bone formation

• bone formation in flat bones - starts in mesenchyme that differentiates into osteoprogenitor cells and then osteoblast to form primary osteoid

• woven collagen- random organization • non-mineralized • collagen type 1

-cytoplasmic processes of osteocytes that maintain contacts with cytoplasmic processes of other osteocytes

used by intramembranous ossification building bone from outside (frosting on cake)

it differentiates in to bone marrow or becomes compact bone via secondary deposition that fills in the marrow spaces

• process by which endosteum forms via first creating a cartilage model of bone to be and then calcification, erosion of this model by bone • LONG BONES of limbs and vertebrae of spine are formed like this

• form a mini version of hyaline cartilage model of bone to be. • once bone forms, it grown interstitially and appositionally but maintains outline

appositional growth- adds to the width of the bone interstitial adds to the length of the bone

• the perichondrium becoming vascularized - when this occurs the osteoprogenitor cells differentiate into osteoblasts and perichondrium converts into periosteum

endochondral ossification- forms long bones of limbs and vertebrae and does so via a cartilage model that is replaced via calcification intramembranous ossification- forms in a membrane of vascularized mesenchyme to create flat bones

the cartilage begins to hypertrophy and produce alkaline phosphatase to calcify thed matrix. because the matrix is calcified no nutrients can enter and the chondrocytes die

diaphysis

epiphysis

• occurs in the diaphysis • hyaline cartilage forms and grows via intersitital and appositional growth • hyaline cartilage is covered by perichondrium that becomes vascularized and cause the chondrogenic cells to differentiate into osteoprogenitor cells and then osteoblasts • osteoblasts lay down bone called bone collar (bone shaft aka diaphysis) via intramembanous ossification because it’s growing by width • as the bone forms, cartilage cells die cuz not getting enough nutrients and are then calcified and replaced by bone (ie cartilage template for bone) • osteogenic bud (blood vessel) enter the bone via osteoclasts and bring osteoprogenitor cells that differenciate into osteoblasts that make bone marrow and new boneosteoprogenitor cells that make bone marrow and new bone

• one epiphysis grows right after birth and then the other follows • bone is grown out via replacement of calcified hyaline cartilage by bone via osteoblasts until the bone reaches the diaphysis. • hyaline cartilage left at articulating edges of bone • hyaline cartilage left at epiphyseal growth plates • THERE IS NO BONE COLLAR

cartilage length: appositional growth/ width- interstitial growth long bone length: endochondrial ossification/ width- appositional growth

• it grows at the epiphyseal growth plates • on epiphysis side, cartilage is constantly being replenished, at diaphysis side, bone is constantly replacing cartilage • plates never change size

start on outside and move in: zone of reserve cartilage- typical hyaline bone zone of proliferation- intense mitosis; chondrocytes form parallel columns of isogenous cells. chondrocytes appear in stacks that look like PENNIES- make new ORGANIC matrix- cartilage grows interstitially zone of hypertrophy- chondrocytes accumulate glycogen and become vacuolated; chondrocytes swell- no addition of new cells or cartilage zone of calcified cartilage-lacunae coalese, calcific of cartilage matrix, chondrocytes die zone of resportion and ossification - calcified cartilage form and osteoprogenitor cells invade and make bone via osteoblasts

• calcified is basophilic (blue) with no cells because they have died • mineralized bone is acidophilic (pink) and have cells because it’s alive

-in the zone of proliferation and it means that the bone grows in length

• occurs at 20 years of age • zone of proliferation slows down but the ossification still occurs and causes the plate to be replaced by bone and eliminates the ability of the bone to grow in length • instead a plate of calcified cartilage and mineralized bone replaces it

appositional growth

• the periosteum has osteoprogenitor cells that differentiate in to osteoblasts that secrete bone matrix AT THE SURFACE OF BONE • OSTEOCLASTS- RESPRB BONE ON INTERNAL SURFACE OF BONE • bone marrow cavity diameter increases and bone diameter increases

• minerals delivered in blood • osteocalcin/sialoproteins bind ca • increase in conc of ca activates osteoblasts to release alkaline phosphatase -> increases PO4 • increases ca more • osteoblasts release matrix vesicles into bone matrix (that now has lots of ca and PO4) • vesicles regulate initial site of mineral deposition in the osteoid • vesicles have a membrane that allow more and more minerals to enter • get crystallization of ca and PO4 to get hydroxyapatite crystals • as the crystals become sharp and large, they damage the membrane and are deposited into the bone matrix