CHAPTER 6-BONES AND SKELETAL TISSUES I. Bone tissue is an ever-changing, growing, developing tissue in the human body. It serves as the major support tissue for the human body. A. Osteology II. FUNCTIONS OF BONE TISSUE A. Support-bone provides a framework for the human body. It supports soft tissues and serves as a region for muscle attachment. B. Protection C. Movement-skeletal muscle attaches to and moves bones. D. Mineral Storage-bone is a reservoir for calcium and phosphate. On demand, bone tissue can release both of these minerals into the bloodstream for use in the human body. E. Blood Cell Production 1. Hemopoiesis (hematopoiesis)-the formation/production of blood cells. This process takes place in red bone marrow. F. Energy Storage 1. Yellow Bone Marrow-associated with bone. This material is composed of adipose tissue and scattered leukocytes. The adipose tissue serves as a source of energy for the human body. III. ORGANIZATION OF THE SKELETAL SYSTEM A. The human skeleton is composed of 206 bones dispersed throughout the body. These bones are classified into two major skeletal divisions: 1. The Axial Skeleton-bones located along the central axis of the body. 2. The Appendicular Skeleton-bones of the extremities. b. Types of Bones in the Human Skeleton-based on shape. 1. Long Bones-are longer than they are wide. a. Are typically curved. The curvature acts to increase their strength which allows them to withstand great stress; thus reducing the chance of fracture. b. The Major Parts of a Long Bone: 1) Diaphysis-the shaft of the bone. 2) Epiphyses-the ends of the bone. These are covered and protected by hyaline cartilage. 3) Metaphysis-the region in a mature bone where the diaphysis meets the epiphysis. This region of the bone contains the epiphyseal plate-a region where cartilage is replaced by bone. The epiphyseal plate is involved in bone growth. 4) Hyaline (Articular) Cartilage-a layer of cartilage that covers the ends of a long bone. The cartilage serves as a shock absorber between bones. 5) Periosteum-a membrane that surrounds the surface of a bone. It is composed of 2 Layers: a) An outer fibrous layer that is composed of dense irregular connective tissue. This layer contains blood vessels, nerves and lymphatic vessels that pass into the bone. b) An inner osteogenic layer that contains elastic fibers, blood vessels and bone cells. c) Overall, the periosteum is involved in bone growth, repair and development. It also serves as a site of attachment for ligaments and tendons. 6) Medullary (Marrow) Cavity-an open space within the diaphysis of a bone. It contains yellow bone marrow. 7) Endosteum-a membrane that covers and lines the medullary cavity of a bone. It contains 2 specialized types of bone cells: osteoprogenitor cells and osteoclasts. 2. Short Bones-are cube-shaped. These are composed of spongy bone tissue except for an outer layer of compact bone tissue. The carpals and tarsals are examples of short bones. 3. Flat Bones-are very thin bones. The cranial bones, sternum and ribs are flat bones. a. These are composed of 2 plates of compact bone tissue that encloses a layer of spongy bone. These bones provide considerable protection and they offer a great surface area for tendon and ligament attachment. 4. Irregular Bones-have complex shapes. The vertebrae of the spinal column and some facial bones are classified as irregular bones. 5. Sesamoid Bones-small bones embedded in tendons in the body. The patella is an example. IV. HISTOLOGY OF BONE TISSUE A. Overall, bone tissue is composed of 4 types of cells that are embedded in a thick, hardened matrix. B. Bone Matrix-is composed of about 25% water, 25% protein, and 50% mineral salts (calcium carbonate and calcium phosphate). 1. Calcification (Mineralization)-the formation of new matrix. This occurs as the above mineral salts accumulate over collagen fibers. The collagen fibers act to provide strength to the matrix. C. 4 Types of Cells in Bone Tissue: 1. Osteoprogenitor cells-unspecialized cells derived from mesenchyme. These cells are capable of undergoing rapid cell division. These can develop into osteoblasts. a. Osteoprogenitor cells are located near blood vessels in the periosteum and endosteum of bone. 2. Osteoblasts-secrete collagen and other materials needed to build bone tissue. These have lost the ability to undergo cell division. These cells function by secreting new bone matrix. a. These are located on the surface of bone tissue. b. When osteoblasts are completely surrounded by matrix, they are referred to as Osteocytes. 