Bone

The term bone refers both to organs of the body such as the femur and mandible, composed of multiple tissue types, and to the bone tissue, or osseous tissue, that makes up most of the mass of bones. There are two forms of osseous tissue: (1) Spongy bone fills the heads of the long bones. Although it is calcified and hard, its delicate slivers and plates give it a spongy appearance. (2) Compact (dense) bone is a more dense calcified tissue with no spaces visible to the naked eye. It forms the external surfaces of all bones, so spongy bone, when present, is always covered by compact bone.

The differences between compact and spongy bone are described in chapter 7. Here, we examine only compact bone (table 5.7). Most specimens you study will probably be chips of dead, dried bone ground to microscopic thinness. In such preparations, the cells are absent but spaces reveal their former locations. Most compact bone is arranged in cylinders of tissue that surround central (haversian20 or osteonic) canals, which run longitudinally through the shafts of long bones such as the femur. Blood vessels and nerves travel through the central canals in life. The bone matrix is deposited in concentric lamellae, onionlike layers around each central canal. A central canal and its surrounding lamellae are called an osteon. Tiny lacunae between the lamellae are occupied in life by mature bone cells, or osteocytes.21 Delicate canals called canaliculi radiate from each lacuna to its neighbors and allow the osteo-cytes to keep in touch with each other. The bone as a whole is covered with a tough fibrous periosteum (PERR-ee-OSS-tee-um) similar to the perichondrium of cartilage.

About a third of the dry weight of bone is composed of collagen fibers and chondroitin sulfate; two-thirds consists of minerals (mainly calcium salts) deposited around the collagen fibers.

16chondro = cartilage, gristle + blast = forming 17lacuna = lake, cavity 18hyal = glass

19peri = around + chondri = cartilage

20Clopton Havers (1650-1702), English anatomist

Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 5 Histology 173

Table 5.6 Types of Cartilage

Figure 5.20 Fetal Skeleton.

Microscopic appearance: Clear, glassy matrix, often stained light blue or pink in tissue sections; fine, dispersed collagen fibers, not usually visible; chondrocytes often in small clusters of three or four cells (cell nests), enclosed in lacunae; usually covered by perichondrium

Representative locations: Forms a thin articular cartilage, lacking perichondrium, over the ends of bones at movable joints; a costal cartilage attaches the end of a rib to the breastbone; forms supportive rings and plates around trachea and bronchi; forms a boxlike enclosure around the larynx; forms much of the fetal skeleton

Functions: Eases joint movements; holds airway open during respiration; moves vocal cords during speech; a precursor of bone in the fetal skeleton and the growth zones of long bones of children

Figure 5.21 External Ear.

Microscopic appearance: Elastic fibers form weblike mesh amid lacunae; always covered by perichondrium Representative locations: External ear; epiglottis Functions: Provides flexible, elastic support

Figure 5.22 Intervertebral Disc.

Microscopic appearance: Parallel collagen fibers similar to those of tendon; rows of chondrocytes in lacunae between collagen fibers; never has a perichondrium Representative locations: Pubic symphysis (anterior joint between two halves of pelvic girdle); intervertebral discs that separate bones of vertebral column; menisci, or pads of shock-absorbing cartilage, in knee joint; at points where tendons insert on bones near articular hyaline cartilage Functions: Resists compression and absorbs shock in some joints; often a transitional tissue between dense connective tissue and hyaline cartilage (for example, at some tendon-bone junctions)

Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

174 Part One Organization of the Body

Table 5.7 Bone

Table 5.8 Blood

Organization Compact Bone Cranium
Picture Osteon And Functions
(a)

Osteon

Concentric —

Central i—Lacunae

Osteon

Concentric —

Central i—Lacunae

Figure 5.23 Compact Bone.

Platelets-, Neutrophils-, Lymphocyte rErythrocytes ,-Monocyte

Figure 5.23 Compact Bone.

Microscopic appearance (compact bone): Calcified matrix arranged in concentric lamellae around central canals; osteocytes occupy lacunae between adjacent lamellae; lacunae interconnected by delicate canaliculi Representative locations: Skeleton Functions: Physical support of body; leverage for muscle action; protective enclosure of viscera; reservoir of calcium and phosphorus

Platelets-, Neutrophils-, Lymphocyte rErythrocytes ,-Monocyte

Figure 5.24 Blood Smear.

Figure 5.24 Blood Smear.

Microscopic appearance: Erythrocytes appear as pale pink discs with light centers and no nuclei; leukocytes are slightly larger, are much fewer, and have variously shaped nuclei, usually stained violet; platelets are cell fragments with no nuclei, about one-quarter the diameter of erythrocytes Representative locations: Contained in heart and blood vessels Functions: Transports gases, nutrients, wastes, chemical signals, and heat throughout body; provides defensive leukocytes; contains clotting agents to minimize bleeding; platelets secrete growth factors that promote tissue maintenance and repair

Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

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