10.12:
Bone Formation by Endochondral Ossification
In the embryonic skeleton, the hyaline cartilage surrounded by perichondrium forms the general shape of bones, called the cartilaginous model.
Most bones, including long bones, replace this cartilage with bone tissue during development through endochondral ossification.
It begins with the perichondrium developing into a periosteum and forming a collar of bone matrix around the diaphysis of the model.
Simultaneously, the chondrocytes at the center become enlarged and direct the calcification of the surrounding matrix.
Because calcification prevents nutrient diffusion, the chondrocytes die, leaving behind cavities.
Next, osteogenic and hematopoietic cells infiltrate these cavities along with the nutrient vessels and nerves.
The osteogenic cells differentiate into osteoblasts, which deposit new bone matrix in these cavities and form the primary ossification center.
As the bone grows outward from the ossification center, the hematopoietic cells give rise to osteoclasts. These osteoclasts degrade the central matrix and form the medullary cavity.
By the twelfth week of gestation, most of the diaphysis is ossified. But, the ossification of epiphyses begins only after birth.
10.12:
Bone Formation by Endochondral Ossification
Bone formation, or ossification, begins around the sixth to seventh week of embryonic development. Most bones develop from a cartilaginous template through the process of endochondral ossification. Cartilage formation begins when clusters of mesenchymal cells differentiate into chondrocytes. These chondrocytes proliferate rapidly and secrete an extracellular matrix that becomes encased in a membrane called the perichondrium. The resulting cartilage model provides a template that resembles the general shape of the future bone.
When the peripheral blood vessels infiltrate the perichondrium, it develops into the periosteum. The mesenchymal cells then differentiate into osteoblasts and secrete bone matrix around the diaphysis, forming the bone collar. As the bone collar forms, the chondrocytes in the center of the diaphysis grow in size. This hypertrophy promotes cartilage matrix calcification, preventing nutrients from diffusing to the chondrocytes. This results in cell death and the disintegration of the surrounding cartilage, forming cavities. The bone collar stabilizes the cartilage model while healthy cartilage continues to grow at the epiphyses, contributing to growth in the size of the bone.
Simultaneously, the periosteal bud containing the nutrient artery and vein, nerve fibers, hematopoietic cells, and osteogenic cells invade the cavities in the diaphysis. The osteogenic cells differentiate into osteoblasts and secrete new bone matrix at this primary ossification center. Osteoclasts, derived from the hematopoietic cells, degrade the central matrix to form the medullary cavity that houses red marrow.
This process of cartilage formation and replacement by bone matrix continues as the bone grows in size. The epiphyses begin ossification from secondary ossification sites, which form after birth. Ossification of cartilage in the epiphyses and diaphysis continues until adolescence, when the bones stop growing.