3. Osteocytes-mature bone cells. These cells have lost the ability to divide. Osteocytes do not secrete bone matrix. They are involved in nutrient/waste exchange between bone and blood. a. These cells regulate the daily activities of bone tissue. 4. Osteoclasts-are involved in bone resorption (the destruction of bone matrix). These play a key role in bone growth and repair. D. 2 Types of Bone Tissue: Compact Bone and Spongy Bone. E. Compact Bone Tissue 1. Compact bone forms the external layer over all bones in the body. It also makes up the diaphysis of long bones. 2. Compact bone is composed of repeating units known as Haversian Systems (Osteons). 3. Structure of a Haversian System: a. Haversian (Central) Canals-run longitudinally in bone tissue. These contain blood vessels and nerves. b. Lamellae-rings of matrix in bone. This is composed of the mineral salts calcium carbonate and calcium phosphate. c. Volkmann’s Canals-run horizontally in bone tissue. These also contain blood vessels and nerves. d. Lacunae-small spaces in the lamellae of compact bone. Osteocytes are in these small spaces. e. Canaliculi-small channels extending from lacunae. These serve as passageways through which nutrients and wastes can pass. F. Spongy Bone Tissue-contains many open spaces. 1. Spongy bone tissue is composed of thin plates of bone known as trabeculae. It does not contain Haversian Systems. 2. The spaces between trabeculae are filled with red bone marrow which is involved in blood cell production. 3. Osteocytes are located in the trabeculae. 4. Spongy bone tissue is found in: short bones, flat and irregular bones and in the epiphyses of long bones. Specifically, spongy bone is found in the sternum, ribs, skulls, and vertebrae. G. Bone contains a large supply of blood. Nutrient arteries carry blood into the diaphysis of long bones. These enter the bone through nutrient foramina. 1. Epiphyseal arteries carry blood into the epiphyses of a bone. V. BONE FORMATION (OSSIFICATION) A. Bone is a dynamic, ever-changing type of tissue. Ossification is the process by which bone forms. B. 2 Patterns of Ossification in the Human Body: 1. Intramembranous Ossification-bone formation directly on or over loose fibrous connective tissue. a. No cartilage stage is present in bones that form in this fashion. b. Steps in Intramembranous Ossification 1) Mesenchyme cells cluster at the site of bone formation and differentiate into osteoprogenitor cells. This cluster of cells is referred to as a center of ossification. 2) Next, the osteoprogenitor cells develop into osteoblasts which secrete bone matrix. As the matrix forms, it develops into trabeculae which fuse together to form spongy bone. Red bone marrow fills the spaces between the trabeculae. 3) Eventually, the surface layers of the spongy bone develop into compact bone. a) Spongy bone remains in the center of the developing bone. 2. Endochondral Ossification-bone formation over hyaline cartilage. a. Most human bones form in this manner. b. Steps in Endochondral Ossification: 1) Mesenchyme cells develop into chondroblasts which secrete the matrix of hyaline cartilage. 2) Next, the cartilage grows via interstitial and appositional growth. 3) Eventually, a nutrient artery grows into the developing hyaline cartilage. This stimulates osteoprogenitor cells to develop into osteoblasts which begin secreting matrix. 