1. Dr. Mohammed Hussein M.B.Ch.B, MSC, PhD, DCH (UK), MRCPCH
Bone
Part 2
Dr. Mohammed Hussein
M.B.Ch.B, MSC, PhD, DCH (UK), MRCPCH
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2. Histogenesis of Bone (Osteogenesis)
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3. Histogenesis of Bone Intramembranous Endochondral
Bone formation during embryonic development may be of two types:
Intramembranous
Endochondral
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4. Intramembranous Bone Formation

5. Intramembranous Bone Formation
Most flat bones are formed by intramembranous bone formation.
Mesenchymal cells differentiate into
Osteoblasts
Osteoblasts secrete bone matrix, forming a network of spicules and trabeculae whose surfaces are populated by these cells.
This region of initial osteogenesis is known as the primary ossification center.
Calcification quickly follows osteoid formation, and osteoblasts trapped in their matrices become osteocytes.
The processes of these osteocytes are also surrounded by forming bone, establishing a system of canaliculi.
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6. Endochondral Bone Formation
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7. Endochondral Bone Formation
Most long bones are formed by endochondral ossification, which involving:
Hyaline cartilage serving as a cartilage model
Cartilage proliferation and calcification
Gradual replacement by bone
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9. Bone Growth in Length
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10. Bone Growth in Length
The chondrocytes of the epiphyseal plate proliferate and participate in the process of endochondral bone formation.
Proliferation occurs at the epiphyseal aspect, and replacement by bone takes place at the diaphyseal side of the plate.
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11. Zone of Reserve Cartilage Zone of Proliferation
Histologically, the epiphyseal plate is divided into five Recognizable Zones, beginning at the epiphyseal side, are as follows:
Zone of Reserve Cartilage
Zone of Proliferation
Zone of Maturation and Hypertrophy
Zone of Calcification
Zone of Ossification
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12. Chondrocytes mature, hypertrophy.
Chondrocytes randomly distributed throughout the matrix are mitotically active.
Reserve Zone
Chondrocytes, rapidly proliferating, form rows of isogenous cells that parallel the direction of bone growth.
Proliferation Zone
Chondrocytes mature, hypertrophy.
Hypertrophy Zone
Hypertrophied chondrocytes die, and cartilage matrix becomes calcified.
Calcification Zone
Osteoprogenitor cells invade the area and differentiate into osteoblasts, which elaborate matrix that becomes calcified on the surface of calcified cartilage. This is followed by resorption of the calcified cartilage.
Ossification Zone
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13. Reserve Zone Proliferation Zone Hypertrophy Zone Calcification Zone
Ossification Zone
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14. At about the 20th year of age:
The rate of mitosis decreases in the zone of proliferation and
The zone of ossification overtakes the zones of proliferation and cartilage reserve.
The cartilage of the epiphyseal plate is replaced by a plate of calcified cartilage/calcified bone complex
Once the epiphyseal plate is resorbed, growth in length is no longer possible.
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15. Bone Repair
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16. Repair of a fractured bone occurs through several stages:
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17. Bone Repair
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18. Clinical Correlations
If segments of bone are lost or damaged so severely that they have to be removed, a “bony union” is not possible; that is, the process of bone repair cannot occur because a bony callus does not form. In cases of this sort, a bone graft is required. Bone grafts can be:
Autografts
Homografts
Heterografts
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19. Maintenance of Blood Calcium Levels
Calcium is vital for:
The activity of many enzymes and
Membrane permeability,
Cell adhesion,
Blood coagulation,
Nerve impulse transmission,
Muscle contraction.
To fulfill all of the necessary functional requirements for which Ca is responsible, a tightly controlled blood level of mg/dL must be maintained.
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22. Rickets
Rickets is a disease in infants and children who are deficient in vitamin D
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24. Osteomalacia
Osteomalacia, or adult rickets, results from prolonged deficiency of vitamin D.
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26. Osteoporosis
Osteoporosis is related to decreasing bone mass, which becomes more serious after menopause when estrogen secretion drops appreciably
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28. Scurvy
Is a condition resulting from a deficiency of vitamin C. One effect is deficient collagen production, causing a reduction in the formation of bone matrix and bone development
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