BIOLOGY (M2-CHAP 6).pdf

Chapter 6 Skeletal System: Bones and Joints Components of Skeletal System Bones Cartilages Tendons Ligaments Functions of the Skeletal System 1. Body support 2. Organ protects 3. Body movement 4. Mineral storage 5....

Chapter 6 Skeletal System: Bones and Joints Components of Skeletal System Bones Cartilages Tendons Ligaments Functions of the Skeletal System 1. Body support 2. Organ protects 3. Body movement 4. Mineral storage 5. Blood cell production Bone Histology Bone, cartilage, tendons, and ligaments of the skeletal system are all connective tissues. Their characteristics are largely determined by the composition of their extracellular matrix. The matrix always contains collagen, ground substance, and other organic molecules, as well as water and minerals. Collagen is a fibrous protein that provides flexibility but resists pulling or compression. Matrix ground substance contains proteoglycans which are water trapping proteins that help cartilage to be smooth and resilient. The extracellular matrix of tendons and ligaments contains large amounts of collagen fibers, making these structures very tough, like ropes or cables. Bone Matrix Bone matrix is about 35% organic and 65% inorganic material by weight. The organic material is primarily collagen and proteoglycans The inorganic material is primarily a calcium phosphate crystal called hydroxyapatite. Collagen fibers lend flexible strength to the bone. The mineral component gives bone compression (weight-bearing) strength. Bone Cells - Osteoblasts Osteoblasts are responsible for the formation of bone and the repair and remodeling of bone. Osteoblasts produce collagen and proteoglycans. Osteoblasts also secrete high concentrations of Ca 2+ and phosphate ions, forming crystals called hydroxyapatite. The formation of new bone by osteoblasts is called ossification. Bone Cells - Osteocytes Osteocytes are cells that maintain bone matrix and form from osteoblast after bone matrix has surrounded it. Osteocytes account for 90–95% of bone cells and are very long- lived.  Osteocyte cell bodies are housed within the bone matrix in spaces called lacunae Their cell extensions are housed in narrow, long spaces called canaliculi Osteoclasts Osteoclasts are bone-destroying cells. They contribute to bone repair and remodeling by removing existing bone, called bone reabsorption. Bone breakdown is important for mobilizing crucial Ca 2+. As bone is broken down, the Ca 2+ goes into the blood. Spongy and Compact Bone Mature bone is called lamellar bone. It is organized into thin, concentric sheets or layers, called lamellae. Bone can be classified according to the amount of bone matrix relative to the amount of space within the bone. Spongy bone has less bone matrix and more space than compact bone, which has more bone matrix and less space. Spongy bone consists of interconnecting rods or plates of bone called trabeculae Between the trabeculae are spaces, which in life are filled with bone marrow and blood vessels. The surfaces of trabeculae are covered with a single layer of cells consisting of osteoblasts with a few osteoclasts. Compact Bone Compact bone, or cortical bone, is the solid, outer layer surrounding each bone. The functional unit of compact bone is an osteon. It is composed of concentric rings of matrix surrounding a central canal. Central canals are lined with endosteum and contain blood vessels, nerves, and loose connective tissue. Lamellae are concentric rings of bone matrix which surround the central canal. Osteocytes are located in spaces called lacunae between the lamellar rings. Small tunnels called canaliculi radiate between lacunae across the lamellae. Canaliculi connect osteocytes to one another, transport nutrients and remove waste. Structure of a Long Bone The diaphysis is the center portion of the bone which is composed of compact bone surrounding a hollow center called the medullary cavity. Some spongy bone tissue lines the medullary cavity. The ends of a long bone are called epiphyses They contain mostly spongy bone, with an outer layer of compact bone. Within joints, the end of a long bone is covered with hyaline cartilage called articular cartilage The epiphyseal plate is located between the epiphysis and the diaphysis. Growth in bone length occurs at the epiphyseal plate. When bone stops growing in length, the epiphyseal plate becomes ossified and is called the epiphyseal line. Bone Marrow Cavities in spongy bone and the medullary cavity in the diaphysis are filled with soft tissue called marrow.  