4) Bone tissue forms as trabeculae over the hyaline cartilage. Osteoclasts form the marrow cavity of the bone. The diaphysis completely replaces the spongy bone with compact bone. VI. BONE GROWTH-IN LENGTH A. Bone growth in length generally ends before the age of 25; however, bones may continue to thicken throughout a person’s life. Length growth may stop earlier in females than in males. B. Events in Length Growth of Bone: 1. Epiphyseal Plate-a layer of hyaline cartilage in the metaphysis of a growing bone. It consists of 4 Major Zones: a. The Zone of Resting Cartilage-cells here anchor the epiphyseal plate to the compact bone of the epiphysis. The cells here are not involved in bone growth. b. The Zone of Proliferating Cartilage-contains actively dividing chondrocytes. As the cells here divide, the epiphysis moves away from the diaphysis. This in turn produces length growth in bone. c. The Zone of Hypertrophic Cartilage-contains mature chondrocytes. d. The Zone of Calcified Cartilage-contains osteoblasts which secrete bone matrix. C. Final Points on Length Growth in Bones: 1. The epiphyseal plate is the only area in a bone where length growth can occur. Eventually, cells in the epiphyseal plate stop dividing. At this point, bone tissue replaces the cartilage. This produces a remnant known as the epiphyseal line. 2. Fractures of the epiphyseal plate can result in a cessation of bone growth. Due to this, a fractured bone may be shorter than its counterpart. 3. Bone growth usually stops before the age of 25. In general, length growth ends earlier in females than in males. VII. BONE GROWTH-IN THICKNESS-this occurs as osteoblasts secrete new matrix to the periosteum of a bone. VIII. HORMONAL REGULATION OF BONE GROWTH A. Human Growth Hormone (HGH)-secreted by the pituitary gland. This hormone regulates bone growth prior to puberty. Oversecretion of this hormone may lead to gigantism; whereas undersecretion may lead to dwarfism. B. At puberty, the sex hormones estrogen and testosterone stimulate changes in the human skeleton. These hormones are responsible for the growth spurt that occurs at puberty. They also stimulate the skeleton to develop into the typical male and female shape. C. Thyroid Hormones-also play a role in bone growth and development. IX. BONE REMODELING-the ongoing replacement of old bone tissue by new bone tissue. A. Bone is an ever-changing type of tissue. Remodeling removes worn and injured bone tissue and replaces it with new, healthy bone tissue. This ensures that bone remain healthy. B. Osteoclasts-bone cells that are responsible for removing old bone matrix (bone resorption). 1. These cells breakdown matrix by secreting protein-digesting enzymes and various acids. 2. Once old bone matrix has been removed, osteoblasts secrete new matrix. 3. Removal of too much matrix can cause osteoporosis; whereas, oversecretion of bone matrix can lead to bone spurs or calcium deposits. C. Alkaline phosphatase-an enzyme that regulates the formation of calcium carbonate and calcium phosphate This enzyme is needed in large supplies for bone remodeling to occur. D. Vitamins and Minerals that are needed for Bone Remodeling to occur: 1. Calcium 2. Vitamin C-needed for the formation of collagen fibers. 3. Vitamin D-needed for the absorption of calcium. 4. Vitamin A-maintains a balance between bone deposit and bone resorption. 5. Magnesium, phosphorous, manganese X. FRACTURE-refers to any break in a bone. A. The repair of a fracture is a slow and painful process. B. Steps in Fracture Repair: 1. A fracture occurs. This breaks the bone and blood vessels around the bone. 2. As bleeding occurs around the fracture, a blood clot forms. This clot is known as a fracture hematoma. This hematoma is fully formed about 8 hours after the fracture. 3. Blood capillaries grow into the damaged area. White blood cells and osteoclasts begin removing old, damaged and dead bone tissue at the site of the fracture. This may take several weeks to occur. 4. Next, capillaries grow into the fracture hematoma. This produces a structure known as a procallus. Osteoprogenitor cells from the periosteum of healthy bone tissue invade the procallus. The Osteoprogenitor cells then develop into: a. Chondroblasts-which secrete cartilage tissue. b. Ostoeblasts-which secrete bone matrix. 5. The procallus develops into a callus which is a cartilaginous mass of tissue that bridges the ends of the broken bone. Osteoblasts secrete matrix over the callus to produce new bone matrix. 6. Remodeling occurs to shape and strengthen the bone. C. Types of Fractures: 1. Partial fracture-the bone is not broken into two or more pieces. 2. Complete fracture-the bone is broken into two or more pieces. 3. Closed (Simple) fracture-the break does not break the skin. 4. Open (Compound) fracture-the fracture does break the skin. 5. Comminuted fracture-the bone has splintered into several small pieces at the site of the fracture. 