This Issue's Rounds Question -
Please tell us:
What is topographical anticipation?

(responses to be published in next issue)

In the last issue of Body Cast, Rounds asked:
How is the repair of a fractured adult bone similar to endochondral bone formation?

The following responses were received:

From Livain Arseneau:

Endochondral ossification is the process responsible for bone growth in vertebrate skeletons, especially in long bones. Endochondral ossification occurs by replacement of hyaline cartilage.

Successful fracture healing typically involves the production of a cartilaginous callus, which is eventually remodelled into new bone. The blood vessels in the advancing front of endochondral ossification are likely to play an important role in the replacement of cartilage within the callus.

From Lhea Burk:

Bones grow in length at the epiphyseal plate by a process that is similar to endochondral ossification. The cartilage in the region of the epiphyseal plate next to the epiphysis continues to grow by mitosis. The chondrocytes next to the diaphysis age and degenerate. Osteoblasts move in and ossify the matrix to form bone. This process continues throughout childhood and adolescent years until the cartilage growth slows and stops. When cartilage growth ceases, usually in the early 20s, the epiphyseal plate completely ossifies so that only a thin line remains and the bones can no longer grow in length.

From MaryAnne Lash:

When an adult fractures a bone, the repair or healing is similar to endochondral bone formation. The similarity starts with the process of endochondral ossification, which is

needed for skeletal development and bone repair. In development, the cells differentiate into chondrocytes that produce a cartilage matrix. This cartilage matrix produces osteoblasts. The osteoblasts use calcified matrix as a pattern for the body to build bone. This pattern is used in bone healing should a fracture occur. So, endochondral ossification is how the body initially produces bone and later uses the pattern to repair itself.

From Cheryl Rivers:

The similarity between the two is without endochondral ossification there would be no bone formation. Endochondral ossification is one of two types of bone formation and is the process responsible for much of the growth in vertebrate skeletons, especially in long bones. Endochondral ossification occurs by replacement of hyaline cartilage. - Wikipedia Encyclopedia

From Heather Wong: Endochondral bone formation

Cause a proliferation and differentiating of stem cells (mesenchymal cells) into chondroblasts that form cartilage and osteoblasts. The production and maturation of cartilage and bone matrix. The differentiation of circulating osteoclasts are precursor cells into osteoclasts (remodelling cells).

Fracture healing

1. Reactive phase - fracture of bone and inflammation

• granulation tissue forms

2. Reparative phase - callus forms

• bone deposit

3. Remodelling - remodel to original bone contour.

From Neuville Yao:

During the reparative phase of fracture repair, there's callus formation and then there is lamellar bone deposition. This phase involves the replacement of the hyaline cartilage and woven bone with "lamellar bone". The replacement process is known as "endochondral ossification".

With respect to the hyaline cartilage and "bony substitution" with respect to the woven bone.

Substitution of the woven bone with lamellar bone precedes the substitution of the hyaline cartilage with lamellar bone. The lamellar bone begins forming soon after the collagen matrix of either tissue becomes mineralized. At this point, "vascular channels" with many accompanying osteoblasts penetrate the mineralized matrix. The osteoblasts form new lamellar bone upon the recently exposed surface of the mineralized matrix. This new lamellar bone is in the form of "trabecular bone". Eventually, all of the woven bone and cartilage of the original fracture callus is replaced by trabecular bone, restoring much, if not all, of the bone's original strength.

From Thomas York:

In a fracture bone, osteoblasts, osteocytes and osteoclasts are the three cell types involved in the development, growth and remodelling of bones.

Osteoblasts are bone-forming cells. Osteocytes are mature bone cells. Osteclasts break down and re-absorb bone.

Hyaline cartilage - somewhat elastic, semitransparent cartilage, characterized by type 2 collagen and proteoglycan aggrecans. Seen in movable joint surfaces and the physis (growth plate).

Endochondral ossification - involves the replacement of hyaline cartilage with bony tissue. Bones are formed as hyaline cartilage models. During the third month after conception, the perichondrium that surrounds the hyaline cartilage models becomes infiltrated with blood vessels and osteoblasts and changes into a periosteum. The osteoblasts form a collar of compact bone around the diaphysis. The cartilage in the centre of the diaphysis begins to disintegrate. Osteoblasts penetrate the disintegrating cartilage and replace it with spongy bone. This forms a primary ossification centre. Ossification continues from this centre toward the ends of the bone. After spongy bone is formed in the diaphysis, osteoclasts break down the newly formed bone to open the medullary canal.

The cartilage in the epiphysis continues to grow so the developing bone increases in length. Ossification in the epiphysis is similar to that in the diaphysis except that the spongy bone is retained instead of being broken down to form the medullary cavity.

Responses were also received from: B. Arseneault, J. Carragher, E. Christiansen, A. Crossman, R. Grenier, C. Griffith, S. Groulx, L. Head, B. Lavallee, N. Lockyer, C. Longphee, D. Longphee, L. Lough, L. MacDonald, B. Matheson, J. Maulucci, G. Marshall, E. Oborowsky, R. Passmore, J. Pike, J. Punwassie, B. Sheppard, A. Tarambikos and A. Wentzell.