6. STRUCTURE OF BONE

6. STRUCTURE OF BONE

• Macroscopic bone structure may be analyzed by considering the parts of a long bone, such as the humerus (the arm bone)

•  A long bone is one that has greater length than width. 

A typical long bone consists of the following parts:

1. The diaphysis ( growing between) 

• the bone’s shaft or body—the long, cylindrical, main portion of the bone.

2. The epiphyses 

• ( growing over; singular is epiphysis) 
• are the proximal and distal ends of the bone.

3. The metaphyses 

• ( meta- between; singular is metaphysis) 
• are the regions between the diaphysis and the epiphyses.

• In a growing bone, each metaphysis contains an epiphyseal (growth) plate , a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length. 
• When a bone ceases to grow in length at about ages 18–21, the cartilage in the epiphyseal plate is replaced by bone; 
• the resulting bony structure is known as the epiphyseal line.

4. The articular cartilage 

• is a thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. 
• Articular cartilage 

1. reduces friction and 
2. absorbs shock at freely movable joints.

5. The periosteum (per -e--OS-te--um; peri- round) 

• surrounds the external bone surface wherever it is not covered by articular cartilage. 
• It is composed of 

1. an outer fibrous layer of dense irregular connective tissue 
2. and an inner osteogenic layer that consists of cells.

•  Some of the cells of the periosteum enable bone to grow in thickness, but not in length. 
• The periosteum also

1. protects the bone, 
2. assists in fracture repair, 
3. helps nourish bone tissue, and
4.  serves as an attachment point for ligaments and tendons.

• It is attached to the underlying bone through perforating (Sharpey’s) fibers, thick bundles of collagen fibers that extend from the periosteum into the extracellular bone matrix.

6. The medullary cavity

•  ( medulla- marrow, pith) or marrow cavity 
• is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow in adults.

7. The endosteum ( endo- within)

• is a thin membrane that lines the internal bone surface facing the medullary cavity. 
• It contains a single layer of cells and a small amount of connective tissue.

HISTOLOGY OF BONE TISSUE

The structure of bone at the microscopic level. 

• Like other connective tissues, bone, or osseous tissue , contains an abundant extracellular matrix that surrounds widely separated cells. 
• The extracellular matrix is about 

1. 25% water, 
2. 25% collagen fibers, and 
3. 50% crystallized mineral salts. 

The most abundant mineral salt is 

• calcium phosphate [Ca3(PO4)2].
•  It combines with another mineral salt, calcium hydroxide [Ca(OH)2], to form crystals of hydroxyapatite [Ca10(PO4)6 (OH)2]. 
• As the crystals form, they combine with still other mineral salts, such as calcium carbonate (CaCO3), and ions such as magnesium, fluoride, potassium, and sulfate. 

• As these mineral salts are deposited in the framework formed by the collagen fibers of the extracellular matrix, 
• they crystallize and the tissue hardens. 
• This process, called calcification ,
•  is initiated by bone-building cells called osteoblasts

• It was once thought that calcification simply occurred when enough mineral salts were present to form crystals. 
• We now know that the process requires the presence of collagen fibers.

• Mineral salts first begin to crystallize in the microscopic spaces between collagen fibers. 
• After the spaces are filled, mineral crystals accumulate around the collagen fibers.

• Although a bone’s hardness depends on the crystallized inorganic mineral salts, 
• a bone’s flexibility depends on its collagen fibers. 

Like reinforcing metal rods in concrete, collagen fibers and other organic molecules provide 

• tensile strength,
• resistance to being stretched or torn apart. 

• Soaking a bone in an acidic solution, such as vinegar, dissolves its mineral salts, causing the bone to become rubbery and flexible. 
•  when the need for particular minerals arises or as part of bone formation or breakdown,  bone cells called osteoclasts secrete enzymes and acids that break down both the mineral salts and the collagen fibers of bone extracellular matrix.

Four types of cells are present in bone tissue: 

• osteogenic cells, 
• osteoblasts, 
• osteocytes, and 
• osteoclasts 

1. Osteogenic cells ( -genic producing) 

• are unspecialized stem cells derived from mesenchyme, the tissue from which almost all connective tissues are formed. 
• They are the only bone cells to undergo cell division; 
• the resulting cells develop into osteoblasts. 

Osteogenic cells are found along 

• the inner portion of the periosteum, 
• in the endosteum, 
• and in the canals within bone that contain blood vessels.

2. Osteoblasts ( -blasts buds or sprouts) 

• are bone-building cells. 
• They synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue, 
• and they initiate calcification . 
• As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes. 
• (Note: The ending -blast in the name of a bone cell or any other connective tissue cell means that the cell secretes extracellular matrix.) 

