Chapter 6 Skeletal System Functions of the skeletal system/bone 1. Support 2. Protection 3. Movement 4. Mineral storage skeletal system is the internal framework of the body protects internal organs muscles & bones work together. muscles move bones as levers bones are made up of minerals, especially calcium phosphate 5. Blood cell formation red marrow contains blood cells Bone is classified by? How the tissue is organized Compact Bone Spongy Bone This forms the outer surface of bone. Dense compact bone Network of interwoven spicules called trabeculae Lots of space Made of same stuff as compact bone, arranged differently Bone shape (4 general shapes) Long Bones Flat Bones Short Bones Irregular Bones Bone Structure Longer than they are wide Most appendicular bones are long bones Femur, metacarpals, etc? Thin & flat, broad, curved Cranial bones, sternum, ribs, scapula Cuboidal, box-like Carpal & tarsal bones Includes "sesamoid bones", which are bones that form within a tendon (where stress is on a tendon). These vary! Complex shape Vertebrae, coxal bones, maxilla, ethmoid, etc? LONG BONES form into a bone with distinct regions?the DIAPHYSIS, EPIPHYSIS, METAPHYSIS and MEDULLARY CAVITY. The diaphysis is the shaft of the long bone, while the epiphyses are the rounded ends. The metaphysis is the area where the diaphysis and epiphyses meet. This region surrounds the epiphyseal plate, which is on the inside. The medullary cavity, located on the inside of the bone, runs at least along the length of the diaphysis, but it can also go up into the epiphysis. It is lined by spongy bone and is hollow space. MEMBRANES surround every bone. The membrane is a double layer called the PERIOSTEUM, while the ENDOSTEUM lines the trabeculae of internal spongy bone. The periosteum is made up of two layers?the outer layer is made up of dense irregular C.T. so that tendons and ligaments can insert. The inner layer is made up of osteoblasts and osteoclasts. How does the periosteum bind to the bone? It uses Sharpey?s (perforating fibers) The periosteum is richly supplied with nerve fibers, blood vessels and lymphatic vessels. The ENDOSTEUM is not as dense as the periosteum and is made up of areolar CT. It also contains osteoblasts and osteoclasts. Structure of compact bone Compact bone is made up of OSTEONS, the functional unit of compact bone. Osteons are cylinders of bone tissue that run parallel to the long axis of a long bone. Each osteon surrounds a CENTRAL CANAL (elevator shaft) that transmits blood vessels and nerve that serve the needs of the osteon's cells. While the central canals go up and down, the PERFORATING CANALS run perpendicular to the long axis, connecting the blood and nerve supply of the PERIOSTEUM to the central canal. Think of Willie Wonka's elevator system that would go up and down and sideways. These canals, like all other internal bone cavities, are lined with endosteum. The osteon up close! Each osteon is made up of concentric layers of matrix called lamellae. Think of a tree ring! Running through this matrix are collagen fibers?the alternating direction of the collagen fibers in each layer of lamellae give the osteon greater strength! Osteocytes occupy spaces called LACUNAE at the junctions of the lamellae. Hairlike canals called CANALICULI connect the lacunae to each other and to the central canal. This is how they get their nutrients. What is an osteocyte? Mature bone cell Resides in the lacunae (lacuna, singular) The lacunae are between the lamellae What about other lammalae? Osteons can't fill the whole space because they are round. Think of apples in a box?there is lots of space around the apples! Something has to fill this space around the osteons?what is it? Two types of lamellae?circumferential and interstitial. CIRCUMFERENTIAL LAMALLAE is deep to the periosteum and superficial to the medullary cavity. Circumferential lamellae look like the rings of a tree and surround the group of osteons on the circumference of the diaphysis. INTERSTITIAL LAMALLAE lie between the osteons. Essentially interstitial lamellae are incomplete osteons (old ones). RED MARROW contains hematopoetic tissue and is the site of blood cell production. The locations vary based on developmental stage. In infants, who need a greater level of blood cell production, the red marrow is in the medullary cavities of all long bones and in all areas of spongy bones. In adults the red marrow is located in the proximal heads of the humerus and femur and in the spongy bone of flat bones (the skull, coxal bone and sternum). So, what fills the remaining areas of the long bones? Yellow marrow (lipid), fills the space and it may revert to red marrow if needed due to blood loss. The Matrix The bone has both organic and inorganic components and together is like reinforced concrete. The ORGANIC COMPONENT of the matrix : Mostly collagen and has to be made by living cells (osteoblasts). Provides tension strength. Without it the bone would be too brittle. The INORGANIC COMPONENT of the matrix: Mainly crystals of calcium/phosphorus salt and is called hydroxyapatite. For weight bearing. Without it the bone would be too soft and flexible. Bone Physiology Bone formation begins around the 8 th week of development. At this stage, the skeletal elements are fibrous membranes and hyaline cartilage. There are two different methods of ossification, one for each type. The fibrous membrane will become bone via INTRAMENBRANOUS OSSIFICATION, while the hyaline cartilage will become bone via ENDOCHONDRAL OSSIFICATION. With intramembranous ossification, bone forms within the fibrous membrane. This is generally for skull bones and clavicles. With endochondral ossification, bone forms from the cartilaginous model. This occurs in long bones, short bones, irregular bones) Intramembranous ossification 1. Embryonic mesenchymal cells form a collagen membrane containing osteochondral progenitor cells, which specialize to become osteoblasts. They produce bone matrix and woven bone develops. . 2. Additional osteoblasts gather on the surfaces of the trabeculae, producing more bone so they become larger and longer. Spongy bone forms. 