endochondral ossification

Endochondral ossification is one of two types of bone formation and is the process responsible for much of the bone growth in vertebrate skeletons, especially in long bones. As the name might suggest (endo - within, chondro - root for cartilage), endochondral ossification occurs by replacement of hyaline cartilage.

Primary centre of ossification

The first site of ossification occurs in the primary centre of ossification, which is in the middle of diaphysis (shaft). The following steps then occur:

Formation of periosteum:Once vascularized, the perichondrium becomes the periosteum. The periosteum contains a layer of undifferentiated cells which later become osteoblasts.
Formation of bone collar: The osteoblast secretes osteoid against the shaft of the cartilage model. This serves as support for the new bone.
Calcification of matrix: Chondrocytes in the primary centre of ossification begin to grow (hypertrophy). They stop secreting collagen and other proteoglycans and begin secreting alkaline phosphatase, an enzyme essential for mineral deposition. Nutrients can no longer diffuse if the matrix becomes sufficiently calcified and the chondrocytes subsequently die. This creates cavities within the bone.
Invasion of periosteal bud: A periosteal bud, which consist of blood vessels, lymph vessels and nerves, invades the cavity left by the chondrocytes. The vascularization utlimately carries hemopoietic cells, osteoblasts and osteoclasts inside the cavity. The hemopoietic cells will later form the bone marrow.
Formation of trabeculae: Osteoblasts use the calcified matrix as a scaffold and begin to secrete osteoid, which forms the bone trabecula. Osteoclasts break down spongy bone to form the medullary (bone marrow) cavity.

Secondary centre of ossification

Cartilage is retained in the the epiphyseal plate, located between the diaphysis (shaft) and the epiphysis (end) of the bone. These areas of cartilage are known as secondary centres of ossification. Cartilage cells undergo the same transformation as above. As growth progresses, the proliferation of cartilage cells in the epiphyseal plate slows and eventually stops. The continuous replacement of cartilage by bone results in the obliteration of the epiphyseal plate, termed the closure of the epiphysis. Only articular cartilage remains.

Appositional bone growth

The growth in diameter of bones around the diaphysis occurs by deposition of bone beneath the periosteum. Osteoclasts in the interior cavity continue to degrade bone until its ultimate thickness is achieved, at which point the rate of formation on the outside and degradation from the inside is constant.

Histology

During endochondrol ossification, four distinct zones can be seen at the light-microscope level.

Zone of resting cartilage. This zone contains normal, resting hyaline cartilage.
Zone of proliferation. In this zone, chondrocytes undergo rapid mitosis, forming dinstinctive looking stacks.
Zone of maturation / hypertrophy. It is during this zone that the chondrocytes undergo hypertrophy (become enlarged). Chondrocytes contain large amounts of glycogen and begin to secrete alkaline phosphatase.
Zone of calcification. In this zone, chondrocytes are either dying or dead, leaving cavities that will later become invaded by bone-forming cells.