Figure 82

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Structure of a typical long bone. The diaphysis (shaft) of a long bone contains a large marrow cavity surrounded by a thick-walled tube of compact bone. A small amount of spongy bone may line the inner surface of the compact bone. The proximal and distal ends, or epiphyses, of the long bone consist chiefly of spongy bone with a thin outer shell of compact bone. The expanded or flared part of the diaphysis nearest the epiphysis is referred to as the metaphysis. Except for the articular surfaces that are covered by hyaline (articular) cartilage, indicated in blue, the outer surface of the bone is covered by a fibrous layer of connective tissue called the periosteum, indicated in pink.

cartilage on articular surface line bone metaphysis-epiphysis -

compact bone

)ongy bone epiphyseal line articular cartilage on articular surface verse is true. Here, the spongy bone is extensive, and the compact bone consists of little more than a thin outer shell (see Fig. 8.1).

Short bones possess a shell of compact bone and have spongy bone and a marrow space on the inside. Short bones usually form movable joints with their neighbors; like long bones, their articular surfaces are covered with hyaline cartilage. Elsewhere, periosteum, a fibrous connective tissue capsule, covers the outer surface of the bone. s s? general structure of bones Outer Surface of Bones

Bones are covered by periosteum, a sheath of dense fibrous connective tissue containing osteoprogenitor cells

Bones are covered by a periosteum except in areas where they articulate with another bone. In the latter case, the articulating surface is covered by cartilage. The per iosteum that covers an actively growing bone consists of an outer fibrous layer that resembles other dense connective tissues and an inner, more cellular layer that contains the osteoprogenitor cells. If active bone formation is not in progress on the bone surface, the fibrous layer is the main component of the periosteum, and the inner layer is not well defined. The relatively few cells that are present, the periosteal cells, are, however, capable of undergoing division and becoming osteoblasts under appropriate stimulus.

In general, the collagen fibers of the periosteum are arranged parallel to the surface of the bone in the form of a capsule. The character of the periosteum is different where ligaments and tendons attach to the bone. Collagen fibers from these structures extend directly, but at an angle, into the bone tissue, where they are continuous with the collagen fibers of the extracellular matrix of the bone tissue. These fibers are called Sharpey's fibers.

Bones that articulate with neighboring bones possess movable (synovial) joints

Where a bone articulates with a neighboring bone, as in synovial joints, the contact areas of the two bones are referred to as articular surfaces. The articular surfaces are covered by hyaline cartilage, also called articular cartilage because of its location and function; articular cartilage is exposed to the joint cavity. This cartilage is not covered with perichondrium.

Bone Cavities

Bone cavities are lined by endosteum, a layer of connective tissue cells that contains osteoprogenitor cells

The lining tissue of both the compact bone facing the marrow cavity and the trabeculae of spongy bone within the cavity is referred to as endosteum. The endosteum is often only one cell layer thick and consists of cells that can differentiate into osteoblasts in response to appropriate stimuli. These osteoprogenitor cells, called endosteal cells, are flattened cells that resemble fibroblasts.

The marrow cavity and the spaces in spongy bone contain bone marrow

Red bone marrow consists of developing blood cells in different stages of development (see page 232) and a network of reticular cells and fibers that serve as a supporting framework for the developing blood cells and vessels. As an individual grows, the amount of red marrow does not increase in proportion to bone growth. In later stages of growth and in the adult, when the rate of blood cell formation has diminished, the tissue in the marrow cavity consists mostly of fat cells; it is then called yelloiv marrow. In response to appropriate stimuli, such as extreme blood loss, yellow marrow can revert to red marrow. In the adult, red marrow is normally restricted to the spaces of spongy bone in a few locations such as the sternum and the iliac crest. Diagnostic bone marrow samples and marrow for transplantation are obtained from these sites.

Mature Bone

Mature bone is composed of structural units called osteons (Haversian systems)

Mature bone is largely composed of cylindrical units called osteons or Haversian systems (Fig. 8.3). The osteons consist of concentric lamellae (sing., lamella) of bone matrix, surrounding a central canal, the osteonal (Haversian) canal, which contains the vascular and nerve supply of the osteon. Canaliculi containing the processes of osteocytes

Inflammation of the joints (arthritis) can be caused by many factors and can produce varying degrees of pain and disability, from the pathologic response of articular cartilage to injury.

Simple trauma to a joint by a single incident or by repeated insult can so damage the articular cartilage that it calcifies and begins to be replaced by bone. This process can lead to ankylosis, i.e., bony fusion in the joint and subsequent loss of motion. The foot and knee joints of runners and football players and hand and finger joints of stringed instrument players are especially vulnerable to this condition.

Immune responses or infectious processes that localize in joints, as in rheumatoid arthritis or tuberculosis, can also damage the articular cartilages, producing both severe joint pain and gradual ankylosis. Surgery that replaces the damaged joint with a prosthetic joint can often relieve the pain and restore joint motion In seriously debilitated individuals.

