Lecture 08

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-organ system consisting of bones -includes cartilage, ligaments, and other connective tissues
-primary organs of the skeletal system -interior cavity of bone contains CT consisting of either red or yellow bone marrow -osseous tissue -marrow -2 types
Primary function of bones
-form the framework of the body
Compact bone (dense or cortical)
-relative dense, white, smooth, and solid -makes up 80% of total bone mass
Spongy bone (cancellous or trabecular)
-internal to compact bone -appears porous -remaining 20% of total bone mass
-covers the external surface of bone -consists of 2 layers
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Outer fibrous layer
-layer of periosteum -dense irregular CT -protect the bone from surrounding structures -anchors blood vessels and nerves -attachment site for tendons and ligaments
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Inner cellular layer
-layer of periosteum -osteoprogenitor cells, osteoblasts, and osteoclasts
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-covers the internal surface of bone within the medullary cavity -an incomplete layer of cells containing osteoprogenitor cells, osteoblasts, and osteoclasts
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Diaphysis (structure of long bone)
-structure of a long bone -long, hollow cylindrical shaft -provides leverage and weight support -an external, relatively thick layer of compact bone and an internal thin layer of spongy bone surrounds the central medullary cavity *(in children: cavity is filled with red marrow) *(in adults: cavity is filled with yellow marrow)
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-structure of a long bone -knobby ends of the long bone -an external, thin layer of compact bone and an internal core filled with spongy bone *(spongy bone resists stresses in all directions) -at the moveable joints, covered in articular cartilage to reduce friction and absorb shock
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-structure of a long bone -region of mature bone between the diaphysis and epiphysis -contains the epiphyseal (growth) plate
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Unique structures of other bones
-short, flat, and irregular differ from long bones in structure -external surface is compact bone -interior is composed of spongy bone *(also called diploe in the flat bone of the skull) -no medullary cavity
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Flat bones
-flat, thin surfaces -may be slightly curved -protect underlying soft tissue and serve as large surface area for muscle attachment -ex: cranial vault of skull, scapulae, sternum, and ribs
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-found within tendons -small, flat, and oval shaped (think of sesame seed) -increase muscle leverage -ex: patella
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Long bones
-longer than they are wide -have an elongated cylindrical shaft (diaphysis) and two ends (epiphysis) -most common bone shape -found in upper and lower limbs (except wrists and ankles)
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Short bones
-same length as width (apprx.) -ex: carpals and tarsals (wrist & ankle) and sesamoid bones, e.g. patella
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Irregular bones
-elaborate, complex shapes -doesn't fit in other categories -ex: vertebrae, bones of skull, and hip bones
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Bone markings
-surface features that characterize each bone in the body
-clefts located when the bone meets another structure -allow blood vessels and nerves to travel along the bone -allows 2 bones to articulate
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-holes in the body -indicate where blood vessels or nerves travel through the bone
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-bony extensions -variety of shapes and sizes -points where muscles, tendons, and ligaments attach to the bone
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-soft connective tissue of the bone
Red marrow
-hematopoietic tissue -in adults, located within: spongy bone of the long bones diploe of flat bones -in children, located within: medullary cavities of long bones
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Yellow marrow
-replaces red marrow in medullary cavities of adults -primarily adipocytes -may revert back to red marrow under extreme anemic conditions
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4 major bone cell types
-osteoclasts (resorbs bone) -osteogenic (stem cell) -osteoblast (forms bone matrix) -osteocyte (maintains bone tissue)
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Osteoprogenitor cells
-mitotic stem cells -can give rise to the osteoblasts -found in the periosteum and endosteum
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-produce new bone matrix (osteoid) -become trapped within calcified osteoid -differentiate into osteocytes
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-mature bone cell -cannot make new bone -reside within lacuna within the bone matrix -connected to the other nearby osteocytes -maintain the bone matrix and detect mechanical stress in the bone matrix
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-large, multinuclear phagocytic cell -derived from a hematopoietic progenitor -located within depressions on the bone surface called resorption lacuna -break down the bone matrix (during resorption)
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Formation of bone matrix
-osteoblasts secrete bone matrix, osteoid -osteoid calcifies -occurs as a result of high plasma Ca2+ levels or when bone needs to respond to greater stress
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Resorption of bone matrix
-osteoclasts destroy bone matrix -releases Ca2+ into blood plasma
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Bone continues to renew and reshape
-recycle ~5-7% of our bone mass every week -occurs at the periosteal and endosteal surfaces of the bone -compact bone is replaced slower than spongy bone
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Composition of bone matrix (Organic)
-osteoid (~1/3rd of the matrix) *(collagen fibers) *(semi-solid ground substance (proteoglycans and glycoproteins) -cells -gives bone tensile strength -resists stretching and twisting
