase present in the cell membranes of osteoblasts plays an important role in this function. Osteoblasts are believed to shed off matrix vesicles that possibly serve as points around which formation of hydroxyapatite crystals takes place. Osteoblasts may indirectly influence the resorption of bone by inhibiting or stimulating the activity of osteoclasts.

Pathological Correlation

A benign tumor arising from osteoblasts is called an osteoma.A malignant tumor arising from the same cells is called an osteosarcoma. Osteosarcomas are most commonly seen in bones adjoining the knee joint. They can spread to distant sites

in the body through the blood stream.

Osteocytes

These are the cells of mature bone. They lie in the lacunae of bone, and represent osteoblasts that have become “imprisoned” in the matrix during bone formation. Delicate cytoplasmic processes arising from osteocytes

establish contacts with other osteocytes and with bone lining cells present on the surface of bone.

In contrast to osteoblasts, osteocytes have eosinophilic or lightly basophilic cytoplasm. This is to be correlated with

The fact that these cells have negligible secretory activity

The presence of only a small amount of endoplasmic reticulum in the cytoplasm.

Osteocytes are present in greatest numbers in young

bone, the number gradually decreasing with age.

Functions

The functions attributed to osteocytes are:

They probably maintain the integrity of the lacunae and canaliculi, and thus keep open the channels for diffusion of nutrition through bone

They play a role in removal or deposition of matrix and of calcium when required.

Osteoclasts

These are bone removing cells. They are found in relation to surfaces where bone removal is taking place (Bone removal is essential for maintaining the proper shape of growing bone). At such locations the cells occupy pits called resorption bays or lacunae of Howship. Osteoclasts are very large cells (20 to 100 μm or even more in diameter). They have numerous nuclei: up to 20 or more. The cytoplasm shows numerous mitochondria and lysosomes containing acid phosphatase. At sites of bone resorption the surface of an osteoclast shows many folds that are described as a ruffled membrane. Removal of bone by osteoclasts involves demineralization and removal of matrix. Bone removal can be stimulated by factors secreted by osteoblasts, by macrophages, or by lymphocytes. It is also stimulated by the parathyroid hormone.

Recent studies have shown that osteoclasts are derived from monocytes of blood. It is not certain whether osteoclasts are formed by fusion of several monocytes, or by repeated division of the nucleus, without division of cytoplasm.

Bone Lining Cells

These cells form a continuous epithelium-like layer on bony surfaces where active bone deposition or removal is not taking place. The cells are flattened. They are present on the periosteal surface as well as the endosteal surface. They also line spaces and canals within bone. It is possible that these cells can change to osteoblasts when bone formation is called for (in other words many of them are osteoprogenitor cells).

Organic and Inorganic Constituents of

Bone Matrix

The ground substance (or matrix) of bone consists of an organic matrix in which mineral salts are deposited.

The Organic Matrix

This consists of a ground substance in which collagen fibers are embedded. The ground substance consists of glycosaminoglycans, proteoglycans, and water. Two special glycoproteins osteonectin and osteocalcin are present in large quantity. They bind readily to calcium ions and, therefore, play a role in mineralization of bone. Various other substances including chondroitin sulfates, phospholipids, and phosphoproteins are also present.

The collagen fibers are similar to those in connective tissue (Type I collagen) (they are sometimes referred to as osteoidcollagen). The fibers are usually arranged in layers, the fibers within a layer running parallel to one another. Collagen fibers of bone are synthesized by osteoblasts.

The matrix of bone shows greater density than elsewhere immediately around the lacunae, forming capsules around them, similar to those around chondrocytes in cartilage. The term osteoid is applied to the mixture of organic ground substance and collagen fibers (before it is mineralized).

The Inorganic Ions

The ions present are predominantly calcium and phosphorus (or phosphate). Magnesium, carbonate, hydroxyl, chloride, fluoride, citrate, sodium, and potassium are also present in significant amounts. Most of the calcium, phosphate and hydroxyl ions are in the form of needleshaped crystals that are given the name hydroxyapatite (Ca10[PO4]6[OH]2). Hydroxyapatite crystals lie parallel to collagen fibers and contribute to the lamellar appearance of bone. Some amorphous calcium phosphate is also present.