Red marrow is the location of blood forming cells Yellow marrow is mostly fat. In the fetus, the spaces within bones are filled with red marrow. Just before birth the red bone marrow starts to get converted to yellow marrow. This continues well into adulthood. In adults, most red bone marrow is in the flat bones. The long bones of the femur and humerus contain yellow marrow. Periosteum The outer surface of a bone is covered by a connective tissue membrane called periosteum. The outer layer of periosteum contains blood vessels and nerves. The inner layer is a single layer of bone cells, including osteoblasts and osteoclasts. Where tendons and ligaments attach to bone, fibers of the tendon or ligament become continuous with those of the periosteum. Endosteum The endosteum is a single cell layer of connective tissue that lines the internal surfaces of all cavities within bones. The endosteum includes osteoblasts and osteoclasts Bone Formation Bone formation in the fetus follows two processes: Intramembranous ossification starts within embryonic connective tissue membranes. Endochondral ossification starts with a cartilage model. Both types of bone formation can result in compact or spongy bone. Intramembranous Ossification Intramembranous ossification occurs when osteoblasts begin to produce bone within connective tissue. This occurs primarily in the bones of the skull. Osteoblasts line up on the surface of connective tissue fibers and begin depositing bone matrix to form trabeculae. The process begins in areas called ossification centers and the trabeculae radiate out from the centers. Usually, two or more ossification centers exist in each flat skull bone and mature skull bones result from fusion of these centers as they enlarge. The trabeculae are constantly remodeled and they may enlarge or be replaced by compact bone. Steps in Intramembranous Ossification 1. Osteoblasts within the center of ossification produce bone matrix around collagen fibers of the connective tissue membrane. Once the osteoblasts are embedded in bone matrix, the osteoblasts become osteocytes. Many tiny trabeculae of woven bone develop. 2. Additional osteoblasts gather on the surfaces of the trabeculae and produce more bone. Trabeculae become larger and longer. Spongy bone forms as the trabeculae join. 3. Cells within the spaces of the spongy bone specialize to form red bone marrow, and cells surrounding the developing bone specialize to form the periosteum. Osteoblasts from the periosteum lay down bone matrix to form an outer surface of compact bone. Endochondral Ossification Endochondral bone formation is bone formation within a cartilage model. The cartilage model is replaced by bone. Initially formed is a primary ossification center, which forms bone in the diaphysis of a long bone. A secondary ossification center forms bone in the epiphysis. Steps in Endochondral Ossification 1. Chondroblasts build a cartilage model, the chondroblasts become chondrocytes. 2. Cartilage model calcifies (hardens). 3. Osteoblasts invade calcified cartilage and a primary ossification center forms in the diaphysis. 4. Secondary ossification centers form epiphysis. 5. Original cartilage model is almost completely ossified and remaining cartilage is articular cartilage. Bone Growth in Width Bone growth occurs by the deposition of new bone lamellae onto existing bone or other connective tissue. As osteoblasts deposit new bone matrix on the surface of bones between the periosteum and the existing bone matrix, the bone increases in width, or diameter. This process is called appositional growth. Bone Growth in Length Growth in the length of a bone, which is the major source of increased height in an individual, occurs in the epiphyseal plate. This type of bone growth occurs through endochondral ossification. Chondrocytes increase in number on the epiphyseal side of the epiphyseal plate. Then the chondrocytes enlarge and die. The cartilage matrix becomes calcified. Much of the cartilage that forms around the enlarged cells is removed by osteoclasts, and the dying chondrocytes are replaced by osteoblasts. The osteoblasts start forming bone by depositing bone lamellae on the surface of the calcified cartilage. This process produces bone on the diaphyseal side of the epiphyseal plate. Bone Remodeling Bone remodeling involves: removal of existing bone by osteoclasts and deposition of new bone by osteoblasts occurs in all bones responsible for changes in bone shape, bone repair, adjustment of bone to stress, and calcium ion regulation Bone Repair 1. Broken bone causes bleeding and a hematoma forms. 