Ossification

Ossification

The process of gradual bone formation by depositing minerals, from the mesenchymal membrane or the cartilaginous model is called ossification.
 
Types of ossification
1. Intramembranius ossification
2. Cartilaginous ossification
 
 
Ossification center
The area of the cluster of osteoblasts with blood supply from where bone start to form is called ossification center.
 
Primary centers:
The primary ossification center is that from where, the bone starts ossified which appears before birth.
The primary ossification center is that from which, the main part of the bone is ossified & is appears before birth, usually during the 8th week of intrauterine life, which forms diaphysis (later called as the shaft of a long bone).
 
Secondary centers:
The secondary ossification center is that from which accessory part of a bone is ossified & is appears mostly after birth, which forms the epiphysis.
 
 Remember:
In case of typical long bone
• Epiphysis develops from the secondary ossification center.
• Diaphysis (shaft) develop from the primary ossification center.
 
 

Law of union of epiphysis/ Law of ossification
• The Epiphyseal center which appears first, unit last with the diaphysis & vice-versa.
• That means epiphysis that ossify first fuse with diaphysis last
& the epiphysis that ossifies last fuses first with diaphysis except fibula.
 
Fibula violet the law of ossification.
• Secondary ossification center appears first in the lower end,
that means epiphysis of lower end of fibula ossify first but also fused with diaphysis first.
• As the upper epiphysis of fibula fused last, the upper end of the fibula is the growing end of the fibula as usual.
• There is no violation in determining the growing end. It's same for all long bones (opposite to the direction of nutrient foramen)
 
 
Law of ossification:
• The epiphysis which is the first to appear, is the last to join, and the epiphysis which is the last to appear is the first to join.
• The only exception is the fibula.
In fibula
• The secondary ossification center appears 1st in the lower end and fused first with diaphysis.
• The secondary ossification center appears later in the upper end & fused last with diaphysis.

 


Overview
Intramembranous ossification
Mesodermal model
(Formed by undifferentiated mesenchyme)
        ↓
Directly converted into bone.
(By mineralization of matrix.)
Endochondral Ossification
Undifferentiated mesenchyme
         ↓
Form Cartilage model
         ↓
Converted to bone later
 
Intramembranous ossification
• If bones are formed directly from the mesenchymal rudiment it is called intramembranous ossification.
• Conversion of the mesenchymal rudiment/mesodermal model into bone is called intramembranous ossification
• & the bones formed are called membrane bone.
 
 
EXAMPLE:
Most of the cranial bones
• Flat bones of face
• Bones that form cranial vault 
(Flat bone of Skull)
 
Base of skull below the highest nuchal line is formed by Intra cartilaginous ossification.
 
Sequence of events:
Appearance of centre of ossification
From mesoderm, undifferentiated mesenchymal cells develop.
 
Mesenchymal cells undergo proliferation & cells become closely packed to form a mesenchymal condensation.
And new blood vessels are formed.
 
Differentiation into osteoblasts
Some mesenchymal cells become enlarge & differentiate into osteoblasts.
 
Formation of osteoid matrix
Osteoblasts secrete the component of the matrix, which is not calcified known as osteoid.
 
Formation of bone matrix
Osteoid becomes calcified with the deposition of calcium phosphate. The process of
calcification results from the release of alkaline phosphatase by osteoblasts.
 
Formation of lamellae & trabeculae of spongy bone
Conversion of spongy bone into compact bone
 
Superficial cells surrounding the mesenchyme again condense to form periosteum.
 
Simplified way
Mesoderm
   |
Mesodermal condensation
   |
Mesenchyme
   |
Vascularization of mesenchyme & differentiation of osteoblast cells
   |
Osteoid matrix
   |
Mineralization of matrix
   |
Bone
 
 
Intra cartilaginous / Endochondral ossification
If the mesenchyme rudiment is first converted into a cartilaginous model which is replaced by bone is called Intra cartilaginous or endochondral ossification.
It is the process by which a cartilage model is converted into bone is called Intra cartilaginous ossification.
 
 
Difference between intramembranous & Intra cartilaginous ossification
Intramembranous ossification Intra cartilaginous ossification
By direct mineralization of matrix secreted by osteoblast By deposition of bone matrix in pre-existing cartilage matrix
Primary center of ossification only present Both primary & secondary center of ossification present
From mesenchymal tissue Modified from hyaline cartilage model
Most of the flat bone ie.
Ribs, Sternum, Clavicles
Bones of the skull that form cranial vault,
Mandible, maxilla, hip bone,
vertebrae
Short & long bones ie. Humerus, femur, tibia, fibula, etc.


Intramembranous ossification
• Mandible
• Maxilla
• Most of the flat bones of the skull that form the cranial vault
• Sternum
• Ribs
• Vertebrae
• Hip bone
Intra cartilaginous ossification
• Short & long bones of the body
• Base of the skull
• Humerus
• Radius & ulna
• Femur
• Tibia & fibula
• Bones of hands & legs