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Composition of bone matrix (Inorganic)
-mineral salts (~2/3rd of the matrix) *primarily Ca3(PO4)2 *Ca3(PO4)2 combines with other salts to form Hydroxyapatite crystals which deposit along the long axis of collagen -harden the matrix -accounts for rigidity and inflexibility -gives bone compression strength
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Osteon of compact bone
-structural unit of compact bone -run parallel with the long axis -weight bearing pillars -resist torsion stress -when viewed in cross-section, has the appearance of a bullseye
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Components of the osteon
-Central canals -Composed of concentric lamella -Osteocytes -Lacunae -Caniculi
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Central canals
-contain small blood vessels & nerve fibers that serve the osteon's cells -run parallel to the long axis of the bone
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Composed of concentric lamella
-collagen fibers running in alternating directions in each lamella -gives bone strength and resilience
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-mature bone cells that maintain the bone matrix -isolated in lacunae
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-contain osteocytes -found at the junctions of the lamellae
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-tiny canals between lacunae -osteocytes connect with one another (by gap junctions) to pass nutrients & waste and to communicate with one another
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Structures of the compact bone
-Perforating (Volkmann) canals -Circumferential lamellae -Interstitial lamellae
Perforating (Volkmann) canals
-connect and supply small blood vessels and nerves to the central canals & medullary cavity -lie at right angles to the long axis of the bone
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Circumferential lamellae
-rings of bone that run the entire circumference of the bone -protect against torsion -external circum. -internal circum.
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External circum. lamellae
-found internal to the periosteum
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Internal circum. lamellae
-found internal to the endosteum
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Interstitial lamellae
-located between intact osteon -incomplete and have no central canal -remnants of resorbed osteons or fill-ins
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Structures of spongy bone
-structure appears more haphazard -open lattice -trabeculae -no osteons *(possess parallel lamellae) *(no blood vessels) *(some canaliculi open to surface to allow the diffusion of nutrients and waste)
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-narrow plates and rods of bone -align along lines of stress -distribute stress throughout
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Bone formation
-ossification or osteogenesis -begins in the embryo through childhood and adolescence -2 types resulting different kinds of bone formations *intramembranous *endochondral
-yields flat bones -bone growth within a membrane -4 steps
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4 steps of Intramembranous ossification
1: Formation of the ossification center -committed osteogenic cells divide to become osteoblasts secreting osteoid -several ossification centers can occur simultaneously 2: Calcification of the osteoid 3: Formation of woven bone and the periosteum -woven bone is immature and unorganized -mesenchyme condenses and forms periosteum -blood vessels form and branch 4: Replacement of woven bone by lamellar bone
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-yields most bones of the skeleton -bone development that replaces cartilage (used as a model for construction) -long bones -bones of the pelvis, vertebrae, and clavicle -6 steps
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6 steps of Endochondral ossification
1: Fetal hyaline cartilage model develops (week 8-12 development) 2: Cartilage calcification & formation of the periosteal collar 3: Primary ossification center forms in the diaphysis 4: Secondary ossification center forms in the epiphysis 5: Bone replaces almost all cartilage -articular cartilage remains on epiphyseal surfaces -cartilage at the epiphyseal plate, the junction between the diaphysis and the epiphysis 6: Lengthwise growth continues until epiphyseal plate closes -cartilage in epiphyseal plate ossify between age 10 and 25
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Interstitial bone growth
-growth in length -dependent upon cartilage growth within the epiphyseal plate *(growth in epiphyseal plate of cartilage on the epiphysis side of the plate) *(chondrocytes become deprived of nutrients and become calcified) *(remodeling of calcified cartilage by the bone tissue on the diaphysis side of the plate -epiphyseal plate closure occurs after adolescence *(chondroblasts in epiphyseal plate divide less frequently and eventually are replaced by bony tissue) *(epiphysis and diaphysis fuse)
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5 cartilaginous zones
-involved in interstitial bone growth -Zone of resting cartilage -Zone of Proliferating cartilage -Zone of hypertrophic cartilage -Zone of calcified cartilage -Zone of ossification
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Zone of resting cartilage
-furthest from medullary cavity of diaphysis -resembles healthy cartilage -secures epiphysis to epiphyseal plate
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Zone of proliferating cartilage
-rapid mitotic division -cells aligned in columns
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Zone of hypertrophic cartilage
-no longer dividing -cells become hypertrophic and resorb matrix
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Zone of calcified cartilage
-mineral deposition of the matrix -chondrocytes die
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Zone of ossification
-vascularization and bone deposition
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Appositional bone growth
-growth in width -bone is added to the external surface -bone is resorbed on the internal surface -rate of addition > rate of resorption *(thicker, stronger bones)
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