Bone Formation

Ossification

Ossification

Intra-membranous Intra- cartilaginous

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1.Intra-membranous ossification`• occurs in flat bones of skull, face and clavicle.

•with no cartilage template

• It takes place within the center of mesenchymal tissue, where a primary center of ossification appears in which the mesenchymal cells differentiate into osteogenic cells and the blood vessels are increased.

•The osteogenic cells divide to form osteoblasts which form bone matrix.

• The osteoblasts that are surrounded by bone matrix are now called osteocytes.

• The new bone extends from the center of ossification outwards in radial manner forming a net of interlacing trabeculae. Thus the mesenchymal membrane is changed into spongy bone.

• The vascular tissue that fills the spaces between the trabeculae of spongy bone forms the bone marrow and the osteogenic cells form the endosteum. The osteogenic cells in the tissue covering the bone plate form the periosteum.

• The osteoblasts present in the periosteum deposit bone in regular layers forming parallel lamellae of compact bone.

Intramembranous ossification

)osteoid is the organic part(

2. Endochondral ossification

• Occurs in long bones in which a temporary

cartilaginous model of the future bone is first

formed.

• The cartilage is then removed and its place is

taken by new bone.

• This process involves the development of primary

and secondary centers of ossification.

Primary center of ossificationIt occurs in the diaphysis of the cartilaginous model during the late

embryonic and early fetal life:1. Chondrocytes within the core of the cartilage model undergo

hypertrophy and calcium salts are deposited around their lacunae.

2. Chondrocytes degenerate due to prevention of diffusion from the matrix, leaving empty spaces.

3. The perichondrium becomes highly vascular causing the transformation of the chondrogenic cells to osteogenic cells which differentiate into osteoblasts. The perichondrium is now called the periosteum.

4. The osteoblasts start to lay down a collar of compact bone around the shaft called periosteal collar.

5. The osteoclasts form perforations in the bone collar that permits the periosteal bud to enter the newly formed spaces in the cartilaginous model. The periosteal bud consists of blood vessels and osteoblasts.

6. The thin walls of the empty lacunae are broken down forming the primary marrow spaces which will be filled by primary red marrow derived from the vascular bud.

7. The subperiosteal bone collar becomes thicker and elongates toward the epiphysis.

8. Osteoblasts that have accompanied the vascular bud start to lay down bone on the walls of the spaces.

9. Gradually, as a result of bone resorption by the osteoclasts and bone deposition by osteoblasts, spongy bone is formed in the center of the shaft, surrounded by compact bone.

10. Later, a large marrow cavity occupied by red marrow appears in the center of the bone.

Secondary center of ossificationThe secondary center of ossification develops at the

epiphysis after birth in a sequence similar to the described for the primary center.

The chondrocytes in the center of epiphysis hypertrophy. The matrix becomes calcified thus the chondrocytes degenerate leaving empty spaces.

Blood vessels and osteogenic cells invade these spaces and the osteoblasts lay down bone matrix on the disintegrating cartilage. Thus, the cartilage in the middle of the epiphysis is replaced by cancellous bone.

When the epiphyses are filled with bone tissue, cartilage remains in two areas; the articular surface and the epiphyseal plates.

The epiphyseal plate: During growth of long bone the following zones are found in the

epiphyseal plate from epiphysis to diaphysis.• Resting zone: It is present next to the epiphyseal cartilage. It is

a layer of hyaline cartilage. The covering perichondrium of this cartilage is very rich in osteogenic cells and blood vessels.

• Proliferative zone: chondrocytes divide rapidly and form columns parallel to the long axis of the bone.

• Hypertrophic zone: chondrocytes accumulate glycogen and increase in size, while the matrix between them is reduced. They produce alkaline phosphatase, which is concerned with the calcification of intercellular matrix.

• Calcification zone: the thin septa of cartilage matrix become calcified by the deposition of hydroxyapatite. Calcification prevents the nutrients from diffusion through the matrix leading to degeneration of chondrocytes.

• Invasion & Ossification zone: blood capillaries & osteogenic cells invade the cavities left by chondrocytes. Osteogenic cells differentiate into osteoblasts, which deposit bone matrix over the calcified cartilage.

(maturation)

Long bone growth

Zone of

ossification

Zone of reserve

Organization of cartilage within the epiphyseal plate of a growing long bone

Bone growth and remodeling• Bone growth is generally associated with partial

resorption of bone tissue and simultaneous lying down of new bone. The growth of long bones is a complex process. The diaphyseal shaft increases in length as a result of osteogenic activity of the epiphyseal plate, and increases in width as a result of formation of new bone by the periosteum on the external surface. At the same time, bone is removed from the internal surface causing the bone marrow cavity to increase in diameter.

• When the cartilage of the epiphyseal plate stops growing, it is replaced by bone tissue around age 20.

Repair of bone fractures (breaks(

•Simple and compound fractures

•Closed and open reduction

Normal bone

Osteoporotic bone

Factors regulating bone growth

Vitamin D: increases calcium from gut Parathyroid hormone (PTH(: increases

blood calcium (some of this comes out of bone(

Calcitonin: decreases blood calcium (opposes PTH(

Growth hormone & thyroid hormone: modulate bone growth

Sex hormones: growth spurt at adolescense and closure of epiphyses