3. Osteocytes ( -cytes cells), 

• mature bone cells, 
• are the main cells in bone tissue 
• and maintain its daily metabolism, such as the exchange of nutrients and wastes with the blood. 
• Like osteoblasts, osteocytes do not undergo cell division. 
• (Note: The ending -cyte in the name of a bone cell or any other tissue cell means that the cell maintains the tissue.)

4. Osteoclasts ( -clast break) 

• are huge cells derived from the fusion of as many as  50 monocytes (a type of white blood cell) 
• and are concentrated in the endosteum. 

• On the side of the cell that faces the bone surface, the osteoclast’s plasma membrane is deeply folded into a ruffled border. 
• Here the cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying bone matrix. 
• This breakdown of bone extracellular matrix, termed resorption , 
• is part of the normal development, maintenance, and repair of bone. 
• (Note: The ending -clast in a bone cell means that the cell breaks down extracellular matrix.) 
•  in response to certain hormones, 

1. osteoclasts help regulate blood calcium level 
2. They are also target cells for drug therapy used to treat osteoporosis 

• Bone is not completely solid but has many small spaces between its cells and extracellular matrix components. 
• Some spaces serve as channels for blood vessels that supply bone cells with nutrients. 
• Other spaces act as storage areas for red bone marrow. 

• Depending on the size and distribution of the spaces, the regions of a bone may be categorized as 

1. compact or 
2. spongy

•  Overall,  about 80% of the skeleton is compact bone and 
• 20% is spongy bone.

Compact Bone Tissue

• Compact bone tissue contains few spaces and 
• is the strongest form of bone tissue. 
• It is found beneath the periosteum of all bones 
• and makes up the bulk of the diaphyses of long bones. 

• provides protection and support and
• resists the stresses produced by weight and movement.

• Blood vessels, lymphatic vessels, and nerves from the periosteum penetrate compact bone through transverse perforating or Volkmann’s (FOLK-mans) canals. 
• The vessels and nerves of the perforating canals connect with those of the medullary cavity, periosteum, and central or haversian canals. 

• The central canals run longitudinally through the bone. 
• Around the central canals are concentric  lamellae—rings of calcified extracellular matrix much like the rings of a tree trunk. 
• Between the lamellae are small spaces called lacunae (little lakes; singular is lacuna),which contain osteocytes. 
• Radiating in all directions from the lacunae are tiny canaliculi ( small channels) filled with extracellular fluid. 

• Inside the canaliculi are slender fingerlike processes of osteocytes  Neighboring osteocytes communicate via gap junctions.
• The canaliculi connect lacunae with one another and with the central canals, forming an intricate, miniature system of interconnected canals throughout the bone. 
• This system provides many routes for nutrients and oxygen to reach the osteocytes and for the removal of wastes.

• The components of compact bone tissue are arranged into repeating structural units called osteons or haversian systems
•  Each osteon consists of a central (haversian) canal with its concentrically arranged lamellae, lacunae, osteocytes, and canaliculi. 
• Osteons in compact bone tissue are aligned in the same direction along lines of stress. 

In the shaft, for example, 

• they are parallel to the long axis of the bone. 
• As a result, the shaft of a long bone resists bending or fracturing even when considerable force is applied from either end. 

• The osteons of a long bone can be compared to a stack of logs; 
• each log is made up of rings of hard material, 
• and together it requires considerable force to fracture them all. 

• The lines of stress in a bone change as a baby learns to walk 
• and in response to repeated strenuous physical activity, such as weight training. 
• The lines of stress in a bone also can change in response to fractures or physical deformity.

• Thus, the organization of osteons is not static but changes over time in response to the physical demands placed on the skeleton.

• The areas between osteons contain interstitial lamellae, which also have lacunae with osteocytes and canaliculi.
•  Interstitial lamellae are fragments of older osteons that have been partially destroyed during bone rebuilding or growth. 

• Lamellae that encircle the bone just beneath the periosteum or encircle the medullary cavity are called circumferential lamellae.

Spongy Bone Tissue

• In contrast to compact bone tissue, spongy bone tissue does not contain osteons. 
• Despite what the name seems to imply, the term “spongy” does not refer to the texture of the bone, only its appearance.

• Spongy bone consists of lamellae arranged in an irregular lattice of thin columns called trabeculae ( little beams; singular is trabecula).
• The macroscopic spaces between the trabeculae help make bones lighter and can sometimes be filled with red bone marrow, which contains numerous small blood vessels. 

• Within each trabecula are lacunae that contain osteocytes. 
• Canaliculi radiate outward from the lacunae. 
• Osteocytes receive nourishment from the blood circulating through the blood vessels in the spaces between trabeculae.

• Spongy bone tissue makes up most of the interior bone tissue of short, flat, and irregularly shaped bones, 
• and most of the epiphyses of long bones. 
• Spongy bone tissue also forms a narrow rim around the medullary cavity of the diaphysis of long bones, where it is covered by endosteum. 

