Bone Formation (Ossification)

SOURCE FOR INFORMATION: http://www.mc.vanderbilt.edu/histology/labmanual2002/labsection1/boneform&synovialjoints03.htm

Intramembranous Bone Formation:
a. More blood vessels form in bone (increased vascularity of bone)

b. Rapid growth of mesenchymal cells. The mesenchymal cells become osteogenic cells, which develop into osteoblasts.

c. Osteoblasts begin to lay down osteoid. Osteoid is the organic part of bone without the inorganic constituent.

d. Osteoblasts either retreat or become entrapped as osteocytes in the osteoid.

e. The osteoid calcifies to form spicules of spongy bone. The spicules unite to form trabeculae. The inorganic salts carried in by the blood vessels supposedly bring about calcification. The salts are deposited in an orderly fashion as fine crystals (hydroxyapatite crystals) intimately associated with the collagenous fibers. These crystals are only visible with the electron microscope.

f. Bone remodeling occurs. Periosteum and compact bone are formed.


http://apbrwww5.apsu.edu/thompsonj/Anatomy%20&%20Physiology/2010/2010%20Exam%20Reviews/Exam%202%20Review/intramembranous%20fig.6.7a.jpg


2. Intracartilaginous (endochondral) Bone Formation: Involves the replacement of a cartilaginous model by bone and is best observed in long bones, such as the humerus or femur. Events of endochondral ossification can be seen in Figure 21 below and include the following:

a. Primary ossification center. The first change indicative of beginning ossification takes place about the center of the future bone shaft. Here the cartilage cells hypertrophy and the cartilage matrix becomes calcified. Subsequently, part of the calcified matrix disintegrates, opening cavities that communicate with the connective tissue and vessels at the surface.

b. Bone collar. The bone collar forms concurrently with the primary ossification center. Cells of the perichondrium begin to form bone. The bone collar holds together the shaft, which has been weakened by the disintegration of the cartilage. The connective tissue about the bone collar, previously a perichondrium, is now called periosteum.

c. Periosteal buds. These are connective tissue buds or "sprouts" containing mesenchymal cells (which give rise to osteogenic cells) and blood vessels, which grow from the periosteum to reach the primary ossification center. Osteoblasts attach to spicules of calcified cartilage in the primary ossification center and begin to produce osteoid. Thus, bone is formed and the process continues toward both epiphyses while this is occurring, the cartilage outside the primary ossification center increases in size by interstitial and appositional growth.

d. Secondary ossification centers. About the time of birth, a secondary ossification center appears in each end (epiphysis) of long bones. Periosteal buds carry mesenchyme and blood vessels in and the process is similar to that occurring in a primary ossification center. The cartilage between the primary and secondary ossification canters is called the epiphyseal plate, and it continues to form new cartilage, which is replaced by bone, a process that results in an increase in length of the bone. Growth continues until the individual is about 21 years old or until the cartilage in the plate is replaced by bone. The point of union of the primary and secondary ossification centers is called the epiphyseal line.


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SOURCE FOR INFORMATION: http://www.mc.vanderbilt.edu/histology/labmanual2002/labsection1/boneform&synovialjoints03.htm