Bone Formation by Endochondral Ossification

A cartilaginous model is a hyaline cartilage template surrounded by perichondrium that forms the general shape of bones during embryonic development. This model provides the structural framework that guides bone formation and is gradually replaced by bone tissue through endochondral ossification, beginning around the sixth to seventh week of gestation.

The bone collar forms when the perichondrium develops into a periosteum after peripheral blood vessels infiltrate it. Mesenchymal cells then differentiate into osteoblasts, which secrete bone matrix around the diaphysis of the cartilaginous model. This bone collar stabilizes the cartilage model while healthy cartilage continues to grow at the epiphyses.

Chondrocytes die because cartilage matrix calcification prevents nutrient diffusion to the cells. As chondrocytes at the center of the diaphysis enlarge and direct calcification of the surrounding matrix, they become isolated from nutrient sources. This results in cell death and disintegration of the surrounding cartilage, creating cavities that are later invaded by osteogenic cells.

The primary ossification center forms when osteogenic and hematopoietic cells infiltrate cavities left by dead chondrocytes, along with nutrient vessels and nerves. Osteogenic cells differentiate into osteoblasts and deposit new bone matrix in these cavities. By the twelfth week of gestation, most of the diaphysis is ossified through this process.

As bone grows outward from the primary ossification center, hematopoietic cells give rise to osteoclasts. These osteoclasts degrade the central bone matrix, creating the medullary cavity that houses red marrow. This process occurs simultaneously with the formation of new bone matrix by osteoblasts at the ossification center.

Secondary ossification sites begin forming after birth in the epiphyses, the ends of long bones. Ossification of cartilage in the epiphyses and diaphysis continues until adolescence, when bones stop growing. This delayed ossification of the epiphyses allows bones to continue increasing in length after birth.

Endochondral ossification replaces a cartilaginous template with bone tissue, beginning with a bone collar formation and primary ossification center in the diaphysis. Unlike bone formation by intramembranous ossification, which forms bone directly from mesenchymal tissue without a cartilage intermediate, endochondral ossification uses cartilage as a temporary scaffold that is gradually replaced by bone.