General Logic and Stages of the Process
Bone development via endochondral ossification is characterized by a sequential change in tissue types. The process proceeds in three main stages:
- Formation of the cartilage model. The future bone is initially laid down as a hyaline cartilage model. In this zone, cells (chondrocytes) are oval-shaped, lie in lacunae in isogenous groups, and the basophilic matrix contains a high amount of glycosaminoglycans.
- Replacement. The cartilage tissue undergoes degradation, and woven bone tissue forms in its place. This forms the primary spongy bone.
- Remodeling. Woven tissue is gradually replaced by mature lamellar bone. Depending on the location within the bone, either secondary spongy bone or compact bone tissue is formed.
Perichondral Ossification of the Diaphysis
The replacement of cartilage with bone (using the development of an embryonic long bone as an example) does not happen all at once. It starts with perichondral ossification—the formation of bone tissue outside the cartilage in the diaphyseal region.
This process begins with the transformation of the sheath: the perichondrium turns into the periosteum, where active osteoblasts appear. These cells synthesize woven bone tissue along blood vessels.
A bone collar—a peculiar cylinder enclosing the cartilage—forms around the diaphysis. The collar actively grows: it thickens in width and extends deeply toward the epiphyses. The main consequence of this process is the disruption of the diffuse nutrition of the underlying hyaline cartilage, which triggers the next phase of ossification.
Endochondral Ossification and Vascular Invasion
Due to the nutritional blockade beneath the bone collar, degenerative changes begin. Chondrocytes swell and transform into hypertrophic (vesicular) cells, and the extracellular matrix becomes mineralized (calcified), acquiring a grayish-purple hue in histological preparations.
Degenerating chondrocytes cease producing anti-angiogenic factors and begin signaling hypoxia. This triggers vascular invasion:
- Blood vessels from the periosteum actively grow into the modified cartilage zone.
- Osteogenic cells migrate along with them.
- Osteoclasts (acting here as chondroclasts) destroy the mineralized cartilage matrix, freeing up space and forming primary cavities (the future medullary cavity).
On the remnants of the destroyed cartilage, osteoblasts form bone trabeculae of woven tissue. The trabeculae themselves are avascular (contain no blood vessels) at this stage.
Zonation and Postembryonic Bone Growth
As ossification progresses toward the epiphyses, the metaphysis forms. On a histological section (from epiphysis to diaphysis), distinct zones are visible: the zone of resting (reserve) hyaline cartilage, the zone of proliferation (columnar cartilage), the zone of hypertrophy (hypertrophic/vesicular cartilage), and the zone of endochondral ossification.
Ossification of the epiphysis itself also occurs endochondrally, while the articular surface remains unossified and covered by cartilage. Postembryonic bone growth continues up to 20 years or more:
- Growth in thickness (width) occurs via the periosteum through appositional growth—laying down new layers of bone externally.
- Growth in length is provided by the epiphyseal plate (growth plate). Bone elongation occurs due to the active division of chondroblasts in the zone of columnar cartilage, while on the diaphyseal side, the cartilage is continuously destroyed and replaced by bone.