Lecture on Osteogenesis by Dr Roomi

8/2/2019 Lecture on Osteogenesis by Dr Roomi

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HISTOLOGY OF BONE

BY

DR. MUDASSAR ALI ROOMI (MBBS, M. Phil.)


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Spongy bone


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Spongy bone

It is idntified by:

1. Trabeculae of the bone

matrix

2. Haemopoitic (bone

marrow) tissue

3. Few Haversian system


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osteogenesis

Also called as ossification, histiogenesis of

bone

Starts at early intrauterine life

Continues into the adulthood.

Bone is derived from mesenchyme tissue.

Mesenchyme is a derivative of mesoderm andit consists of a meshwork of star shaped cells

which bear many processes.


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Methods of osteogenesis

Bones develop from mesenchyme by twomethods:

1. Intramembranous method of ossification

2. Intracartilaginous (endochondral) method of

ossification

Ossification

Intra-membranous Intra- cartilaginous

1ry

2ry


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Methods of osteogenesis

Intramembranous method of

ossification

Mesenchyme is directly

converted into bone

without any interveningstage of cartilage formation.

Also called as direct method

of ossification.

Bones: flat bones of skull,bones of face and clavicle

develop by this method.

Intracartilaginous method of

ossification

Mesenchyme is first convertedinto cartilage which serves as atemporary supporting

framework. The cartilagemodel is then replaced bybone.

Also called as indirect methodof ossification.

Also called as endochondralossification.

Most of the bones of bodydevelop by this process.


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Intra-membranous ossification`

Begins in the 2nd month of intra uterine life.

Mesenchyme is condensed in the form of a membrane.

within the center of mesenchymal tissue, center of ossification

appears in which the mesenchymal cells differentiate into osteogenic

cells.

This membrane is vascularized at several points.

The osteogenic cells divide to form osteoblasts which form bone

matrix (osteoid) which is then calcified under the influence of ALP.

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 network of trabeculae. Thus the mesenchymal

membrane is changed into spongy bone.


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The vascular tissue that fills the spaces (marrow cavities)

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.


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Intra-membranous ossification


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Intra-membranous ossification


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Intracartilaginous ossification


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Starts from hyaline

cartilage model

Two important events

occur during this process:1. Destruction and removal

of hyaline cartilage

2. Formation of bone

tissue in the spaceformerly occupied by

the cartilage.


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Primary centre of

ossification: located in

the centre of shaft

secondary centre ofossification: located in

the ends of bone


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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 undergohypertrophy and calcium salts are deposited around their lacunae.

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

3. The perichondrium becomes highly vascular causing thetransformation of the chondrogenic cells to osteogenic cells whichdifferentiate into osteoblasts. The perichondrium is now called theperiosteum.

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 cartilaginousmodel. The periosteal bud consists of blood vessels and osteoblasts.


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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.


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Some important terms

Epiphysis

diaphysis

Metaphysis Epiphyseal cartilage or growth plate

Epiphyseal line

Interstitial bone growth Appositional bone growth


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Secondary centre of ossification


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epiphyseal cartilage is divided into five zones


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epiphyseal cartilage is divided into five zones

A plate of epiphyseal cartilage is divided into five zones, starting from theepiphyseal side of cartilage:

1. The resting zone consists of hyaline cartilage with typical chondrocytes.

2. In the proliferative zone, chondrocytes begin to divide rapidly and formcolumns of stacked cells parallel to the long axis of the bone.

3. The hypertrophic cartilage zone contains swollen chondrocytes.4. In the calcified cartilage zone, loss of the chondrocytes by apoptosis isaccompanied by calcification of the septa of cartilage matrix.

5. In the ossification zone, bone tissue first appears. Capillaries andosteoprogenitor cells originating from the periosteum invade the cavitiesleft by the chondrocytes. Many of these cavities will be merged and

become the marrow cavity. The osteoprogenitor cells form osteoblasts,which settle in a discontinuous layer over the septa of calcified cartilagematrix. The osteoblasts deposit osteoid over the spicules of calcifiedcartilage matrix, forming bone.


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Achondroplasia

An autosomal-

dominant genetic

disorder that causes

dwarfism. It is a genetic problem

which leads to

abnormal cartilage

formation and shortstature.


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Osteoporosis

A bone disease

characterized by

reduced bone mineral

density, thinned bonecortex, and trabeculae.

It causes an increased

risk of fracture,

especially inpostmenopausal

women.

Lecture on Osteogenesis by Dr Roomi

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