6. Greenstick fracture-a partial fracture in which one side of the bone is broken and the other side is twisted. These are common in young children. 7. Spiral fracture-occurs when the bone is twisted. 8. Stress fracture-microscopic fracture often caused by repeated stress. 9. Compression fracture-bones are forced together. This crushes healthy bone tissue. D. The Clavicle is the most commonly broken bone in the human body. XI. BONE AND CALCIUM HOMEOSTASIS A. Bone is the major calcium reservoir in the human body. Bones stores 99% of the body’s calcium. B. Uses of Calcium in the Human Body: 1. Regulation of muscle contraction 2. Impulse formation and conduction in the nerve tissue 3. Blood clotting 4. Serves as a cofactor for many enzymes C. Parathyroid Hormone (PTH)-hormone that regulates calcium exchange between bone and the blood. 1. This hormone is secreted by the parathyroid gland. It aids in regulating bond remodeling. 2. When calcium levels fall in the blood, PTH stimulates the activity of osteoclasts in bone tissue. a. The osteoclasts respond by increasing their rate of bone resorption. As this occurs, calcium is released from the bone into the bloodstream; thus, increasing calcium levels in the blood. b. PTH also increases calcium recovery in the kidneys. Once again, this aids in returning calcium to the bloodstream. D. Calcitonin-secreted by cells in the thyroid gland. 1. When calcium levels rise above normal in the bloodstream, calcitonin responds by decreasing the activity of osteoclasts. It also increases the activity of osteoblasts. Overall, this reduces calcium levels in the bloodstream. XII. DISORDERS/MEDICAL TERMINOLOGY ASSOCIATED WITH THE SKELETAL SYSTEM A. Osteoporosis-a condition of porous bone. It is characterized by decreased bone mass and increased susceptibility to fracture. Has been treated with calcium and vitamin D supplements. Exercise appears to reduce (an in some cases prevent) the onset of osteoporosis. B. Paget’s disease-accelerated remodeling. Causes weak areas in bone tissue. C. Osteoarthritis-the degeneration of hyaline cartilage. D. Osteomalacia-disorders in which the bones are inadequately mineralized. For whatever reason, calcium is not deposited in bone tissue. E. Rickets-a form of osteomalacia that occurs in children. Is usually associated with vitamin D deficiency. Drinking vitamin D fortified milk usually alleviates this illness. F. Achondroplasia-a type of dwarfism. Involves defective cartilage and endochondral bone growth in which the bones of the limbs are too short. G. Bony spur-abnormal projection from a bone due to bone overgrowth. H. Ostealgia-pain in a bone. I. Osteitis-inflammation of a bony tissue. J. Osteomyelitis-inflammation of bone and bone marrow caused by bacterial infection. K. Osteosarcoma-bone cancer. Usually metastasizes to the lungs and liver. Limb amputation is the usual treatment. Survival rate is less than 50% ; even when detected early. L. Pathologic fracture-fracture in a diseased bone involving slight or no physical trauma. M. Traction-placing tension on a body part to keep the parts in proper alignment. Usually associated with severe fractures or damage to the vertebral column. DISORDERS/MEDICAL TERMINOLOGY ASSOCIATED WITH THE AXIAL SKELETON 1. Herniated disc-characterized by the protrusion of the inner layer of an intervertebral disc. Can be repaired via surgery. 2. Abnormal curves of the Vertebral Column: a. Scoliosis-a lateral bending of the vertebral column, usually in the thoracic region. b. Kyphosis-an exaggeration of the throacic curve of the vertebral column. Is often referred to as “round shoulders.” c. Lordosis-an exaggeration of the lumbar curve of the vertebral column. 3. Laminectomy-surgical removal of a vertebral lamina to relieve the symptoms of a ruptured disc. 4. Spina bifida-genetic disorder in which the lamina of the vertebral column fail to unite at the midline. 5. Spinal fusion-surgical procedure involving the insertion of bone chips to immobilize and stabilize a portion of the vertebral column. This is usually associated with herniated discs and vertebral fractures.
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