Endochondral ossification

Section of fetal bone of cat. ir. Irruption of the subperiosteal tissue. p. Fibrous layer of the periosteum. o. Layer of osteoblasts. im. Subperiosteal bony deposit. (From Quain’s “Anatomy,” E. A. Schäfer.)

Endochondral ossification is one of the two processes during fetal development of the mammalian skeletal system resulting in the creation of bone tissue. It is also an essential process during the rudimentary formation of long bones, the growth of the length of long bones, and the healing of bone fractures that are not too rigidly immobilized. Unlike intramembranous ossification, that is the other process, cartilage is present during endochondral ossification. When present in fracture healing this cartilage bridging and ensheatning the fracture ends is called callus.

Growth of the cartilage model

The cartilage model would grow in length by continuous cell division of chondrocytes, which is accompanied by further secretion of extracellular matrix. This is called interstitial growth. The process of appositional growth occurs when the cartilage model would also grow in thickness which is due to the addition of more extracellular matrix on the periphery cartilage surface, which is accompanied by new chondroblasts that develop from the perichondrium.

Primary center of ossification

The first site of ossification occurs in the primary center of ossification, which is in the middle of diaphysis (shaft). Then:

• Formation of periosteum: Once vascularized, the perichondrium becomes the periosteum. The periosteum contains a layer of undifferentiated cells (osteoprogenitor cells) which later become osteoblasts.
• Formation of bone collar: The osteoblasts secrete osteoid against the shaft of the cartilage model (Appositional Growth). This serves as support for the new bone.
• Calcification of matrix: Chondrocytes in the primary center of ossification begin to grow (hypertrophy). They stop secreting collagen and other proteoglycans and begin secreting alkaline phosphatase, an enzyme essential for mineral deposition. Then calcification of the matrix occurs and apoptosis of the hypertrophic chondrocytes occurs. This creates cavities within the bone. The exact mechanism of chondrocyte hypertrophy and apoptosis is currently unknown.
• Invasion of periosteal bud: The hypertrophic chondrocytes (before apoptosis) secrete Vascular Endothelial Cell Growth Factor that induces the sprouting of blood vessels from the perichondrium. Blood vessels forming the periosteal bud invade the cavity left by the chondrocytes and branch in opposite directions along the length of the shaft. The blood vessels carry hemopoietic cells, osteoprogenitor cells and other cells inside the cavity. The hemopoietic cells will later form the bone marrow.
• Formation of trabeculae: Osteoblasts, differentiated from the osteoprogenitor cells that entered the cavity via the periosteal bud, use the calcified matrix as a scaffold and begin to secrete osteoid, which forms the bone trabecula. Osteoclasts, formed from macrophages, break down spongy bone to form the medullary (bone marrow) cavity.

Secondary center of ossification

Cartilage is retained in the epiphyseal plate, located between the diaphysis (the shaft) and the epiphysis (end) of the bone. These areas of cartilage are known as secondary centers 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. Mineralisation of articular cartilage and its replacement by bone continues in the adult, though at a much reduced rate than in growing animals.

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

Part of a longitudinal section of the developing femur of a rabbit. a. Flattened cartilage cells. b. Enlarged cartilage cells. c, d. Newly formed bone. e. Osteoblasts. f. Giant cells or osteoclasts. g, h. Shrunken cartilage cells. (From “Atlas of Histology,” Klein and Noble Smith.)

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

1. Zone of resting cartilage. This zone contains normal, resting hyaline cartilage.
2. Zone of proliferation. In this zone, chondrocytes undergo rapid mitosis, forming distinctive looking stacks.
3. 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.
4. Zone of calcification. In this zone, chondrocytes are either dying or dead, leaving cavities that will later become invaded by bone-forming cells. Chondrocytes here die when they can no longer receive nutrients or eliminate wastes via diffusion. This is because the calcified matrix is much less hydrated than hyaline cartilage.

See also

• Intramembranous ossification
• Ossification