Another common cause of damage to articular cartilages is the deposition of crystals of uric acid in the joints, particularly those of the toes and fingers. This condition is known as gouty arthritis or, more simply, gout. Gout has become more common because of the widespread use of thiazide diuretics in the treatment of hypertension. In genetically predisposed individuals, gout is the most common side effect of these drugs. Gout causes severe, unbearable pain because of the sharp crystals in the joint. The irritation also causes the formation of calcareous deposits that deform the joint and limit its motion.

are generally arranged in a radial pattern with respect to the canal. The system of canaliculi that opens to the os-teonal canal also serves for the passage of substances between the osteocytes and blood vessels. Between the osteons are remnants of previous concentric lamellae, called interstitial lamellae (Fig. 8.3). Because of this organization, mature bone is also called lamellar bone.

The long axis of an osteon is usually parallel to the long axis of the bone. The collagen fibers in the concentric lamellae in an osteon are laid down parallel to one another in any given lamella but in different directions in adjacent lamellae. This arrangement gives the cut surface of lamellar bone the appearance of plywood and imparts great strength to the osteon.

Lamellar bone is also found at sites other than the osteon. Circumferential lamellae follow the entire inner and outer circumferences of the shaft of a long bone, appearing much like the growth rings of a tree (see Fig. 8.3). Perforating canals (Volkmann's canals) are channels in lamellar bone through which blood vessels and nerves travel from the periosteal and endosteal surfaces to reach the osteonal canal; they also connect osteonal canals to one another. They usually run at approximately right angles to the long axis of the osteons and of the bone (Fig. 8.3). Volkmann's canals are not surrounded by concentric lamellae, a key feature in their histologic identification.

Mature spongy bone is structurally similar to mature compact bone

Mature spongy bone is similar in structure to mature compact bone except that the tissue is arranged as trabec-ulae or spicules; numerous interconnecting marrow spaces of various size are present between the bone tissue. The matrix of the bone is lamellated. If the trabeculae are sufficiently thick, they will contain osteons.

The blood supply to the shaft of a long bone is chiefly by arteries that enter the marrow cavity through nutrient foramina

Nutrient foramina are openings in the bone through which blood vessels pass to reach the marrow. The greatest numbers of nutrient foramina are found in the diaph-ysis and epiphysis (Fig. 8.4). Metaphyseal arteries supplement the blood supply to the bone. Drainage of the bone is by veins that leave through the nutrient foramina or through the bone tissue of the shaft and out through the periosteum.

The nutrient arteries that supply the diaphysis and epiphysis arise clevelopmentally as the principal vessel of the periosteal buds. The metaphyseal arteries, in contrast, arise developmentally from periosteal vessels that become incorporated into the metaphysis during the growth process, i.e., through the widening of the bone.

The blood supply to bone tissue is essentially centrifugal

The blood that nourishes bone tissue moves from the marrow cavity into and through the bone tissue and out via periosteal veins; thus its flow is in a centrifugal direction. With respect to nourishment of the bone tissue itself, Volkmann's canals provide the major route of entry for osteonal artery collagen fibers inner circumferential lamellae endosteum osteon osteonal artery collagen fibers inner circumferential lamellae endosteum osteon

Osteon Flow Chart

outer circumferential lamellae

Volkmann's canal lamellae of bone outer circumferential lamellae

Volkmann's canal lamellae of bone osteonal endosteum Haversian canal osteocyte and lacuna periosteum FIGURE 8.3

Diagram of a section of compact bone removed from the shaft of a long bone. The concentric lamellae and the Haversian canal that they surround constitute an osteon (Haversian system). One of the Haversian systems in this diagram is drawn as an elongated cylindrical structure rising above the plane of the bone section. It consists of several concentric lamellae that have been partially removed to show the perpendicular orientation of collagen fibers in adjacent layers. Interstitial lamellae result from bone remodeling and formation of new Haversian systems. The inner and outer surfaces of the compact bone in this diagram show additional lamellae-the outer and inner circumferential lamellae-that are arranged in broad layers. The inner circumferential lamella is covered by a thin layer of endosteum that faces the marrow cavity, similar to the outer surface of the bone, which is covered by periosteum. Branches of nutritional arteries accompanied by small veins are shown within the Haversian and Volkmann's canals. These arteries also supply the periosteum, endosteum, and bone marrow.

vessels to pass through the compact bone. The smaller blood vessels enter the Haversian canals, which contain a single arteriole and a venule or a single capillary. A lesser supply to the bone tissue arises from periosteal vessels, which usually provide for only the outermost portions of the compact bone. Bone tissue lacks lymphatic vessels; only the periosteal tissue is provided with lymphatic drainage.

Immature Bone

Bone tissue initially formed in the skeleton of a developing fetus is called immature bone. It differs from mature bone in several respects (Fig. 8.5):

• Immature bone does not display an organized lamellated appearance. On the basis of its collagen fiber arrangement, such bone is designated nonlamellar. Nonlamellar bone is also referred to as bundle or woven bone because of the interlacing arrangement of the collagen fibers.

• Immature bone contains relatively more cells per unit area than does mature bone.

• The cells in immature bone tend to be randomly arranged, whereas cells in mature bone tend to be arranged with their long axes in the same direction as the lamellae.

Sister Sleeping Prone Bone

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