The development of bone is called osteogenesis. It is based on the mesenchyme, which is poorly differentiated connective tissue. It begins at the end of the second month of the intrauterine period, when osteoblasts start to develop from the mesenchymal cells. The bones of the calvaria (the dome-like upper part of skull) and those of the face develop around connective tissue through intramembranous ossification, while others develops in cartilage, either perichondrally (from fibrous connective tissue) or endochondrally (occurring within cartilage). When the development of bone begins, all the other types of tissues are already present in the body of the embryo.
The bones that originate from connective tissue are known as primary bones. They go through two developmental stages: membranous and bony. Bones developing in cartilage, on the other hand, are called secondary bones, and they pass through three stages: connective tissue, cartilaginous, and bony.
In intramembranous ossification, islets of ossification appear at the site of future bones as condensation of mesenchymal cells, which take part in the formation of fibrous fibers, and a great number of blood vessels. When osteoblasts differentiate from the mesenchymal cells, they produce intercellular substance, which consists of calcium salts and ossein, which is the collagen component of bone. The fibrous fibers are impregnated with this substance, with the osteoblasts being embedded in them. Then the osteoblasts develops into the mature cells of bony tissue, which are called the osteocytes.
Perichondrial ossification takes place in a similar cells at the expense of the cells of the perichondrium (irregular connective tissue). Endochondral ossification takes place through growth of blood vessels from the surrounding mesenchyme into the cartilaginous germs of bones. Paradoxically, the process of osteogenesis continuous with the formation of osteoblasts (bone destroyers), which will have a key role in bone resorption later in life, when the infant becomes an adult.
Cartilaginous tissue, with a great number of ossification nuclei called primary, prevails after birth in the skeleton of the newly born. Secondary ossification nuclei will emerge later. The primary, like the secondary nuclei, appear earlier in girls than in boys. In tubular bones, the ossification nuclei first arise in the central parts of the diaphysis, and then in the epiphysis.
