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Chapter 6

Chapter 6 Bones and Skeletal Tissue.doc

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Anatomy and Physiology
Jacqueline Carnegie

Chapter 6: Bones and Skeletal Tissue -the human skeleton is initially made up of cartilages and fibrous membranes and are soon replaced by bone -the remaining cartilage is found in regions where flexible skeletal tissue is needed Structure, Types, and Locations of Cartilage Skeletal Cartilage -made up of some variety of cartilage tissue -consists primarily of water (up to 80%) which allows it to compress and return to its original shape (i.e.: gives it resilience) -contains no nerves or blood vessels -surrounded by a layer of dense irregular connective tissue known as the perichondrium -this acts as a girdle to resist outward expansion when cartilage is compressed -contains blood vessels that allow nutrients to diffuse through the matrix to reach the cartilage cells -this limits cartilage thickness -hyaline, elastic, and fibrocartilage make up the 3 types of cartilage tissue in the body -cartilage cells are known as chondrocytes, and are encased in small cavities called lacunae within an extracellular matrix containing a jellylike ground substance and fibres -chondroblasts are immature cartilage cells and actively form cartilage; have a high metabolic rate -chondrocytes are mature cartilage cells and maintain cartilage; have a low metabolic rate -skeletal cartilage contains all 3 types of cartilage Hyaline Cartilage -provides support with flexibility and resilience -most abundant skeletal cartilage -only has fine collagen fibres in the matrix -includes articular cartilages which covers the ends of most bones at movable joints, costal cartilages which connect the ribs to the sternum, respiratory cartilages which form the skeleton of the larynx and reinforce respiratory passageways, and nasal cartilages which support the external nose Elastic Cartilage -contain more stretchy elastic fibers than hyaline cartilage and can stand up to repeated bending -found only in the external ear and the epiglottis Fibrocartilage -highly compressible and has great tensile strength -intermediate of hyaline and elastic cartilage -consists of parallel rows of chondrocytes alternating with thick collagen fibres -found in sites that are subjected to heavy pressure and stretch (i.e.: menisci of the knee and intervertebral discs) Growth of Cartilage -cartilage has a flexible matrix that can accommodate mitosis -ideal for laying down the embryonic skeleton and provide for skeletal growth -appositional growth involves cartilage-forming cells in the surrounding perichondrium that secrete new matrix against the external face of the existing cartilage tissue -interstitial growth involves the lacunae-bound chondrocytes dividing and secreting new matrix in order to expand the cartilage from within -cartilage growth ends during adolescence when the skeleton stops growing -calcified cartilage is not bone; cartilage and bone are distinct tissues Classification of Bones -206 named bones in the body -axial skeleton forms the long axis of the body and includes the bones of the skull, vertebral column, and rib cage -involved in protecting, supporting, or carrying other body parts -appendicular skeleton consists of the bones of the upper and lower limbs and girdles (shoulder bones and hip bones) which attach the limbs to the axial skeleton -limb bones help with locomotion and manipulation of the environment -bones are classified by their shape as long, short, flat, and irregular 1) Long Bones -longer than they are wide -has a shaft and 2 ends -all limb bones are long bones except the patella (knee cap), wrist, and ankle bones 2) Short Bones -cube shaped -e.g.: wrist and ankle bones -sesamoid bones are a special type of short bone that form in a tendon (i.e.: the patella) and shaped like a sesame seed -act to alter the direction of pull of a tendon 3) Flat Bones -thin, flattened, and slightly curved -i.e.: sternum (breastbone), scapulae (shoulder blades), ribs, and most skull bones 4) Irregular Bones -complicated shapes -i.e.: vertebrae and hip bones Functions of Bones 1) Support -bones provide a framework that supports the body and cradles its soft organs 2) Protection -skull protects the brain -vertebrae surround the spinal cord -rib cage helps protect the vital organs of the thorax 3) Movement -skeletal muscles attach to bones by tendons and use bones as levers to move the body and its parts -joint shape allows for certain movements 4) Mineral and Growth Factor Storage -bone acts as a reservoir for minerals (i.e.: calcium and phosphate) -stored minerals are released into the bloodstream for distribution; deposits and withdrawals of minerals to and from the bones go on almost continuously -mineralized bone matrix stores growth factor, bone morphogenic proteins, etc. 5) Blood Cell Formation -most blood cell formation (hematopoiesis) occurs in marrow cavities 6) Triglyceride (fat) Storage -fat is stored in bone cavities and is a source of