1Chapter 6- Part II• Bone Development• Bone growth and hormonal regulation of bone growth•Bone remodelingBone remodeling– Regulation of blood Ca2+• Bone Repair• Osteomalacia, Rickets, Osteoporosis– Causes and TreatmentsCan you identify all of these parts of a bone? What is the definition of• Ossification• CalcificationFormation of the Bony Skeleton• Begins at week 8 of embryonic development• Ossification–Intramembranous ossification – bone develops from a fibrous membraneEd h d l ifi tibf b li–Endochondral ossification–bone forms by replacing hyaline cartilageEndochondral and Intramembranous ossification• Both processes are more similar than they are different– Same cells participate and do similar things•At biochemical and cellular level many of the•At biochemical and cellular level, many of the same events occur• Differences:– Site of activity– Organization of activity– Numbers of centers of ossification– What is replacedIntramembranous ossification – Also called dermal ossification– Occurs in fibrous membrane of mesenchyme– Flat bones of the skull, mandible, and clavicleYou lucky ducks don’t have to know the steps ypof this process!2The Birth of Bone• When new bone is born, either during development or regeneration, it often starts out as spongy bone (even if it will later be pgy (remodeled into compact bone)Endochondral OssificationNote: you DO have to know this one• Begins in the second month of development• Uses hyaline cartilage “bones” as models for bone construction then ossifies cartilage into bone Cbiihli•Common, as most bones originate as hyaline cartilage• This is like a “trick” the body uses to allow long bones to grow in length when bones can only grow by appositional growthBone formation in a chick embryo• Stained to represent hardened bone (red) and cartilage (blue)• : This image is the cover illustration from The Atlas of Chick Development by Ruth Bellairs and Mark Osmond, published by Academic Press (New York) in 1998Bone formation in a human embryoEndochondral Ossification• Bone replaces a cartilaginous model (hyaline)• Occurs in most bones 1. Cells of the perichondrium differentiate to osteoblasts and form a bone collar around the outside (a periosteum)2. Chondrocytes at center of model grow large, cause calcification to happen all around themselves, and then they die, leaving behind big spaces (fancy term= cavitation)3 Spaces invaded byperiosteal bud(blood vessels nerve lymphatic3. Spaces invaded by periosteal bud, (blood vessels, nerve, lymphatic vessel, osteoclasts, osteoblasts)• Osteoclasts and -blasts arrive. Set up primary ossification center, and bone growth occurs (as spongy bone, trabeculae) spreads along shaft of bone, toward ends.4. Primary ossification center grows, and a medullary cavity is carved out. Cartilage continues to grow at the epiphyseal side of the shaft. 5. At or after birth, the centers of each epiphysis also begin to be replaced by bone, each end is a secondary ossification center• At the epiphyseal plate (growth plate), as long as cartilage growth outpaces replacement by bone, bone length continues. HyalinecartilagePrimaryifi tiDeterioratingcartilagematrixEpiphysealblood vesselSecondaryossificatoncenterMedullarycavityEpiphysealplatecartilageArticularcartilageSpongyboneStages of Endochondral OssificationFormation ofbone collararound hyalinecartilage model.ossificationcenterBone collar1Cavitation ofthe hyaline carti-lage within thecartilage model.2Invasion ofinternal cavitiesby the periostealbud and spongybone formation.3Formation of themedullary cavity asossification continues;appearance of sec-ondary ossificationcenters in the epiphy-ses in preparationfor stage 5.4Ossification of theepiphyses; whencompleted, hyalinecartilage remains onlyin the epiphyseal platesand articular cartilages.53Endochondral Ossification…and the race is onWhen secondary ossification is complete, where would you find hyaline cartilage? Endochondral Ossification: Step 5 (Elongation)• Epiphyses fill with spongy bone but cartilage remains at two sites:–ends of bones within the joint cavity = articular cartilage– cartilage at the metaphysis = epiphyseal cartilage (plate)Figure 6–9 (Step 6)What occurs at the epiphyseal plate, or growth plate? …and the race is onPostnatal Bone Growth• Growth in length of long bones– Cartilage on the side of the epiphyseal plate closest to the epiphysis is relatively inactiveCil bi hhffhb–Cartilage abutting the shaft of the bone organizes into a pattern that allows fast, efficient growth – Cells of the epiphyseal plate proximal to the resting cartilage form three functionally different zones: growth, transformation, and osteogenic Functional Zones in Long Bone Growth•Growthzone – cartilage cells undergo mitosis, pushing the epiphysis away from the diaphysis• Transformation zone –older cells enlarge, the matrix becomes calcified, cartilage cells die, and the matrix begins to deteriorate• Osteogenic zone – new bone formation occursGrowth in Length of Long BoneFigure 6.94Growth PlateEpiphyseal cartilages undergo ossification and become epiphyseal linesMore on Postnatal bone growth• Remember that bone growth can only occur from the outside (appositional growth). So this type of endochondral growth is a way yp g yfor bones to grow from the inside and lengthen because it is the cartilage that is growing, not the boneKey Concept• As epiphyseal cartilage grows through the division of chondrocytes it pushes the ends of the bone outward in length. • At the “inner” (shaft) side of the epiphyseal plate, recently born cartilage gets turned into bone, but as long as the cartilage divides and extends as fast or faster than it gets turned into bone, the bone will grow longerLong Bone Growth and Remodeling• Growth in length – cartilage continually grows and is replaced by bone as shown • Remodeling – bone is resorbed and added by appositional growth as shown – compact bone thickens and strengthens long bones with layers of circumferential lamellaeLong Bone Growth and RemodelingFigure 6.105Appositional Growth Epiphyseal Lines• When long bone stops growing, between the ages of 18 – 25:– epiphyseal cartilage disappears ih l l l–epiphyseal plate closes– visible on X-rays as an epiphyseal line• At this point, bone has replaced all the cartilage and the bone can no longer grow in lengthBone growth