ENDOCHONDRAL OSSIFICATION: HISTOLOGY AND PROCESS - ANATOMY AND PHYSIOLOGY - 2025

The endochondral ossification and intramembranous ossification are two mechanisms of bone formation during embryonic development. Both mechanisms give rise to histologically identical bone tissue.

Endochondral ossification requires a cartilage mold and is the ossification mechanism for most of the long and short bones in the body. This process of bone formation occurs in two stages: 1) a miniature model of hyaline cartilage is formed; 2) cartilage continues to grow and serves as a structural skeleton for bone formation. Cartilage is reabsorbed as it is replaced by bone.

Graphic representation of the structure of hyaline cartilage (Source: Kassidy Veasaw via Wikimedia Commons)

It is called endochondral because ossification occurs from the inside out, to differentiate it from perichondral ossification that occurs outside (from the perichondrium) inward.

Ossification means bone formation. This bone formation is produced by the action of osteoblasts that synthesize and secrete the bone matrix, which is then mineralized.

Ossification begins at a site in the cartilage that is called the ossification center or bone nucleus. There may be several of these centers that rapidly fuse to form a primary ossification center from which the bone will develop.

Histology

In the fetus, in the region where bone is to be formed, a model of hyaline cartilage develops. Hyaline cartilage is formed by the differentiation of mesenchymal cells. It contains type II collagen and is the most abundant in the body. From this cartilage, ossification occurs.

Cartilage formation

In the regions where the cartilage is to be formed, the mesenchymal cells are grouped and modified, losing their extensions and becoming rounded. This is how chondrification centers are formed. These cells transform into chondroblasts, secrete matrix and become trapped, forming the so-called "gaps".

The matrix-surrounded chondroblasts that form the gaps are called chondrocytes. These cells divide and, as they secrete matrix, they separate, forming new gaps and as a consequence, generating cartilage growth.

This type of growth occurs from the inside out and is called interstitial growth. The mesenchymal cells that surround the cartilage differentiate into fibroblasts and go on to form the perichondrium that surrounds the cartilaginous skeleton.

Bone formation

Initially, the cartilage grows, but then the chondrocytes in the center hypertrophy, accumulate glycogen, and form vacuoles. This phenomenon reduces the matrix partitions, which in turn calcify.

This is how the process of bone formation begins from a primary ossification center that, through a sequential process, replaces the cartilage that is reabsorbed and bone is formed.

The secondary centers of ossification form at the ends of the bony epiphyses by a mechanism similar to that of the ossification of the diaphyses, but they do not form the bony collar.

In this case, the osteoprogenitor cells that invade the cartilage of the epiphysis transform into osteoblasts and begin to secrete the matrix, which eventually ends up replacing the cartilage of the epiphysis with bone.

Endocrinal ossification process

- Main processes

Endochondral ossification is accomplished through seven processes that are described below.

Formation of hyaline cartilage

A model of hyaline cartilage covered with a perichondrium is formed. This occurs in the embryo, in the region where the bone will later develop. Some chondrocytes hypertrophy and then die, and the cartilage matrix calcifies.

The primary center of ossification is formed

The middle membrane of the diaphysis is vascularized in the perichondrium. In this process, the perichondrium becomes the periosteum and the chondrogenic cells become osteoprogenitor cells.

Formation of a bone collar

The newly formed osteoblasts synthesize matrix and form a bony collar just below the periosteum. This collar prevents the diffusion of nutrients towards the chondrocytes.

Formation of medullary cavities

Chondrocytes within the center of the diaphysis that had hypertrophied, not receiving nutrients, die and degenerate. This leaves confluent empty vacuoles in the center of the diaphyses that then form the medullary cavities of the bone.

The osteogenic bud and the onset of calcification

Osteoclasts begin to form “holes” in the subperiosteal bone collar through which the so-called osteogenic bud enters. The latter is made up of osteoprogenitor cells, hematopoietic cells, and blood vessels. This starts calcification and bone production.

Formation of a complex consisting of cartilage and calcified bone

Histologically, calcified cartilage stains blue (basophilic) and calcified bone stains red (acidophilus). Osteoprogenitor cells give rise to osteoblasts.

Bone growth process (Source: derivative work: Chaldor (talk) Illu_bone_growth.jpg: Fuelbottle via Wikimedia Commons)

These osteoblasts make the bone matrix that is deposited in the calcified cartilage, then that newly formed matrix is ​​calcified and at that time the complex of calcified cartilage and bone is produced.

Resorption process

Osteoclasts begin to resorb the calcified cartilage and bone complex as the subperiosteal bone thickens, growing in all directions within the diaphysis. This resorption process increases the size of the medullary canal.

The thickening of the subperiosteal bone collar grows towards the epiphyses and, little by little, the cartilage of the diaphyses is completely replaced by bone, leaving cartilage only in the epiphyses.

- Secondary ossification centers

1- Here the ossification of the epiphyses begins. This occurs in the same way that occurs in the primary center of ossification, but without forming the subperiosteal bony ring. The osteoblasts deposit matrix on the calcified cartilage.

2- The bone grows in the epiphyseal plate. The articular surface of the bone remains cartilaginous. Bone grows at the epiphyseal end of the plate and bone is added at the diaphyseal end of the plate. The cartilaginous epiphyseal plate remains.

3- When bone growth ends, the cartilage of the epiphyseal plate no longer proliferates. Growth continues until the epiphysis and diaphysis unite with consolidated bone, replacing the cartilage of the epiphysis with bone.

This growth process lasts several years before it is complete, and in the process the bone is constantly being remodeled.

References

1. Eroschenko, VP, & Di Fiore, MS (2013). DiFiore's atlas of histology with functional correlations. Lippincott Williams & Wilkins.
2. Gartner, LP, & Hiatt, JL (2010). Concise Histology E-Book. Elsevier Health Sciences.
3. Hiatt, JL (2006). Color atlas of histology. Lippincott Williams & Wilkins.
4. Mackie, E., Ahmed, YA, Tatarczuch, L., Chen, KS, & Mirams, M. (2008). Endochondral ossification: how cartilage is converted into bone in the developing skeleton. The international journal of biochemistry & cell biology, 40 (1), 46-62.
5. Nathalie Ortega, Danielle J. Behonick, and Zena Werb. (2004) Matrix remodeling during endochondral ossification. Trends Cell Biol.; 14 (2): 86–93.