stored energy for the body Bone Structure -bones are organs (they contains several types of tissue) -osteogenic cell = stem cell -osteablast = matrix-synthesizing cell responsible for bone growth -osteocyte = mature bone cell that maintains the matrix -osteoclast = bone-resorbing cell Gross Anatomy Bone Markings -external surfaces of bones display projections, depressions, and openings that serve as sites of muscle, ligament, and tendon attachment, as joint surfaces, or as conduits for blood vessels and nerves -projects that grow outward from the bone surface include heads, trochanters, spines, and others -bone projections are indications of the stresses created by muscles attached to and pulling on them or are modified surfaces where bones meet and form joints -depressions and openings include fossae, sinuses, foramina, and grooves -they allow passage of nerves and blood vessels Bone Textures: Compact and Spongy Bone -compact bone makes up the external layer -spongy bone makes up the layer that is internal to the external layer -has a honeycomb of small needle-like or flat pieces called trabeculae -open spaces in trabeculae are filled with red or yellow bone marrow Structure of Long Bone Diaphysis -tubular shaft -forms the long axis of the bone -made up of a thick collar of compact bone that surrounds a central medullary cavity (marrow cavity) -in adults, the medullary cavity contains fat (yellow marrow) and is called yellow marrow cavity Epiphyses -bone ends -more expanded than the diaphysis -compact bone forms the exterior and spongy bone forms the interior -the joint surface of each epiphyses is covered with a layer of articular (hyaline) cartilage which cushions the opposing bone ends during joint movement and absorbs stress -an epiphyseal line is found between the diaphysis and epiphysis of an adult long bone -it is a remnant of the epiphyseal plate (a disc of hyaline cartilage that grows during childhood to lengthen the bone -the metaphysis is the region where the diaphysis and epiphysis meet Membranes -the external surface of the entire bone except the joint surface is covered by a glistening white, double-layered membrane called the periosteum -the outer fibrous layer is dense irregular connective tissue -the inner osteogenic layer, abutting the bone surface, consists primarily of bone-forming cells called osteoblasts that secrete bone matrix elements and bone-destroying cells called osteoclasts -osteogenic cells give rise to osteoblasts -periosteum is supplied with nerve fibres, lymphatic vessels, and blood vessels and enter the diaphysis via the nutrient foramina -periosteum is secured to the underlying bone by perforating (Sharpey's) fibres and provides anchoring points for tendons and ligaments -internal bone surfaces are covered with a delicate connective tissue membrane called the endosteum -the endosteum covers the trabeculae of spongy bone and lines the canals that pass through the compact bone -contains osteoblasts and osteoclasts Structure of Short, Irregular, and Flat Bones -all consist of thin plates of periosteum-covered compact bone on the outside and endosteum-covered spongy bone on the inside -have no shaft or epiphyses -contain bone marrow between their trabeculae but have no significant marrow cavity -in flat bones, the spongy bone is called the diploe Hematopoietic Tissue in Bones -red marrow is found within trabecular cavities of spongy bone of long bones and in the diploe of flat bones -these cavities are known as red marrow cavities -in infants, the medullary cavity of the diaphysisand all areas of spongy bone contain red bone marrow -in adults, the fat containing medullary cavity extends into the epiphysis; little red marrow is present in spongy bone cavities -blood cell productiong occurs only in the heads of the femur and humerus -red marrow found in diploe of flat and irregular bones are more active in hematopoiesis Microscopic Anatomy of Bone Compact Bone -compact bone is riddled with microscopic passageways that serve as conduits for nerves, blood vessels, and lymphatic vessels -the structural unit of compact bone is called osteon or Haversian system -each osteon is an elongated cylinder oriented parallel to the long axis of the bone -osteons are weight bearing pillars -an osteon is a group of hollow tubules of bone matrix, one placed outside the next like the growth rings of a tree trunk -each matrix tube is a lamella and compact bone is often called lamellar bone -all of the collagen fibres in a particular lamella run in a single direction; the collagen fibres in an adjacent lamella run parallel to each other but in different directions than other adjacent collagen fibres -this allows for torsional resistance -tiny crystals of bone salts align with the collagen fibres and also alternate their direction in adjacent lamellae -a central canal or Haversion canal runs through each osteon -it contains small blood vessels and nerve fibres that serve the needs of osteon cells -perforating canals or Volkmann's canals
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