Osteoblasts

Ossification involves the formation of bone by the activity of osteoblasts and osteoclasts, the addition of minerals and salts. Calcium compounds must be present for ossification to take place, Osteoblasts do not make these minerals, but must take them from the blood and deposit them in the bone. By the time we are born, many of our bones have been at least partly ossified.
Endochrondral ossification is very important in foetal development (second month) it is the process, in which most hyaline cartilage is replaced by bone, cartilage is the main component of which the foetus is made up and is said to be used as “moulds” for the developing bones to take shape. During endochrondral ossification, chondrocytes increase greatly in size - undergo hypertrophy then cease and disintegrate, the extracellular matrix that construct is overtaken with the growth of blood vessels, osteoclasts, bone marrow cells and osteoblasts all around the site. The blood vessels grow around either end of the bone primarily intertwining with the cartilage to get a strong blood supply to the bone for extra growth/strength. The perichondrial cells convert to osteoblasts
In long bones the growth and elongation continue from birth through adolescence. Elongation is achieved by the activity of two cartilage plates, called epiphyseal plates, located between the diaphysis and the epiphyses of the bones. These plates expand, forming new cells, and increasing the length of the shaft. In this manner, the length of the shaft increases at both ends. In humans, different bones stop lengthening at different ages, but ossification is fully complete between the ages of 18-30. In contrast to the lengthening of bones the thickness and strength of bone must continually be maintained by the body. Old bone must be replaced by new bone all the time. This is accomplished as bone is continually deposited by osteoblasts, while at the same time; it is continually being broken down and digested by the body by osteoclasts. Factors affecting bone formation are Nutrition, exposure to sunlight, hormonal secretions and physical exercise.