2. A callus forms which is a bone cartilage network between and around the bone fragments. 3. Woven, spongy bone replaces the callus. 4. Compact bone replaces the spongy bone. Bone and Calcium Homeostasis Calcium is a critical nutrient involved in many physiological processes including: Stimulation and regulation of skeletal and cardiac muscle contraction Exocytosis of cellular molecules, including those important for neural signaling Bone is a major storage site for calcium. Movement of calcium in and out of bone helps determine blood levels of calcium. Calcium moves into bone as osteoblasts build new bone. Calcium moves out of bone as osteoclasts break down bone. Calcium homeostasis is maintained by parathyroid hormone (PTH) and calcitonin. Parathyroid Hormone (PTH) Secreted by the parathyroid gland Increases formation and activation of osteoclasts, the principal bone-reabsorbing cells Stimulates reabsorption of Ca 2+ from urine in the kidney, reducing the amount of Ca 2+ excreted in the urine. Indirectly increases Ca 2+ uptake from the small intestine through the activation of calcitriol. Calcitonin Secreted from C cells in the thyroid gland when blood Ca2+ levels are too high Rapidly lowers blood Ca 2+ levels by inhibiting osteoclast activity Skeletal Anatomy The average adult has 206 bones. Bones are segregated into the axial skeleton and the appendicular skeleton. The axial skeleton consists of the bones of the skull, the auditory ossicles, the hyoid bone, the vertebral column, and the thoracic cage. The appendicular skeleton consists of the bones of the upper limbs, the lower limbs, and the two girdles. The term girdle, refers to the two zones where the limbs are attached to the body. These two zones are the pectoral girdle and the pelvic girdle. Bone Shapes There are four bone shape classifications: long, short, flat, and irregular. Long bones are longer than they are wide; examples are upper and lower limb bones. Short bones are approximately as wide as they are long; examples are the bones of the wrist and ankle. Flat Bones Flat bones have a relatively thin, flattened shape; examples are bones of the skull and sternum. Irregular bones include the vertebrae and facial bones, which have shapes that do not fit readily into the other three categories. Skeletal Terminology Foramen: hole Example - foramen magnum Fossa: depression Example - glenoid fossa Process: projection Example - mastoid process  Condyle: smooth, rounded end Example - occipital condyle Meatus or canal: canal-like passageway Example - external auditory meatus Tubercle or tuberosity: lump of bone Example - greater tubercle Axial Skeleton The axial skeleton is composed of the skull, the vertebral column, and the thoracic cage. The skull has 22 bones divided into those of the braincase and those of the face. The braincase, which encloses the cranial cavity, consists of 8 cranial bones that immediately surround and protect the brain. The bony structure of the face has 14 facial bones. Thirteen of the facial bones are rather solidly connected to form the bulk of the face. The mandible, however, forms a freely movable joint with the rest of the skull. There are also three auditory ossicles in each middle ear (six total). Cranial Sutures The cranial bones are connected by immovable joints called sutures There are four principal sutures: coronal sagittal lambdoid squamous Cranial Bones - Frontal bone Anterior part of cranium, the ‘forehead” Parietal bones Sides and roof of cranium Occipital bones Posterior portion and floor of cranium Temporal bones Inferior to parietal bones on each side of the cranium Temporomandibular joint Cranial Bones - Sphenoid bone Forms part of cranium floor, lateral posterior portions of eye orbits, lateral portions of cranium anterior to temporal bones Sella turcica Ethmoid bone Anterior portion of cranium, including medial surface of eye orbit and roof of nasal cavity Nasal conchae Facial Bones - Maxillae Forms the upper jaw, anterior portion of hard palate, part of lateral walls of nasal cavity, floors of eye orbits Maxillary sinus Palatine bones Form posterior portion of hard palate, lateral wall of nasal cavity Facial Bones - Zygomatic bones Cheek