3. Cells inside spongy bone specialize to form red marrow and cells surrounding the developing bone specialize to form periosteum. Osteoblasts from periosteum lay down bone matrix to form outer surface of compact bone. 4. Remodeling converts woven bone into lamellar bone and contributes to the final shape Endochondral ossification (cartilage model) Calcify hyaline cartilage, eat it away, and then rebuild it into lamellar bone. 1. Chondroblasts form a cartilage model and it?s surrounded by perichondrium, except where joints will form. 2. Perichondrium turns into periosteum and a bony collar is produced. Internally, the chondrocytes hypertrophy and calcified cartilage forms. 3. Primary ossification center forms as osteoblasts and blood vessels invade the calcified cartilage. Osteoblasts lay down bone matrix forming spngy bone. 4. Process of bone collar formation, cartilage calcification and spongy bone production continues. Calcified cartilage begins to form in the epiphyses. A medullary cavity begins to form in the center of the diaphysis. 5. Secondary ossification centers form in the epiphyses of long bones. 6. Original cartilage model almost completely ossified. Unossified cartilage becomes the epiphyseal plate and the articular cartilage. 7. In mature bone the epiphyseal plate becomes completely ossified and becomes the epiphyseal line. All cartilage in the epiphyses (except for the articular cartilage) has become bone. Bone Growth (?growth in length) Bone growth occurs until the late teens/early 20s. It occurs at the epiphyseal plate. Bone growth takes place because cartilage on the epiphyseal plate continues to grow, pushing the epiphyses away from the diaphysis. The older chondrocytes hypertrophy and their lacunae erode and enlarge. Subsequently the surrounding cartilage matrix calicifies. Osteoclasts and osteoblasts convert cartilage to bone. Osteoclasts remove bone at diaphysis side extending the medullary cavity. Cartilage formation and replacement continue at the same rate and growth continues to occur. At the end of the growth, chondrocyte division slows and bone formation "catches up". The epiphyseal plate closes. Bone Growth (Growth in width?appositional growth) The widening of bone does not involve cartilage. It is used to increase the diameter of bone, producing a thicker, stronger bone that is not too heavy. The osteoblasts in perosteum lay down a layer of bone on the external surface while the osteoclasts in endosteum remove bone from the internal surface. The result is the medullary cavity gets wider and the diameter increases. The thickness of the bone wall increases, due to the laying down activity of the osteoblasts outpacing that of the osteoclasts. Bone Repair 1. Hematoma formation o Usually the hematoma forms a clot o Disruption of blood vessels to central canals (bone tissue adjacent to o fracture site dies). o Tissues around the site usually inflamed and swollen. 2. Callus formation o Mass of tissue forms to connect the broken ends of bone and form a collar around the break. o Internal callus (fibers and cartilage , forms woven bone) o External callus (cartilage, forms woven bone) 3. Callus ossification o Woven,spongy bone replaces the internal and external calluses. 4. Bone remodeling o Compact bone replaces the woven bone and part of the internal callus is removed, restoring the medullary cavity. Bone Fractures 1. Open (compound): bone break with open wound and bone may be protruding. 2. Closed (simple): skin not violated. 3. Complete: separates bone into two or more fragments. 4. Incomplete: does dot separate bone into two or more fragments. 5. Spiral: take a helical (spiral) course around the bone. o Lots of surface area for repair and easy to stabilize. 6. Transverse: occur at right angles to the long axis. o Very little surface area for repair and hard to stabilize (especially from rotation). o Non-unions very common. 7. Greenstick: incomplete fracture occurring on the convex side of the curve. o Common in immature bones. 8. Epiphyseal: occurs across a growth plate, typically only in immature bones. o Often heal quickly, but may cause premature closure of growth plate. 9. Comminuted: complete fracture where bone breaks into fragments. o Difficult to stabilize o Require hardware o Fragments can die or become infected. 10. Compression: crushing of spongy bone o Typically in vertebrae 11. Depression: common skull fracture. Bone Growth Regulation Bone growth is regulated primarily by hormones. In childhood, growth is stimulated by the pituitary GROWTH HORMONE, while the THYROID GLAND modulates the effect of the growth hormone. Later, at the time of a growth spurt or at the end of it when the epiphyseal plate closes, SEX HORMONES (estrogen and testosterone) regulate growth. Estrogen causes quicker closure of the epiphyseal plate than testosterone does. Bone Remodeling For bone remodeling to occur, bone deposit and resorption occur at the same rate. What controls bone remodeling? HORMONAL MECHANISM maintains calcium homeostasis in the blood; and RESPONSE TO MECHANICAL STRESS keeps bones strong where stresses are acting. Some facts about bone remodeling: o Occurs throughout life o Constantly changing, the skeleton is replaced frequently o Spongy bone replaced every 3-4 years o Compact bone replaced every 10 years o Influencing factors o Mineral needs of the body o Stresses on bone o Mechanisms o Hormonal. PARATHYROID HORMONE?PTH (from the parathyroid gland when blood calcium levels are too low) stimulates and increase in the number of osteoclasts, which break down bone and elevate blood calcium levels. CALCITONIN (from the thyroid gland when blood calcium levels are too high) decreases osteoclast activity by binding to receptors on the osteoclasts o Mechanical (relates to stresses). Bone is laid down in regions where forces are the greatest? -walls of shafts of long bones -center of shaft more than the ends -trabeculae form along lines of stress -bony projections occur at muscle attachment þÿ þÿ þÿ þÿ
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