• Spongy bone is always covered by a layer of compact bone for protection.
• At first glance, the structure of the osteons of compact bone tissue appears to be highly organized, 
• and the trabeculae of spongy bone tissue appear to be randomly arranged. 

• However, the trabeculae of spongy bone tissue are precisely oriented along lines of stress, a characteristic that helps bones resist stresses and transfer force without breaking. 

• Spongy bone tissue tends to be located where bones are not heavily stressed or where stresses are applied from many directions.

• Spongy bone tissue is different from compact bone tissue in two respects. 

1. First, spongy bone tissue is light, 

• which reduces the overall weight of a bone so that it moves more readily when pulled by a skeletal muscle. 

2. Second, the trabeculae of spongy bone tissue support and protect the red bone marrow. 

• The spongy bone tissue in the hip bones, ribs, sternum (breastbone), vertebrae (backbones), and the ends of long bones is where red bone marrow is stored and, 
• thus, where hemopoiesis (blood cell production) occurs in adults.

CLINICAL CONNECTION 

Bone Scan

• A bone scan is a diagnostic procedure that takes advantage of the fact that bone is living tissue. 

• A small amount of a radioactive tracer compound that is readily absorbed by bone is injected intravenously. 
• The degree of uptake of the tracer is related to the amount of blood flow to the bone. 

• A scanning device (gamma camera) measures the radiation emitted from the bones, 
• and the information is translated into a photograph that can be read like an x-ray on a monitor. 

• Normal bone tissue is identified by a consistent gray color throughout because of its uniform uptake of the radioactive tracer. 
• Darker or lighter areas may indicate bone abnormalities. 
• Darker areas called “hot spots” are areas of increased metabolism that absorb more of the radioactive tracer due to increased blood flow. 

Hot spots may indicate 

• bone cancer,
•  abnormal healing of fractures, 
• or abnormal bone growth. 

Lighter areas called “cold spots” 

• are areas of decreased metabolism that absorb less of the radioactive tracer due to decreased blood flow. 

Cold spots may indicate problems such as 

• degenerative bone disease, 
• decalcified bone, 
• fractures,
• bone infections, 
• Paget’s disease, 
• and rheumatoid arthritis.

•  A bone scan detects abnormalities 3 to 6 months sooner than standard xray procedures and exposes the patient to less radiation. 
• A bone scan is the standard test for bone density screening, particularly important in screening for osteoporosis in females. 

BLOOD AND NERVE SUPPLY OF BONE

• Bone is richly supplied with blood. 

Blood vessels, 

• which are especially abundant in portions of bone containing red bone marrow, 
• pass into bones from the periosteum. 

We will consider the blood supply of a long bone such as the mature tibia (shin bone) 

• Periosteal arteries  accompanied by nerves enter the diaphysis through many perforating (Volkmann’s) canals 
• and supply the periosteum and outer part of the compact bone .

• Near the center of the diaphysis, a large nutrient artery passes through a hole in compact bone called the nutrient foramen. 

• On entering the medullary cavity, the nutrient artery divides into proximal and distal branches that supply both the inner part of compact bone tissue of the diaphysis and the spongy bone tissue and red marrow as far as the epiphyseal plates (or lines). 

• Some bones, like the tibia, have only one nutrient artery; 
• others like the femur (thigh bone) have several. 

• The ends of long bones are supplied by the  

1. metaphyseal and
2. epiphyseal arteries, 

• which arise from arteries that supply the associated joint. 

The metaphyseal arteries  

• enter the metaphyses of a long bone and, together with the nutrient artery, supply the red bone marrow and bone tissue of the metaphyses. 

The epiphyseal arteries 

•  enter the epiphyses of a long bone 
• and supply the red bone marrow and bone tissue of the epiphyses.

Veins that carry blood away from long bones are evident in three places: 

(1) One or two nutrient veins 

• accompany the nutrient artery 
• and exit in the diaphysis; 

(2) numerous epiphyseal veins and metaphyseal veins 

• accompany their respective arteries 
• and exit in the epiphyses; and 

(3) many small periosteal veins

•  accompany their respective arteries 
• and exit in the periosteum.

Nerves accompany the blood vessels that supply bones.

•  The periosteum is rich in sensory nerves, some of which carry pain sensations. 
• These nerves are especially sensitive to tearing or tension, which explains the severe pain resulting from a fracture or a bone tumor. 

• For the same reason there is some pain associated with a bone marrow needle biopsy. 
• In this procedure, a needle is inserted into the middle of the bone to withdraw a sample of red bone marrow to examine it for conditions such as

1. leukemias, 
2. metastatic neoplasms, 
3. lymphoma, 
4. Hodgkin’s disease,
5. and aplastic anemia. 

• As the needle penetrates the periosteum, pain is felt. 
• Once it passes through, there is little pain.