OSSIFICATIONINTRAMEMBRANOUS DIRECT METHOD

Source of most flat bones.Begin in 2nd month of i.u life.Formation of a membrane by

condensation of mesenchyme in the area.

Vascularisation of the mesenchyme

Differentiation into osteoblast.Formation of osteoidCalcification of osteoid.Formation of spongy bone.

continuedFusion of ossification centres and replacement of orignal

c.tissue.

Fontanelles of newborn are unossified c.tissue. Spaces between trabeculae filled with lamellar bone so

inner and outer tables of skull bone form. Entire primordium is surrounded by dense mesenchyme which gives fibrous layer of periosteum.

INTRACARTILAGINOUS OR ENDOCHONDRAL OSSIFICATIONSource of long and short bones.Formation of primary centre of ossification within a piece of

hayline cartilage which resemble a small version of bone in diaphysis.

Degeneration of local cartilage by programmed cell death.Penetration of blood vessels and mesenchymal cells.Osteoprogenitor cells penetrate that area derived from

perichondrium around that area. Formation of primary bone by osteoblast that surround

cartilaginous matrix remanants.Appearance of secondary centres of ossification in the extremities

of cartilage model.Formation of cavities that are filled with bone marrow.

CONTINUECartilage remains in two region in secondary ossification

centres.Articular cartilage, which persist throught adult life and do

not contribute to bone growth.Epiphyseal cartilage which connect two epiphysis to the

diaphysis. It is responsible for the growth in length of the bone, it disappear in adults that’s why growth ceases in adulthood.

ENDOCHONDRAL ossificationFormation of long bone on a model made of cartilage

Two basic events in endochondral ossification

Destruction and removal of the hyaline cartilage except at joint surfaces.

Formation of bone tissue in the spaces previously occupied by cartilage.

CONTINUED

.

EPIPHYSEAL CARTILAGE

Resting zone, consist of hyaline cartilage.

Proliferative zone, chondrocytes divide rapidly and form colums of packed cells.

Hypertrophic cartilage zone, consist of large chondrocytes with accumulated glycogen ,resorbed matrix is in the form of thin septa.

CONTINUECalcified cartilage

zone ,chondrocyte die,thin septa calcified by deposition of hydroxyapatite crystals.

Ossification zone, endochondral bone tissue appears,blood capillaries and osteoprogenitor cells formed which differentiate into osteoblasts which deposit bone matrix over the septa of calcified cartilage matrix.

THREE DIMENSIONAL SHAPE OF BONE IN THE EPIPHYSEAL PLATE AREA

Rates of two opposing events i.e proliferation and destruction are equal epipheal plate donot change in thickness

It is displaced away from the middle of the diaphysis resulting in growth in length of the bone.

CLINICAL APPLICATIONRicket, ca deficency in

children.Bone matrix does not calcify normaly and epiphyseal plate distorted ,ossification process Is hindred, bone growth is slow.

Osteomalacia ,ca deficency in adults characterised by deficent calcification of recently formed bone .

Osteoprosis ,found in immobilised patients and in postmenopausal women.It is an imbalance in skeletal turnover ,bone resorption exceeds bone formation.

Osteopetrosis or marblebone.a genetic disorder charterised by dense heavy bone ,osteoclast lack ruffled border defective bone resorption.