The Skeletal System: Bone Tissue Checkpoint Questions and Answers

Describe the role of bones in blood cell production.

Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis (hēm-ō-poy-ē-sis; hemo- = blood; -poiesis = making). Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers.

How do mechanical stresses strengthen bone tissue?

When placed under stress, bone tissue becomes stronger through increased deposition of mineral salts and production of collagen fibers by osteoblasts.

Why is it important to engage in weight-bearing exercises before the epiphyseal plates close?

Weight-bearing activities, such as walking or moderate weight lifting, help build and retain bone mass. Adolescents and young adults should engage in regular weight-bearing exercise prior to the closure of the epiphyseal plates to help build total mass prior to its inevitable reduction with aging.

How do red bone marrow and yellow bone marrow differ in composition and function?

Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve. With increasing age, much of the bone marrow changes from red to yellow. Red Bone Marrow: developing blood cells, adipocytes, fibroblasts, and macrophalges. Pelvis, ribes, sternum, vertebrae, skull and ends of long bones, fetal skeleton. Makes red and white cells and platelets.

Diagram the parts of a long bone, and list the functions of each part.

1. The diaphysis (dī-AF-i-sis = growing between) is the bone's shaft or body—the long, cylindrical, main portion of the bone. 2. The epiphyses (e-PIF-i-sēz = growing over; singular is epiphysis) are the proximal and distal ends of the bone. 3. The metaphyses (me-TAF-i-sēz; 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 (ep′-i-FIZ-ē-al), a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length (described later in the chapter). When a bone ceases to grow in length at about ages 14-24, 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 reduces friction and absorbs shock at freely movable joints. Because articular cartilage lacks a perichondrium and lacks blood vessels, repair of damage is limited. 5. The periosteum (per-ē-OS-tē-um; peri- = around) is a tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage. It is composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells. Some of the cells enable bone to grow in thickness, but not in length. The periosteum also protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons. The periosteum is attached to the underlying bone by perforating fibers or Sharpey's fibers, thick bundles of collagen that extend from the periosteum into the bone extracellular matrix. 6. The medullary cavity (MED-ul-er-ē; medulla- = marrow, pith), or marrow cavity, is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels in adults. This cavity minimizes the weight of the bone by reducing the dense bony material where it is least needed. The long bones' tubular design provides maximum strength with minimum weight. 7. The endosteum (end-OS-tē-um; endo- = within) is a thin membrane that lines the medullary cavity. It contains a single layer of bone-forming cells and a small amount of connective tissue.

List the four types of cells in bone tissue and their functions.

1. Osteoprogenitor cells (os′-tē-ō-prō-JEN-i-tor; -genic = producing) are unspecialized bone 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. Osteoprogenitor 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 (OS-tē-ō-blasts′; -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 (described shortly). 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 (OS-tē-ō-sīts′; -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 and monitors the tissue.) 4. Osteoclasts (OS-tē-ō-klasts′; -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 extracellular bone matrix. This breakdown of bone extracellular matrix, termed resorption (rē-SORP-shun), is part of the normal development, maintenance, and repair of bone. (Note: The ending -clast means that the cell breaks down extracellular matrix.)In response to certain hormones, osteoclasts help regulate blood calcium level (see Section 6.7). They are also target cells for drug therapy used to treat osteoporosis (see Disorders: Homeostatic Imbalances at the end of this chapter).

What is a bone scan and how is it used clinically?

A bone scan is used to: Diagnose a bone tumor or cancer. Determine if a cancer that began elsewhere in your body has spread to the bones.

What factors contribute to the hardness and tensile strength of bone?

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.

Define remodeling, and describe the roles of osteoblasts and osteoclasts in the process.

