Human Anatomy Part 1 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2002

Special Section
Osteology, osteologia [the study of bones] - General Osteology
Development of Bones

The Development of Bone tissue occurs in two ways. Flat bones (such as the BONES OF THE Skull vault) are formed from mesenchyme without a preceding cartilaginous stage (direct osteogenesis). Tubular bones, on the other hand, develop on the site of a cartilaginous model (indirect osteogenesis).

Direct Osteogenesis

Mesenchymal Cells actively proliferate at the site of the future bone, accompanied by numerous Blood Vessels (skeletogenic islet). These cells produce Collagen fibers and ground substance, leading to The formation of osteoid—an unmineralized intercellular matrix. Cells enclosed within the osteoid become osteocytes, while those located on its surface transform into osteoblasts, which continue to produce the intercellular substance. As the osteoblasts become embedded within it, they, in turn, become osteocytes. Thus, developing bones exhibit growth from the periphery, known as appositional growth. Osteoid has the property of attracting calcium salts, the deposition of which results in its mineralization.

The periosteum is formed from the mesenchyme surrounding the bone in its immediate vicinity. Initially, a bone consisting of reticulofibrous (coarse-fibered) bone tissue is formed, which is gradually replaced by lamellar bone. This process is carried out with the active participation of osteoclasts—large multinucleated cells that destroy the mineralized intercellular matrix of bone or Cartilage tissue. When lamellae are arranged concentrically around blood vessels, osteons are formed. The first generations of osteons are also destroyed and replaced by osteons of subsequent generations. Externally, on the side of the periosteum, external general (circumferential) lamellae are formed, surrounding the bone.

Indirect Osteogenesis

As already noted, a cartilaginous model of future tubular bones appears at their site, formed in the same manner as under conditions of direct osteogenesis. During cartilage growth, the cartilage Cells of the diaphysis increase in volume, and calcium salts are deposited in the surrounding intercellular matrix. This leads to impaired trophic supply of the cartilage cells from the perichondrium. The cartilage cells of the diaphysis gradually degenerate, the surrounding intercellular matrix is also partially destroyed, and the cartilage becomes spongy. Blood vessels along with mesenchyme penetrate such cartilage from the perichondrium. The perichondrium thickens, osteoblasts appear in it, and it transforms into the periosteum. Due to the osteoblasts in the middle part of the diaphysis, perichondral (around-cartilage) bone is formed, which hinders the diffusion of nutrients, causing further destruction of the diaphanous cartilage. The perichondral bone, initially located inside the diaphysis, spreads in proximal and distal directions, completely enclosing the diaphysis. Consequently, only the ends of the bone, which represent the epiphyses, remain cartilaginous. The cartilage cells at the border of the epiphysis and diaphysis assemble into columns oriented along the length of the bone. In these columns, chondrocytes proliferate on the epiphyseal side and degenerate on the diaphyseal side. These two opposing processes drive bone growth in length. Therefore, the epiphyseal cartilage with its columns of cartilage cells has been termed the growth plate (epiphyseal line), which persists until the Organism completes its growth. As for bone growth in thickness, it occurs due to the periosteum, whose osteoblastic activity increases the diameter of the diaphysis.

The process of bone tissue formation within the cartilage precursor is called endochondral (from Greek endon – inside) ossification (Fig. 21). Here, bone trabeculae are formed, initially located around the remnants of the destroyed diaphyseal cartilage. Osteoclasts, which arise in the Connective Tissue that has penetrated the cartilage along with blood vessels, destroy the endochondral bone, resulting in the formation of so-called resorption cavities, and ultimately – the Bone Marrow cavity. From the connective tissue and blood vessels that have penetrated here, the bone marrow is formed, where hematopoietic stem cells settle and hematopoiesis begins.

At the end of embryonic development, ossification centers also appear in the cartilaginous epiphyses of the Femur and Tibia. The dynamics of their formation is generally the same as the appearance of ossification points in the diaphysis. However, during the Ossification of the epiphyses, cartilage tissue persists on their surface.

As in direct osteogenesis, indirect osteogenesis initially produces reticulofibrous bone tissue, which is then replaced by lamellar bone. Bone lamellae surrounding longitudinally oriented vessels form primary osteons. During further bone development, they are completely or partially destroyed and replaced by secondary osteons. Interstitial lamellae located between osteons in the compact bone substance are fragments of earlier-formed osteons. Following the appearance of primary osteons, external circumferential lamellae are formed through the action of the periosteum.