About 65% of the dry weight of bone is accounted for by inorganic salts, and 35% by organic ground substance and collagen fibers. (Note that these percentages are for dry weight of bone. In living bone about 20% of its weight is made up by water). About 85% of the total salts present in bone are in the form of calcium phosphate; and about 10% in the form of calcium carbonate. Ninetyseven percent of total calcium in the body is located in bone.

The calcium salts present in bone are not “fixed”. There is considerable interchange between calcium stored in bone and that in circulation. When calcium level in

Chapter 7 Bone 63

blood rises calcium is deposited in bone; and when the level of calcium in blood falls calcium is withdrawn from bone to bring blood levels back to normal. These exchanges take place under the influence of hormones (parathormone produced by the parathyroid glands, and calcitonin produced by the thyroid gland).

Pathological Correlation

The nature of mineral salts of bone can be altered under certain!" of foramina leading to compression of nerves, or even of the spinal cord; and abnormalities in histological structure. The

Normal calcium and phosphorus are easily substituted by radioactive calcium (45Ca) or radioactive phosphorus (32P) if the latter are ingested. Calcium can also be replaced by radioactive strontium, radium or lead. Presence of radioactive substances in bone can lead to the formation of tumors and# $ exposed to radioactivity from any source: in particular from a nuclear explosion.

Radioisotopes of calcium and phosphorus can also serve a$ bone is being formed. Areas of such deposits can be localized, radioactive isotopes can be used for study of the patterns of

Added Information

Role of Inorganic Salts in Providing Strength to Bone

Mineral salts can be removed from bone by treating it with weak acids. Chelating agents (e.g. ethylene diamine tetraacetic acid or EDTA) are also used. The process, called or a rib, can even be tied into a knot. This shows that the rigidity of bone is mainly due to the presence of mineral salts in it.

Conversely, the organic substances present in bone can be destroyed by heat (as in burning). The form of the bone remains intact, but the bone becomes very brittle and breaks easily. It follows that the organic matter contributes substantially to the strength of bone. Resistance to tensile forces is mainly due to the presence of co

TYPES OF BONE

Based on histology

Compact boneSpongy bone

Fig. 7.5: How lamellae constitute bone (Schematic representation)

Based on maturity

Mature/lamellar boneImmature/woven bone

Based on manner of development

Cartilage boneMembrane bone.

Lamellar Bone

When we examine the structure of any bone of an adult, we find that it is made up of layers or lamellae (Fig. 7.5). This kind of bone is called lamellar bone. Each lamellus is a thin plate of bone consisting of collagen fibers and mineral salts that are deposited in a gelatinous ground substance. Even the smallest piece of bone is made up of several lamellae placed over one another. Between adjoining lamellae we see small flattened spaces or lacunae.

Each lacuna contains one osteocyte. Spreading out from each lacuna there are fine canals or canaliculi that communicate with those from other lacunae (Fig. 7.6). The canaliculi are occupied by delicate cytoplasmic processes of osteocytes.

The lamellar appearance (Fig. 7.5) of bone depends mainly on the arrangement of collagen fibers. The fibers

Fig. 7.6: Relationship of osteocytes to bone lamellae

(Schematic representation)

of one lamellus run parallel to each other; but those of adjoining lamellae run at varying angles to each other. The ground substance of a lamellus is continuous with that of adjoining lamellae.

Woven Bone

In contrast to mature bone, newly formed bone does not have a lamellar structure. The collagen fibers are present in bundles that appear to run randomly in different directions, interlacing with each other. Because of the interlacing of fiber bundles this kind of bone is called woven bone. All newly formed bone is woven bone. It is later replaced by lamellar bone.

Clinical Correlation

Paget’s disease of bone or osteitis deformans

# $ % &' * + % /011 % & disease of bone is an osteolytic and osteosclerotic bone disease of uncertain etiology involving one (monostotic) or more bones (polyostotic). The condition affects predominantly males over the67

Structure of Cancellous Bone (Spongy Bone)

Cancellous bone is made up of a meshwork of bony plates or rods called trabeculae. Each trabeculus is made up of a number of lamellae (described above) between which there are lacunae containing osteocytes. Canaliculi, containing the processes of osteocytes, radiate from the lacunae (Fig. 7.7).

The trabeculae enclose wide spaces that are filled in by bone marrow (Fig. 7.7 and Plate 7.1). They receive nutrition from blood vessels in the bone marrow.

