Chapter 6: The Skeletal System: Bone Tissue Flashcards
What are the six main functions of the skeletal system?
1. Support – The skeletal system is the structural framework for the body.
2. Protection – Protects internal organs such as the brain, heart, and lungs.
3. Assistance in Movement – When muscles contract, they pull on bones to produce movement.
4. Mineral Homeostasis (storage and release) – Stores minerals such as calcium and phosphorus and released them into the bloodstream on demand.
5. Blood Cell Production – Red bone marrow produces red blood cells, white blood cells, and platelets in a process called hemopoiesis.
6. Triglyceride Storage – Yellow bone marrow consists mainly of adipose cells, which store triglycerides.
What are the 7 parts of a long bone?
A long bone is one that has greater length than width. The parts of a long bone are:
- The diaphysis is the bone’s shaft or body - the long, cylindrical, main portion of the bone.
- The epiphyses are the proximal and distal ends of the bone.
- The metaphyses 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 14–24, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure is known as the epiphyseal line.
- 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.
- The periosteum is a tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage. Some of the cells enable bone to grow in thickness, but not in length. The periosteum 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
- The medullary cavity, or marrow cavity, is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels. This cavity minimizes the weight of the bone by reducing the dense bony material where it is least needed.
- The endosteum 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.
Why is bone tissue classified as a connective tissue?
Like other connective tissues, bone, or osseous tissue, contains an abundant extracellular matrix that surrounds widely separated cells. The most abundant mineral salt is calcium phosphate [Ca3(PO4)2], which combines with another mineral salt, calcium hydroxide [Ca(OH)2], to form crystals of hydroxyapatite in the process of calcification.
What are the 4 types of cells present in bone tissue?
1. Osteoprogenitor cells 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 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, 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 osteo-blasts, 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 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 bone resorption, is part of the normal development, maintenance, and repair of bone. (Note: The ending -clast means that the cell breaks down extra-cellular matrix.) As you will see later, in response to certain hormones, osteoclasts help regulate blood calcium level.
Compare and contrast compact and spongy bone tissue.
Compact bone tissue contains few spaces and is the strongest form of bone tissue. Compact bone tissue is composed of repeating structural units called osteons, or haversian systems. Each osteon consists of concentric lamellae arranged around an osteonic (haversian or central) canal. Resembling the growth rings of a tree, the concentric lamellae are circular plates of mineralized extracellular matrix of increasing diameter, surrounding a small network of blood vessels and nerves located in the central canal. 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. Between the concentric lamellae are small spaces called lacunae which contain osteocytes. Radiating in all directions from the lacunae are tiny canaliculi which are filled with extracellular fluid. Inside the canaliculi are slender finger-like 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. Osteons in compact bone tissue are aligned in the same direction and are parallel to the length of the diaphysis. As a result, the shaft of a long bone resists bending or fracturing even when considerable force is applied from either end. The areas between neighboring osteons contain lamellae called 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. Blood vessels and nerves from the periosteum penetrate the compact bone through transverse interosteonic (Volkmann’s or perforating) canals. Arranged around the entire outer and inner circumference of the shaft of a long bone are lamellae called circumferential lamellae. They develop during initial bone formation. The circumferential lamellae directly deep to the periosteum are called external circumferential lamellae. They are connected to the periosteum by perforating (Sharpey’s) fibers. The circumferential lamellae that line the medullary cavity are called internal circumferential lamellae.
Spongy Bone Tissue
In contrast to compact bone tissue, spongy bone tissue, also referred to as trabecular or cancellous bone tissue, does not contain osteons. 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. Each trabecula consists of concentric lamellae, osteocytes that lie in lacunae, and canaliculi that radiate outward from the lacunae. Spongy bone tissue is different from compact bone tissue in two respects. First, spongy bone tissue is light, which reduces the overall weight of a bone. This reduction in weight allows the bone to move more readily when pulled by a skeletal muscle. Second, the trabeculae of spongy bone tissue support and protect the red bone marrow. Spongy bone in the hip bones, ribs, sternum (breastbone), vertebrae, and the proximal ends of the humerus and femur is the only site where red bone marrow is stored and, thus, the site where hemopoiesis (blood cell production) occurs in adults.
Describe the 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.
Periosteal arteries, small arteries accompanied by nerves, enter the diaphysis through many interosteonic (Volkmann’s or perforating) 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 (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. 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 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.