Bone remodeling is the ongoing replacement of old bone tissue by new bone tissue. It involves bone resorption, the removal of minerals and collagen fibers from bone by osteoclasts, and bone deposition, the addition of minerals and collagen fibers to bone by osteoblasts. Thus, bone resorption results in the destruction of bone extracellular matrix, while bone deposition results in the formation of bone extracellular matrix. During the process of bone resorption, an osteoclast attaches tightly to the bone surface at the endosteum or periosteum and forms a leakproof seal at the edges of its ruffled border (see Figure 6.2). Then it releases protein-digesting lysosomal enzymes and several acids into the sealed pocket. The enzymes digest collagen fibers and other organic substances while the acids dissolve the bone minerals. Working together, several osteoclasts carve out a small tunnel in the old bone. The degraded bone proteins and extracellular matrix minerals, mainly calcium and phosphorus, enter an osteoclast by endocytosis, cross the cell in vesicles, and undergo exocytosis on the side opposite the ruffled border. Now in the interstitial fluid, the products of bone resorption diffuse into nearby blood capillaries. Once a small area of bone has been resorbed, osteoclasts depart and osteoblasts move in to rebuild the bone in that area.

How do hormones act on bone to regulate calcium homeostasis?

Ca2+ exchange is regulated by hormones, the most important of which is parathyroid hormone (PTH) secreted by the parathyroid glands (see Figure 18.13). This hormone increases blood Ca2+ level. PTH secretion operates via a negative feedback system . If some stimulus causes the blood Ca2+ level to decrease, parathyroid gland cells (receptors) detect this change and increase their production of a molecule known as cyclic adenosine monophosphate (cyclic AMP). The gene for PTH within the nucleus of a parathyroid gland cell (the control center) detects the intracellular increase in cyclic AMP (the input). As a result, PTH synthesis speeds up, and more PTH (the output) is released into the blood. The presence of higher levels of PTH increases the number and activity of osteoclasts (effectors), which step up the pace of bone resorption. The resulting release of Ca2+ from bone into blood returns the blood Ca2+ level to normal. PTH also acts on the kidneys (effectors) to decrease loss of Ca2+ in the urine, so more is retained in the blood. And PTH stimulates formation of calcitriol (the active form of vitamin D), a hormone that promotes absorption of calcium from foods in the gastrointestinal tract into the blood. Both of these actions also help elevate blood Ca2+ level. Another hormone works to decrease blood Ca2+ level. When blood Ca2+ rises above normal, parafollicular cells in the thyroid gland secrete calcitonin (CT) (kal-si-TŌ-nin). CT inhibits activity of osteoclasts, speeds blood Ca2+ uptake by bone, and accelerates Ca2+ deposition into bones. The net result is that CT promotes bone formation and decreases blood Ca2+ level.

How are compact and spongy bone tissues different in microscopic appearance, location, and function?

Compact bone tissue is composed of repeating structural units called osteons, or haversian systems (ha-VER-shan). Each osteon consists of concentric lamellae arranged around a central canal or haversian canal. Resembling the growth rings of a tree, the concentric lamellae (la-MEL-ē) are circular plates of mineralized extracellular matrix of increasing diameter, surrounding a small network of blood vessels and nerves located in the central canal (Figure 6.3a). These tubelike units of bone generally form a series of parallel cylinders that, in long bones, tend to run parallel to the long axis of the bone. In contrast to compact bone tissue, spongy bone tissue, also referred to as trabecular or cancellous bone tissue, does not contain osteons (Figure 6.3b, c). Spongy bone tissue is always located in the interior of a bone, protected by a covering of compact bone. It consists of lamellae that are arranged in an irregular pattern of thin columns called trabeculae (tra-BEK-ū-lē = little beams; singular is trabecula).Spongy bone tissue makes up most of the interior bone tissue of short, flat, sesamoid, and irregularly shaped bones. In long bones it forms the core of the epiphyses beneath the paper-thin layer of compact bone, and forms a variable narrow rim bordering the medullary cavity of the diaphysis. Spongy bone is always covered by a layer of compact bone for protection.

What are the major events of intramembranous ossification and endochondral ossification, and how are they different?

Development of the ossification center. At the site where the bone will develop, specific chemical messages cause the cells of the mesenchyme to cluster together and differentiate, first into osteoprogenitor cells and then into osteoblasts. The site of such a cluster is called an ossification center. Osteoblasts secrete the organic extracellular matrix of bone until they are surrounded by it. Calcification. Next, the secretion of extracellular matrix stops, and the cells, now called osteocytes, lie in lacunae and extend their narrow cytoplasmic processes into canaliculi that radiate in all directions. Within a few days, calcium and other mineral salts are deposited and the extracellular matrix hardens or calcifies (calcification). Formation of trabeculae. As the bone extracellular matrix forms, it develops into trabeculae that fuse with one another to form spongy bone around the network of blood vessels in the tissue. Connective tissue associated with the blood vessels in the trabeculae differentiates into red bone marrow. Development of the periosteum. In conjunction with the formation of trabeculae, the mesenchyme condenses at the periphery of the bone and develops into the periosteum. Eventually, a thin layer of compact bone replaces the surface layers of the spongy bone, but spongy bone remains in the center. Much of the newly formed bone is remodeled (destroyed and reformed) as the bone is transformed into its adult size and shape.

Explain how bone growth in length differs from bone growth in thickness.

In length, E. Plate cartlige is replaced by bone on diaphyseal side, which lengthens diapysis. Addition of new bone tissue by periosteal osteoblasts (appositional growth) causes growth in thickness.

How is a bone marrow needle biopsy performed? What conditions are diagnosed through this procedure?

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 leukemias, metastatic neoplasms, lymphoma, Hodgkin's disease, and aplastic anemia. As the needle penetrates the periosteum, pain is felt.

Which bones contain red bone marrow?

It is present in developing bones of the fetus and in some adult bones, such as the hip (pelvic) bones, ribs, sternum (breastbone), vertebrae (backbones), skull, and ends of the bones of the humerus (arm bone) and femur (thigh bone). In a newborn, all bone marrow is red and is involved in hemopoiesis.

Why is bone considered a connective tissue?

Like other connective tissues, bone, or osseous tissue (OS-ē-us), contains an abundant extracellular matrix that surrounds widely separated cells. The extracellular matrix is about 15% water, 30% collagen fibers, and 55% crystallized mineral salts.

How could the metaphyseal area of a bone help determine the age of a skeleton?

Metaphysis has the epiphyseal plate the activity of the epiphyseal plate is the only way that the diaphysis can increase in length. When adolescence comes to an end (at about age 18 in females and age 21 in males), the epiphyseal plates close; that is, the epiphyseal cartilage cells stop dividing and bone replaces all remaining cartilage. The epiphyseal plate fades, leaving a bony structure called the epiphyseal line. Closure of the epiphyseal plate is a gradual process and the degree to which it occurs is useful in determining bone age, predicting adult height, and establishing age at death from skeletal remains, especially in infants, children, and adolescents.

Which part of a bone contains sensory nerves associated with pain?

Nerves accompany the blood vessels that supply bones. The periosteum is rich in sensory nerves, some of which carry pain sensations.

What factors affect bone growth and bone remodeling?

Normal bone metabolism—growth in the young and bone remodeling in the adult—depends on several factors. These include adequate dietary intake of minerals and vitamins, as well as sufficient levels of several hormones. During adulthood, sex hormones contribute to bone remodeling by slowing resorption of old bone and promoting deposition of new bone.

What is the composition of extracellular matrix of bone tissue?

The extracellular matrix is about 15% water, 30% collagen fibers, and 55% 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] (hī-drok-sē-AP-a-tīt). 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.

Describe the zones of the epiphyseal (growth) plate and their functions, and the significance of the epiphyseal line.

The hyaline cartilage that covers the epiphyses becomes the articular cartilage. Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal (growth) plate, the region responsible for the lengthwise growth of long bones. The activity of the epiphyseal plate is the only way that the diaphysis can increase in length. As a bone grows, chondrocytes proliferate on the epiphyseal side of the plate. New chondrocytes replace older ones, which are destroyed by calcification.

Explain the location and roles of the nutrient arteries, nutrient foramina, epiphyseal arteries, and periosteal arteries.