The trabeculae are covered externally by vascular endosteum containing osteoblasts, osteoclasts and osteoprogenitor cells.

Fig. 7.8: Haversian system in compact bone

(Schematic representation)

Compact bone consists of several such osteons. Between adjoining osteons there are angular intervals that are occupied by interstitial lamellae (Fig. 7.9 and Plate 7.2). These lamellae are remnants of osteons, the greater parts of which have been destroyed (as explained later). Near the surface of compact bone the lamellae are arranged parallel to the surface: these are called circumferential lamellae (Plate 7.2).

When we examine longitudinal sections through compact bone (Fig. 7.9) we find that the Haversian canals (and, therefore, the osteons) run predominantly along the length of the bone. The canals branch and anastomose with each other. They also communicate with the marrow cavity, and with the external surface of the bone through channels that are called the canals of Volkmann. Blood vessels and nerves pass through all these channels so that compact bone is permeated by a network of blood vessels that provide nutrition to it.

Comparison between cancellous and compact bone

There is an essential similarity in the structure of cancellous and compact bone. Both are made up of lamellae. The difference lies in the relative volume occupied by bony lamellae and by the spaces. In compact bone the spaces are small and the solid bone is abundant; whereas in cancellous bone the spaces are large and actual bone tissue is sparse (Plates 7.1 and 7.2).

Added Information

Osteons

During bon a clear lamellar structure, but consist of woven bone. Such osteons are described as primary osteons (or atypical Haversian systems). Subsequently, the primary osteons are replaced by secondary osteons (or typical Haversian systems) having the structure already described.

Fig. 7.9: Transverse section and longitudinal section through compact bone to show Haversian canals and the Volkmann's canal (Schematic representation)

Contd...

We have seen that osteons run in a predominantly longitudinal direction (i.e. along the long axis of the shaft). However, this does not imply that osteons lie parallel to each other. They may follow a spiral course, may branch, or may join other osteons. In transverse sections osteons may appear circular, oval or ellipsoid.

The number of lamellae in each osteon is highly variable. The average number is six.

eitherlongitudinally or circumferentially (relative to the long! " lamellae is alternately longitudinal and circumferential. It is arrangement is obvious in sections. It has been claimed# $ % $ %&$ % $ %

The place where the periphery of one osteon comes in contact with another osteon (or with interstitial lamellae) is marked by the presence of a cement line. Along this line inorganic matrix. The various lacunae within an osteon are connected with one another through canaliculi that also communicate with the Haversian canal.

The peripheral canaliculi of the osteon do not (as a rule) communicate with those of neighboring osteons: they form loops and turn back into their own osteon. A few canaliculi that pass through the cement line provide communications leading to interstitial lamellae.

! osteocytes and their processes. The remaining space is* ! is in communication with the Haversian canal and provides a pathway along which substances can pass from blood vessels in the Haversian canal to osteocytes.

PLATE 7.2: Compact Bone

A

B

Structure of compact bone as seen in a transverse section.

(A) As seen in drawing; (B) Photomicrograph

Contd...

When a transverse section of compact bone is examined with polarized light each osteon shows two bright bands that cross each other. This phenomenon is called birefringence. It is an indication of the very regular arrangement of+ # lamellus.

FORMATION OF BONE

All bones are of mesodermal origin. The process of bone formation is called ossification. We have seen that formation of most bones is preceded by the formation of a cartilaginous model, which is subsequently replaced by bone. This kind of ossification is called endochondral

osteons(or Haversian systems)

At the center of each osteon there is a Haversian canal

Around the canal there are concentric lamellae of bone amongst cytes)

Delicate canaliculi radiate from the lacunae; these contain cytoplasmic processes of osteocytes

!

Near the surface of compact bone, the lamellae are arranged "

#$ % ! canal may be seen.

Note: "

Key

1.Haversian system (osteon)

2.Haversian canal

3.Concentric lamellae

&"

5.Volkmann's canal

*" +

ossification; and bones formed in this way are called cartilage bones. In some situations (e.g. the vault of the skull) formation of bone is not preceded by formation of a cartilaginous model. Instead bone is laid down directly in a fibrous membrane. This process is called intramembranous ossification; and bones formed in this way are called membrane bones. The bones of the vault of the skull, the mandible, and the clavicle are membrane bones.

Intramembranous Ossification

The various stages in intramembranous ossification are as follows: