The Skeletal System Bones and Cartilage Bone 2 Types Spongey Compact The 2 Membranes like a honeycomb composed of needle like structures called TRABECULAE structural units of spongy bone composed of OSTEONS structural units of compact bone Structure of a Long Bone Epiphyses expanded ends of long bones Diaphysis shaft long axis of a long bone spongy bone surrounded by a thin layer of compact bone composed of a thick collar of compact bone which surrounds a Medullary Cavity contains red bone marrow in childhood and yellow bone marrow in adulthood Hematopoiesis the process by which blood cells and platelets are formed occurs only in red bone marrow Membranes Endosteum and Periosteum It contains osteoblasts and osteoclasts Endosteum covers the internal surfaces of bone such as the canals Periosteum is double layered composed of the outer fibrous layer and the inner osteogenic layer The fibrous layer is composed of dense irregular connective tissue The osteogenic layer contains of osteoblasts and osteoclasts The periosteum is attached to compact bone by tough collagenous fibers called the SHARPEY S perforating FIBERS The Bone Cells Osteoblasts bone forming cells secrete bone tissue Osteogenic cells give rise to osteoblasts Osteocytes matured osteoblasts Osteoclasts bone resorbing cells destroy bone tissue Microscopic Structure of Compact Bone Composed of osteons structural units of compact bone Each osteon is an elongated cylinder consisting of concentric tubes called LAMELLAE hence compact bone is also known as Lamellar bone The collagen fibers in adjacent lamellae run in opposite directions to resist twisting HAVERSIAN CANAL Central canal runs in the core of each osteon contains blood vessels and nerves Perforating or Volkmann s canals connect blood vessels and nerves between the periosteum and the Haversian canals LACUNAE shallow cavities in the solid bone matrix that house the osteocytes CANALICULI tiny canals that connect lacunae to each other and to the Haversian canal to allow for transfer of substances from the blood vessel in the Haversian canal Ossification Osteogenesis development of the bony skeleton from the embryonic skeleton 2 forms PRENATAL AND POSTNATAL Prenatal bone development occurs before birth 2 types i INTRAMEMBRANOUS OSSIFICATION ii ENDOCHONDRAL OSSIFICATION Intramembranous ossification Endochondral Ossification develops from FIBROUS CONNECTIVE TISSUE MEMBRANE derived directly from mesenchyme and results in the formation of MEMBRANE BONES cranial bones and clavicles Note all membrane bones are flat bones Derived from HYALINE CARTILAGE produced by chondroblasts chondroblasts are derived from mesenchyme The bones formed from endochondral ossification are called endochondral or cartilage bones all bones in the body except the cranial bones and the clavicles Hyaline cartilage is ossified into bone After endochondral ossification hyaline cartilage still persists in two areas in the long bones as the 1 2 Articular cartilage capping the ends of the epiphyses of long bones Epiphyseal plates at the junctions of the epiphyses and the diaphysis of a long bone Postnatal Bone Growth Longitudinal Bone Growth Ossification that occurs after birth 2 types Longitudinal bone growth and Appositional bone growth Longitudinal bone growth Linear bone growth increases the length of long bones height Appositional bone growth increases the width diameter of all bones Involves the EPIPHYSEAL PLATES New hyaline cartilage is added on at the epiphyseal faces of the epiphyseal plates New bone tissue is added on at the diaphyseal faces of the epiphyseal plates Results in lengthening of the diaphysis of the long bone lengthening of the long bone The amount of new hyaline cartilage added on the epiphyseal face the amount of bone tissue formed on the diaphyseal face hence the width thickness of the epiphyseal plates does NOT change Appositional Bone Growth Sequence of events All bones widen and increase in diameter thickness via appositional bone growth Osteoblasts in the osteogenic layer of the periosteum secrete new bone tissue onto the external surface of the bone Osteoclasts in the endosteum slightly resorb bone tissue in the internal surface of the bone Overall more new bone tissue is added onto the external surface and old bone tissue is slightly resorbed from the internal surface resulting in a thicker but lighter bone Hormonal Control of Postnatal Bone Growth Growth hormone stimulates hepatocytes to produce Insulin like growth factors IGFs IGFs stimulate chondroblasts to produce hyaline cartilage on the epiphyseal faces of the epiphyseal plates and bone formation on the diaphyseal faces Sex steroid hormones testosterone in the male and the estrogens in the female synergize with growth hormone to cause growth spurt Towards the end of adolescence the sex steroid hormones antagonize the actions of growth hormone and the epiphyseal plates become ossified EPIPHYSEAL PLATE CLOSURE height determined Bone Remodeling Adult bones constantly undergo bone formation on the periosteal surface and bone resorption on the endosteal surface Bone Remodeling In healthy adults the bone density remains constant because Rate of Bone formation rate of bone resorption If the rate of resorption outpaces the rate of formation OSTEOPOROSIS Functions of Bone Remodeling To maintain calcium homeostasis To allow for bone repair after fractures Factors that Control Bone Remodeling 2 factors Hormonal control and mechanical stress Hormonal Control Under hypercalcemic conditions CALCITONIN is released to stimulate osteoblasts to produce bone tissue and stimulate mineralization uses calcium from blood Under hypocalcemic conditions PARATHYROID HORMONE PTH is released to stimulate osteoclasts to cause bone resorption to release calcium from bones into blood I 25 dihydroxyvitamin D stimulates calcium absorption from the small intestine Mechanical Stress Bones remodel grow in response to mechanical stresses placed on the bones WOLFF S LAW Forms of Evidence in Support of Wolff s Law Bone attachment sites for active skeletal muscles appear thicker projections such as trochanters spines Bones of the upper limb often used are thicker than the less used limb bones in the right arm of a right handed individual are thicker than bones in the left arm and vice versa Long bones are thickest in the middle region of the diaphysis where bending stresses are greatest Bedridden individual not subjected to the stresses of walking or
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