Nerves accompany the blood vessels that supply bones. The periosteum is rich in sensory nerves, some of which carry pain sensations.
Differentiate between intramembranous and endochondral ossification.
The process by which bone forms is called ossification or osteogenesis. The two patterns of bone formation, which both involve the replacement of a pre-existing connective tissue with bone, do not lead to differences in the structure of mature bones, but are simply different methods of bone development. In the first type of ossification, called intramembranous ossification, bone forms directly within mesenchyme, which is arranged in sheetlike layers that resemble membranes. In the second type, endochondral ossification, bone forms within hyaline cartilage that develops from mesenchyme.
Describe the process of Intramembranous Ossification.
Intramembranous ossification is the simpler of the two methods of bone formation:
1. 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.
2. 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.
3. 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.
4. 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.
Describe the process of Endochondral Ossification.
The replacement of cartilage by bone is called endochondral ossification. It proceeds as follows:
1. Development of the cartilage model. At the site where the bone is going to form, specific chemical messages cause the cells in mesenchyme to crowd together in the general shape of the future bone, and then develop into chondroblasts. The chondroblasts secrete cartilage extracellular matrix, producing a cartilage model (future diaphysis) consisting of hyaline cartilage. A covering called the perichondrium develops around the cartilage model.
2 Growth of the cartilage model. Once chondroblasts become deeply buried in the cartilage extracellular matrix, they are called chondrocytes. The cartilage model grows in length by continual cell division of chondrocytes, accompanied by further secretion of the cartilage extracellular matrix. This type of cartilaginous growth, called interstitial (endogenous) growth (growth from within), results in an increase in length. In contrast, growth of the cartilage in thickness is due mainly to the deposition of extracellular matrix material on the cartilage surface of the model by new chondroblasts that develop from the perichondrium. This process is called appositional (exogenous) growth, meaning growth at the outer surface. As the cartilage model continues to grow, chondrocytes in its mid-region hypertrophy (increase in size) and the surrounding cartilage extracellular matrix begins to calcify. Other chondrocytes within the calcifying cartilage die because nutrients can no longer diffuse quickly enough through the extracellular matrix. As these chondrocytes die, the spaces left behind by dead chondrocytes merge into small cavities called lacunae.
3. Development of the primary ossification center. Primary ossification proceeds inward from the external surface of the bone. A nutrient artery penetrates the perichondrium and the calcifying cartilage model through a nutrient foramen in the mid-region of the cartilage model, stimulating osteoprogenitor cells in the perichondrium to differentiate into osteoblasts. Once the perichondrium starts to form bone, it is known as the periosteum. Near the middle of the model, periosteal capillaries grow into the disintegrating calcified cartilage, inducing growth of a primary ossification center, a region where bone tissue will replace most of the cartilage. Osteoblasts then begin to deposit bone extracellular matrix over the remnants of calcified cartilage, forming spongy bone trabeculae. Primary ossification spreads from this central location toward both ends of the cartilage model.
4. Development of the medullary (marrow) cavity. As the primary ossification center grows toward the ends of the bone, osteoclasts break down some of the newly formed spongy bone trabeculae. This activity leaves a cavity, the medullary (marrow) cavity, in the diaphysis (shaft). Eventually, most of the wall of the diaphysis is replaced by compact bone.
5. Development of the secondary ossification centers. When branches of the epiphyseal artery enter the epiphyses, secondary ossification centers develop, usually around the time of birth. Bone formation is similar to what occurs in primary ossification centers. However, in the secondary ossification centers spongy bone remains in the interior of the epiphyses (no medullary cavities are formed here). In contrast to primary ossification, secondary ossification proceeds outward from the center of the epiphysis toward the outer surface of the bone.
6. Formation of articular cartilage and the epiphyseal (growth) plate. 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.
Explain how bone grows in length.
During infancy, childhood, and adolescence, bones throughout the body grow in thickness by appositional growth, and long bones lengthen by the addition of bone material on the diaphyseal side of the epiphyseal plate by interstitial growth.
The growth in length of long bones involves the following two major events: (1) interstitial growth of cartilage on the epiphyseal side of the epiphyseal plate and (2) replacement of cartilage on the diaphyseal side of the epiphyseal plate with bone by endochondral ossification. The activity of the epiphyseal plate is the only way that the diaphysis can increase in length.