The nutrient arteries, nutrient foramina, epiphyseal arteries, and periosteal arteries. are located along the diaphysis. Periosteal arteries (per-ē-OS-tē-al), small arteries accompanied by nerves, enter the diaphysis through many perforating (Volkmann's) canals and supply the periosteum and outer part of the compact bone (see Figure 6.3a). Near the center of the diaphysis, a large nutrient artery passes through a hole in compact bone called the nutrient foramen (foramina is plural). On entering the medullary cavity, the nutrient artery divides into proximal and distal branches that course toward each end of the bone. These branches supply both the inner part of compact bone tissue of the diaphysis and the spongy bone tissue and red bone 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 metaphyseal and epiphyseal arteries, which arise from arteries that supply the associated joint. The metaphyseal arteries (met-a-FIZ-ē-al) 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 (ep′-i-FIZ-ē-al) 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 through the diaphysis; (2) numerous epiphyseal veins and metaphyseal veins accompany their respective arteries and exit through the epiphyses and metaphyses, respectively; and (3) many small periosteal veins accompany their respective arteries and exit through the periosteum.

How does the skeletal system function in support, protection, movement, and storage of minerals?

The skeletal system performs several basic functions: 1. Support. The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles. 2. Protection. The skeleton protects the most important internal organs from injury. For example, cranial bones protect the brain, and the rib cage protects the heart and lungs. 3. Assistance in movement. Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement. This function is discussed in detail in Chapter 10. 4. Mineral homeostasis (storage and release). Bone tissue makes up about 18% of the weight of the human body. It stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone. Bone tissue stores about 99% of the body's calcium. On demand, bone releases minerals into the blood to maintain critical mineral balances (homeostasis) and to distribute the minerals to other parts of the body.

Describe one situation in which these sensory neurons are important.

These nerves are especially sensitive to tearing or tension, which explains the severe pain resulting from a fracture or a bone tumor.

List the types of fractures and outline the four steps involved in fracture repair.

Types of fractures: Open (compound), Comminuted, Greenstick, Impacted, Potts, Colles. Reactive phase. This phase is an early inflammatory phase. Blood vessels crossing the fracture line are broken. As blood leaks from the torn ends of the vessels, a mass of blood (usually clotted) forms around the site of the fracture. This mass of blood, called a fracture hematoma (hē′-ma-TŌ-ma; hemat- = blood; -oma = tumor), usually forms 6 to 8 hours after the injury. Because the circulation of blood stops at the site where the fracture hematoma forms, nearby bone cells die. Swelling and inflammation occur in response to dead bone cells, producing additional cellular debris. Phagocytes (neutrophils and macrophages) and osteoclasts begin to remove the dead or damaged tissue in and around the fracture hematoma. This stage may last up to several weeks. Reparative phase: Fibrocartilaginous callus formation. The reparative phase is characterized by two events: the formation of a fibrocartilaginous callus and a bony callus to bridge the gap between the broken ends of the bones. Blood vessels grow into the fracture hematoma and phagocytes begin to clean up dead bone cells. Fibroblasts from the periosteum invade the fracture site and produce collagen fibers. In addition, cells from the periosteum develop into chondroblasts and begin to produce fibrocartilage in this region. These events lead to the development of a fibrocartilaginous (soft) callus (fi-brō-kar-ti-LAJ-i-nus), a mass of repair tissue consisting of collagen fibers and cartilage that bridges the broken ends of the bone. Formation of the fibrocartilaginous callus takes about 3 weeks. Reparative phase: Bony callus formation. In areas closer to well-vascularized healthy bone tissue, osteoprogenitor cells develop into osteoblasts, which begin to produce spongy bone trabeculae. The trabeculae join living and dead portions of the original bone fragments. In time, the fibrocartilage is converted to spongy bone, and the callus is then referred to as a bony (hard) callus. The bony callus lasts about 3 to 4 months. Bone remodeling phase. The final phase of fracture repair is bone remodeling of the callus. Dead portions of the original fragments of broken bone are gradually resorbed by osteoclasts. Compact bone replaces spongy bone around the periphery of the fracture. Sometimes, the repair process is so thorough that the fracture line is undetectable, even in a radiograph (x-ray). However, a thickened area on the surface of the bone remains as evidence of a healed fracture.

Would children raised in space ever be able to return to Earth?

the strength of the unstressed bones diminishes because of the loss of bone minerals and decreased numbers of collagen fibers. Children raised in space subjected to the microgravity of space also lose bone mass, but would be able to return because the main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of gravity.