bones Also form floor and lateral wall of each eye orbit Lacrimal bones Medial surfaces of eye orbits Nasal bones Form bridge of nose Facial Bones - Vomer In midline of nasal cavity Forms nasal septum with the ethmoid bone Inferior nasal conchae Attached to lateral walls of nasal cavity Mandible Lower jawbone Only movable skull bone Paranasal Sinuses Several of the bones associated with the nasal cavity have large cavities within them, called the paranasal sinuses which open into the nasal cavity. The paranasal sinuses are: Frontal Ethmoid Sphenoid Maxillary Hyoid Bone The hyoid bone is an unpaired, U-shaped bone that is not part of the skull and has no direct bony attachment to the skull or any other bones. The hyoid bone has the unique distinction of being the only bone in the body that does not articulate with another bone. The hyoid bone provides an attachment for some tongue muscles, and it is an attachment point for important neck muscles that elevate the larynx. Vertebral Column The vertebral column, or spine, is the central axis of the skeleton, extending from the base of the skull to slightly past the end of the pelvis. In adults, it usually consists of 26 individual bones, grouped into five regions. The adult vertebral column has four major curvatures: cervical, thoracic, lumbar and sacrococcygeal. The cervical region curves anteriorly. The thoracic region curves posteriorly. The lumbar region curves anteriorly The sacral and coccygeal regions together curve posteriorly Vertebral Column 7 cervical vertebra 12 thoracic vertebra 5 lumbar vertebra 1 sacrum 1 coccyx Atlas: 1st vertebra holds head Axis: 2nd vertebra rotates head Functions of Vertebral Column Supports body weight Protects the spinal cord Allows spinal nerves to exit the spinal cord Provides a site for muscle attachment Provides movement of the head and trunk Thoracic Cage Protects vital organs 12 pair of ribs Sternum: breastbone True ribs: attach directly to sternum by cartilage False ribs: attach indirectly to sternum by cartilage Floating ribs: not attached to sternum Pectoral Girdle and Upper Limb Scapula: shoulder blade Clavicle: collar bone Upper Limb Bones Humerus: upper portion of forelimb Ulna: forearm Radius: forearm Carpals: wrist Metacarpals: Hand Phalanges: 13 Pelvic Girdle Where lower limbs attach to the body Pelvis: includes pelvic girdle and coccyx Ischium: inferior and posterior region of hip bone Ilium: most superior region of hip bone Acetabulum: hip socket (joint) of hip bone Lower Limb Bones Femur: thigh Patella: knee cap Tibia: Larger bone of lower leg Fibula: Smaller bone lower leg Tarsals: ankle Metatarsals: foot Phalanges: toes and fingers Articulations Articulations (joints) are where two bones come together. Joints can be classified structurally as fibrous, cartilaginous, or synovial, according to the major connective tissue type that binds the bones together and whether a fluid-filled joint capsule is present. Joints are also be classified in functional categories according to their degree of motion as synarthroses, amphiarthroses, or diarthroses. Structural Classification of Joints Fibrous joint: united by fibrous connective tissue subclasses are sutures, syndesmosis, and gomphoses Cartilaginous: united by means of cartilage subclasses are synchondroses and symphysis Synovial: joined by a fluid cavity Most joints of the appendicular skeleton Functional Classification of Joints Synarthrosis: non-movable joint Example – skull bone articulations Amphiarthrosis: slightly movable joint Example - between vertebrae Diarthrosis: freely movable joint Example - knee, elbow, and wrist articulations Synovial Joint Synovial joints are surrounded by fluid filled joint cavity. The cavity is created by the joint capsule and is full of synovial fluid. The joint capsule helps hold the bones together while still allowing for movement. The joint capsule consists of two layers: an outer fibrous capsule and an inner synovial membrane. The fibrous capsule is the outer layer of the joint capsule. It consists of dense irregular connective tissue and is continuous with the fibrous layer of the periosteum that covers the bones united at the joint. The synovial membrane is the inner layer of the joint capsule. It lines the joint cavity, produces synovial fluid, a viscous lubricating liquid. Effects of Aging on the Skeletal System and Joints 1. Decreased Collagen Production 2. Loss of Bone Density 3. Degenerative Changes