Bone Ossification & Growth

Bone

Types of bone:

Woven is a mechanically weak, immature form characterised by a random organization of coarse collagen fibres. It is the first form of bone made during development and in fracture repair.
Lamellar is found in mature, remodelled bone and is mechanically strong. It has of layers of parallel collagen fibres and a highly organized infrastructure.

Lamellar bone may be formed as a solid mass in compact (cortical) bone or as an open, sponge-like network in cancellous (trabecular or spongy) bone. Most bones are organized such that they have a rigid, outer cortical shell of compact bone and an inner cancellous zone, the strength of which is provided by its connecting meshwork of trabeculae (L. trabs- beam). The spaces between trabeculae and the central medullary cavity are filled with bone marrow where haematopoiesis (blood cell formation) occurs (red bone marrow) or adipocytes are found (yellow bone marrow).
Compact bone is organised into lengthwise bony columns (Haversian systems or osteons) in which concentric bony lamellae surround a neurovascular central channel called a Haversian canal. Haversian canals are interconnected by perpendicular Volkmann's canals that also bring the blood supply from the surface of the bone. Osteocytes are trapped within the bone lamellae in spaces called lacunae. Canaliculi are tiny channels interconnecting osteocytes in lacunae and Haversian canals. Bone remodelling occurs during growth and also in the adult in response to demands on calcium homeostasis and changing functional stresses. Osteoclasts bore a tunnel called a resorption canal which carries capillaries and osteoblasts that lay down concentric layers of bone to form a new osteon. Between osteons are interstitial lamellae that are remnants of the previous osteons. At both the external and internal surfaces of compact bone circumferential bone lamellae are formed.
In cancellous bone, the vascular function of Haversian canals is replaced by blood sinusoids in the marrow and bone remodelling occurs at the surfaces of the trabeculae.

 

 

 

Lining Haversian canals and internal surfaces of the marrow cavities is the endosteum, a layer of stem cells and inactive osteoblasts. A fibrocollagenous connective tissue layer, the periosteum surrounds most outer surfaces of bone and contains numerous stem cells and the neurovascular supply of the bone.

1.Ground section of Bone
2. Ground section of Bone

 

ENDOCHONDRAL OSSIFICATION

The process of bone formation from cartilage starts with the appearance of a primary centre of ossification.

Primary centre of ossification.

Note the primary centre of ossification in a potential short bone developing from cartilage in the foot.

3. Low power view showing full length of cartilage with primary ossification centre in the middle.
4. Higher power view showing normal cartilage on the left and the primary ossification centre on the right.
5. Higher power view showing cells in the ossification centre undergoing hypertrophy and degeneration.
A collar of bone has formed at the periphery.
6. Very high power view of the edge of the primary ossification centre showing cartilage hypertrophy and degeneration of chondrocytes and the collar of bone. Note also that blood vessels have invaded the perichondrium turning it into a periosteum.

 

Epiphyseal growth plate

Following the establishment of a secondary centre of ossification in the epiphysis of a developing long bone the articular cartilage becomes separated from the cartilaginous growth plate.

7. Low power view showing epiphysis, metaphysis and diaphysis.
8. Higher power showing bone formed at the secondary ossification site in the middle.
The articular cartilage is on the right and the growth plate (darkly stained) is on the left.
9. This view shows a little epiphyseal bone on the right, the darkly staining growth plate next to it and mineralising cartilage to the left. On the right margin of the growth plate the cartilage is resting. Moving left the cells are first proliferating then maturing and degenerating. Beyond the degeneration the cartilage is becoming mineralised.
10. Higher power of the growth plate (left) adjacent to epiphyseal bone (right). Note blood capillaries in the bone adjacent to the growth plate. The latter shows resting cartilage, proliferation and the columns of cells which are maturing and leading to degeneration on the left.
11. Degenerating columns of cells on the right and mineralising cartilage on the left.
12. Bone deposition on mineralised cartilage.

INTRAMEMBRANOUS OSSIFICATION

The formation of bone from connective tissue.

13. Low power view of developing flat bones of the skull with sutural joints between.
14. A sutural joint in the middle with bone on both sides. Notice osteocytes within the bone and osteoblasts on the surface. The sutures are important sites for growth of these bones.
15. Low power view of the developing human mandible showing well formed bone adjacent to an oval shaped cartilage.
16. Higher power showing that the cartilage is separate from the bone. The latter develops intramembranously in the mandible. Many features which are characteristic of developing bone are visible.


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