Epichondrial Ossification PDF

2. Endochondral ossification most bones are born like this Osteoblasts replace chondrocytes (which have died) Nature of the original support structure: hyaline cartilage model ...

2. Endochondral ossification most bones are born like this Osteoblasts replace chondrocytes (which have died) Nature of the original support structure: hyaline cartilage model  perichordium osteoblast Endochondral Ossification Step 1 size eases as the cartilage enlarges, chondrocytes near the center of the shaft in teaser greately in O air O perichond The matrix is reduced to a series of struts that soon begin to calcify ium hollow The enlarged chondrocytes then die and disintegrate, leaving cavities within the cartilage We are making hyaline artilage, the ones on the middle is old and starts dying C Bloodvessels Step 2 blood vessels grow around the edges of the cartilage, and the edges of the cartilage, The cells of the perichondrium converts to osteoblasts. The shaft of the cartilage then becomes ends heated in the superficial layer of the bone Theated covered Step 3 the blood vessels penetrate the cartilage Osteoblast starts producing spongy bone at the primary ossification centre Step 4 remodelling occurs as growth continues, creating a marrow cavity. The bone of the shaft become thicker and further growth increases length and diameter. Osteoclasts they break down trabeculae forming medullary cavity cartilageleft Osteoblast continue to add bone at ends of the cavity Step 5 capillaries and osteoblasts migrate into epiphyses This creates secondary ossification Centers Step 6 soon the epiphyses are filled with spongy bone. An articulate cartilage remains exposed to the joint cavity; over time it will be reduced to a thin superficial layer At each metaphysis, an epiphyseal cartilage separates the epiphysis from diaphysis state of most bones at birth Postnatal growth of bones Growth in length chondrocytes (cartilage cells) in the epiphyseal plate continue to divide and grow, adding length to them bone. ◦ Chondrocytes closets to the cavity dies Osteoblasts (builders) ◦ Migrate towards epiphysis replacing cartilage with bone Bone developers from hyaline cartilage, which becomes fully ossified after birth: ◦ Calcilfication ◦ Deterioration of chondrocytes (die), leave cavities ◦ Replacing with osteoid tissue (osteoblast and clast) Bone growth is under hormonal control during development, bone grows longitudinally through the formation of new cartilage through the epiphyseal plate, this process is complete after puberty. Lateral bone growth is achieved by osteoblasts in the periosteum laying down new bone Osteoclast in the medullary cavity breaking down old bone Summary 1. Development if cartilage model: mesenchymal cells develop into chiondroblastss, which forms the cartilage model 2. Chondrocytes produce a substance that inhibits the formation of blood vessels limiting O2 and nutrient delivery 0 3. Chondrocytes in shafts enlarge and eventually die p 4. Chondrocytes in the primary ossification site enlarge and stop secreting collagen and 0111 other proteoglycans sand begin secreting alkaline phosphatase an enzyme required for mineral deposition. 5. Calcification of the matrix occurs 6. Blood vessels grow into perichondrium transforming it into periosteum containing Osteoprogenitor cells which become osteoblasts 7. Osteoblasts secrete osteoid and create bone collar against shaft of cartilage model -> generate collar of bone o 8. Mature chondrocytes can not divide and die leaving cavities 9. Osteoblasts in ossification sites produce osteoid and turn into osteocytes as mineral deposits surround and harden around them How are new osteons and central canals formed during appositional growth? (Width-wise growth) 1. Ridges in periosteum create groove for periosteum blood vessel O 2. Periosteum ridges fuse, forming an endosteum-lined tunnel they have osteoblastwiththem 0 3. Osteoblasts in endosteum build new concentric lamellae inward toward center of tunnel, forming a new osteon Gets 4. Bone growns outwards as osteoblasts in periosteum build new circumferential lamellae. Osteon formation repeats as new periostea’s ridges fold over blood vessels Ben Enlargement of medullary cavity with increased Brien diameter resulting from appositional growth increase the diameter of the long bone, forms new circumferential lamellae Involved both osteoblast and osteoclasts Growth in width by appositional growth where you have osteoblast in periosteum ◦ Osteoblast under the periosteum lay down bone Osteoclast break down bone on inside of medullary cavity ◦ Bone increases in diameter, cavity enlarges ◦ In the midddle, starts dissolving bone to extend diameter At Puberty 000 under influence of sex hormones, growth hormine, thyroid hormone Cartilage in epiphyseal plate is reaplaced with bone -> No further growth in length Bone remodeling O O bone remodeling occurs through life and involves resorption of old bone and deposition of new bone where it is needed Remodeling occurs in response to ◦ Bone growth (to maintain proper thickness and shape) ◦ Blood Ca++ levels (storage) ◦ Mechanical stress on the bone Why do sharks have a skeleton made of cartilage instead of bone? weight is reduce -> saves on energy Blood Supply to the bone Nerve Supply periosteum Endosteum And the bone itself is richly supplied with sensory nerve endings Hormonal and Nutritional Requirements for Normal Bone growth Hormonal 1. Growth Hormone from the pituitary gland ( at the epiphyseal layers) , promotes osteoblast activity 2. Thyroid hormone thyroxine ( at the epiphyseal layer) Stimulates cell metabolism, promotes osteoblast activity 3. Calcitriol produced by kidneys; derived form vitamin D3 o Allows normal absorption of Ca++ and PO4 ion from the digestive tract 4. Sex hormones estrogen and testosterone Promote growth, cause osteoblasts to produce bone faster When production fo sex hormones increase at puberty, epiphyseal plates become narrower Nutritional Requirements for bone growth 1. Calcium and phosphate salts to prevent rickets (children) and osteomalacia (adults) 2. Vitamin C required for collagen synthesis Stimulates osteoblast differentiation 3. Vitamin A E stimulates osteoblast activity 4. Vitamin K and B12 required for synthesis of proteins in normal bone = large in collagen Regulation of Bone Remodeling Hormonal maintain a clacium ion in blood and interstitial fluid at 9-11 mg/100 ml blood Two organs monitor calcium levels ◦ Para (beside)thyroid glands ‣ Releases parathyroid hormone in response to lower than normal Ca++ in blood ‣ Effects (cannot inhibit calcitonin secretion) Stimulates osteoclast activity ◦ Disssolve calcium ion off the bone, which will go into the blood and help the levels go up Increase the rate of absorption of Ca++ from intestine Decrease the rate of excretion by kidney (don’t pee too much) ◦ thyroid glands ‣ Release the hormone calcitonin ‣ In response to highe than normal Ca++ in blood Inhibit tha activity of osteoclast Increase rate of kidneys excretion Ca++ Bone remodelling: response to mechanical stress Wolff’s law: bones are remodelled in response to forces placed on them How does it work stress generates electrical currents in bone Osteocytes are mechanosensors and secrete chemicals that alter the activities of osteoblasts and osteoclasts (make bone more dense and thicker) Net effect of Wolff’s law bones that are used to stressed become stronger ◦ The trabeculae is shaped to cope with stress Unstressed bones are remodelled to become weaker When stress is applied to the bone, the minerals in the bone produce a weak electrical field that attract osteoblast. -> result in the bone make thicker Repair of Fractures Treatments realign bone pieces = reduction of movement (i.e immobilize) Open (compound ) = term for fracture in which the broken bone breaks through the skin o