Microscopic Structure

Recall from chapter 5 (page 158) that bone cells called osteocytes (os'te-o-sltz) are located in tiny, bony chambers called lacunae, which form concentric circles around central canals (Haversian canals). Osteocytes transport nutrients and wastes to and from nearby cells by means of cellular processes passing through canali-culi. The intercellular material of bone tissue is largely collagen and inorganic salts. Collagen gives bone its strength and resilience, and inorganic salts make it hard and resistant to crushing.

Compact Bone

In compact bone, the osteocytes and layers of intercellular material concentrically clustered around a central canal form a cylinder-shaped unit called an osteon (os'te-on) sometimes called an Haversian system (fig. 7.4). Many of these units cemented together form the substance of compact bone (fig. 7.5). The orientation of the osteons resists compressive forces.

Each central canal contains blood vessels and nerve fibers surrounded by loose connective tissue. Blood in these vessels nourishes bone cells associated with the os-teonic canal via gap junctions between osteocytes.

Central canals pervade bone tissue longitudinally. Transverse perforating canals (Volkmann's canals) interconnect them. These canals contain larger blood vessels and nerves that allow the smaller blood vessels and nerve fibers in the osteonic canals to communicate with the surface of the bone and the medullary cavity (see fig. 7.4).

Spongy Bone

Spongy bone is also composed of osteocytes and intercellular material, but the bone cells do not aggregate around central canals. Instead, the cells lie within the trabeculae and get nutrients from substances diffusing into the canaliculi that lead to the surface of these thin, bony plates.

Severe bone pain is a symptom of sickle cell disease, which is inherited. Under low oxygen conditions, abnormal hemoglobin (an oxygen-carrying protein) bends the red blood cells that contain it into a sickle shape, which obstructs circulation. X rays can reveal blocked arterial blood flow in bones of sickle cell disease patients.

Explain how bones are classified.

List five major parts of a long bone.

How do compact and spongy bone differ in structure?

Describe the microscopic structure of compact bone.

Microscopic Structure Compact Bone

Figure

Compact bone is composed of osteons cemented together by bone matrix. The trabeculae of spongy bone contain osteocytes arranged in lamellae, but lacking central canals.

Electron Microscopy Bone

Central canal

Lacuna Canaliculus

Central canal

Lacuna Canaliculus

Figure 7.5

Scanning electron micrograph of a single osteon in compact bone (575x).

Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, by R. G. Kessel and R. H. Kardon. © 1979 W. H. Freeman and Company.

Electron Micrograph Muscle Tissue

Intra-

membranous bones forming

Endochondral bones forming

Figure 7.6

Intra-

membranous bones forming

Endochondral bones forming

Figure 7.6

(a) Note the stained bones of this fourteen-week fetus. (b) Bones can fracture even before birth. This fetus has numerous broken bones because of an inherited defect in collagen called osteogenesis imperfecta characterized by abnormally brittle bones. Often parents of such children are unfairly accused of child abuse when their children frequently break bones.

Structure Blood Vessels
(b)

Bone Development and Growth

Parts of the skeletal system begin to form during the first few weeks of prenatal development, and bony structures continue to grow and develop into adulthood. Bones form by replacing existing connective tissue in one of two ways. Some bones originate within sheetlike layers of connective tissues; they are called intramembranous bones. Others begin as masses of cartilage that are later replaced by bone tissue; they are called endochondral bones (fig. 7.6).

Intramembranous Bones

The broad, flat bones of the skull are intramembranous bones (in"trah-mem'brah-nus bonz). During their development (osteogenesis), membranelike layers of unspe-cialized, or primitive, connective tissues appear at the sites of the future bones. Dense networks of blood vessels supply these connective tissue layers, which may form around the vessels. These primitive cells enlarge and differentiate into bone-forming cells called osteoblasts (os'te-o-blasts), which, in turn, deposit bony matrix around themselves. As a result, spongy bone forms in all directions along blood vessels within the layers of primitive connective tissues. Later, some spongy bone may become compact bone as spaces fill with bone matrix.

As development continues, the osteoblasts may become completely surrounded by matrix, and in this manner, they become secluded within lacunae. At the same time, matrix enclosing the cellular processes of the os-

Microscopic Structure Lacuna

Figure

Transmission electron micrograph (artificially colored) of an osteocyte isolated within a lacuna (4,700x).

Figure

Transmission electron micrograph (artificially colored) of an osteocyte isolated within a lacuna (4,700x).

teoblasts gives rise to canaliculi. Once isolated in lacunae, these cells are called osteocytes (fig. 7.7).

Cells of the primitive connective tissue that persist outside the developing bone give rise to the periosteum. Osteoblasts on the inside of the periosteum form a layer of compact bone over the surface of the newly formed spongy bone.

This process of replacing connective tissue to form an intramembranous bone is called intramembranous ossification. Table 7.1 lists the major steps of the process.

Major Steps in Bone Development

Major Steps in Bone Development

Intramembranous Ossification

Endochondral Ossification

1. Sheets of primitive connective tissue appear at sites of future bones.

2. Primitive connective tissue cells collect around blood vessels in these layers.

3. Connective tissue cells differentiate into osteoblasts, which deposit spongy bone.

4. Osteoblasts become osteocytes when bony matrix completely surrounds them.

5. Connective tissue on the surface of each developing structure forms a periosteum.

6. Osteoblasts on the inside of the periosteum deposit compact bone over the spongy bone.

1. Masses of hyaline cartilage form models of future bones.

2. Cartilage tissue breaks down. Periosteum develops.

3. Blood vessels and differentiating osteoblasts from the periosteum invade the disintegrating tissue.

4. Osteoblasts form spongy bone in the space occupied by cartilage.

5. Osteoblasts become osteocytes when bony matrix completely surrounds them.

6. Osteoblasts beneath the periosteum deposit compact bone around spongy bone.

Cartilaginous Calcified model cartilage

Developing periosteum

Compact bone developing

Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • ARTEMIA TREVISAN
    Which structure is characteristic of spongy bone?
    6 years ago
  • Riccardo
    What are the parts of the microscopic bone structures?
    6 years ago
  • arto rahkamo
    Which characteristic describes compact bone?
    6 years ago
  • LEE
    How does compact bone differ from spongy bone?
    6 years ago
  • lukas
    What is MICROSCOPIC STRUCTURE?
    5 years ago
  • elizabeth
    How the microscopic structure of spongy bone differs from that of compact bone?
    5 years ago
  • kiros
    What microscopic structure contains nerves and blood vessels and runs through compact bone?
    5 years ago
  • taija
    What structures in Compact Bone Blood Vessels?
    5 years ago
  • rahel
    Which compact bone house the blood vessel?
    4 years ago
  • ambrogio
    What are thin celled microscopic blood vessels called?
    4 years ago
  • PAULI
    What are the two microscopic structures that house the blood vessels in bone?
    4 years ago
  • MARI PUTKONEN
    What structures houses blood vessels?
    4 years ago
  • Marigold
    What are the microscopic structures of compact and spongy bone?
    4 years ago
  • junior
    What lacunae form around?
    3 years ago
  • Rhiannon
    What are microscopic vessels located in tissues called?
    3 years ago
  • Demsas Rezene
    Which anatomical structure in the compact bone house blood vessels?
    3 years ago

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