The epiphyseal (growth) plate is a layer of hyaline cartilage in the metaphysis of a growing bone that consists of four zones:
1. Zone of resting cartilage. This layer is nearest the epiphysis and consists of small, scattered chondrocytes. The term “resting” is used because the cells do not function in bone growth. Rather, they anchor the epiphyseal plate to the epiphysis of the bone.
2. Zone of proliferating cartilage. Slightly larger chondrocytes in this zone are arranged like stacks of coins. These chondrocytes undergo interstitial growth as they divide and secrete extracellular matrix. The chondrocytes in this zone divide to replace those that die at the diaphyseal side of the epiphyseal plate.
3. Zone of hypertrophic cartilage. This layer consists of large, maturing chondrocytes arranged in columns.
4. Zone of calcified cartilage. The final zone of the epiphyseal plate is only a few cells thick and consists mostly of chondrocytes that are dead because the extracellular matrix around them has calcified. Osteoblasts lay down bone extracellular matrix, replacing the calcified cartilage by the process of endochondral ossification. Recall that endochondral ossification is the replacement of cartilage with bone. As a result, the zone of calcified cartilage becomes the “new diaphysis” that is firmly cemented to the rest of the diaphysis of the bone.
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.
Explain how bone grows in thickness.
Like cartilage, bone can grow in thickness (diameter) only by appositional growth:
- At the bone surface, periosteal cells differentiate into osteoblasts, which secrete the collagen fibers and other organic molecules that form bone extracellular matrix. The osteoblasts become surrounded by extracellular matrix and develop into osteocytes. This process forms bone ridges on either side of a periosteal blood vessel. The ridges slowly enlarge and create a groove for the periosteal blood vessel.
- Eventually, the ridges fold together and fuse, and the groove becomes a tunnel that encloses the blood vessel. The former periosteum now becomes the endosteum that lines the tunnel.
- Osteoblasts in the endosteum deposit bone extracellular matrix, forming new concentric lamellae. The formation of additional concentric lamellae proceeds inward toward the periosteal blood vessel. In this way, the tunnel fills in, and a new osteon is created.
- As an osteon is forming, osteoblasts under the periosteum deposit new circumferential lamellae, further increasing the thickness of the bone. As additional periosteal blood vessels become enclosed as in step 1 , the growth process continues.
Describe the process involved in bone remodeling.
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.
Bone remodeling depends on adequate intakes of minerals (calcium and phosphorus, and smaller amounts of magnesium, fluoride, and manganese) and vitamins (Vitamin A stimulates activity of osteoblasts. Vitamin C is needed for synthesis of collagen, the main bone protein. Vitamin D helps build bone by increasing the absorption of calcium from foods in the gastrointestinal tract into the blood. Vitamins K and B 12 are also needed for synthesis of bone proteins). It also depends on hormones including insulin-like growth factors (IGFs – especially during childhood), thyroid hormones, insulin, and sex hormones (which become extremely active at puberty).
What are the 3 factors that affect bone growth and remodelling?
1. Sufficient intake of minerals such as calcium and phosphorus.
2. Sufficient intake of vitamins such as A, C, D, K, and B12.
3. Growth hormones such as insulin-like growth factors (IGFS), thyroid hormones, insulin, and sex hormones (after puberty).
Describe the importance of calcium in the body.
Both nerve and muscle cells depend on a stable level of calcium ions (Ca 2+) in extracellular fluid to function properly. Blood clotting also requires Ca 2+. Also, many enzymes require Ca2+ as a cofactor.
Describe how calcium levels are regulated by the body.
Bone is the body’s major calcium reservoir, storing 99% of total body calcium. The role of bone in calcium homeostasis is to help “buffer” the blood Ca2+ level, releasing Ca2+ into blood plasma (using osteoclasts) when the level decreases, and absorbing Ca 2+ (using osteoblasts) when the level rises.
Ca 2+ exchange is regulated by negative feedback systems caused by hormones, the most important of which is parathyroid hormone (PTH), which can increase the number and activity of osteoclasts. 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.
Another hormone works to decrease blood Ca2+ level. When blood Ca2+ rises above normal, parafollicular cells in the thyroid gland secrete calcitonin (CT). CT inhibits activity of osteoclasts, speeds blood Ca2+ uptake by bone, and accelerates Ca2+ deposition into bones.
