After birth, a person's bones grow in length and thickness. Interstitial growth is the lengthening of the bone resulting from the growth of cartilage and its replacement with bone tissue. A person grows taller because of interstitial growth. This growth occurs at the epiphyseal plate and continues until the person reaches the teenage years. Most bones stop growing at this point, but some, such as those in the nose and lower jaw, continue to lengthen throughout a person's life.
Interstitial growth is similar to endochondral ossification in that it needs cartilage to be present in order to happen. An increase in length happens through events at four distinct locations:
- At the proliferation zone, cells pile themselves up and divide quickly. This pushes the epiphyses away from the diaphysis.
- At the hypertrophic zone, cartilage cells in the pile swell in size (called hypertrophy), and their lacunae get larger as well. This results in the formation of large spaces.
- At the calcification zone, cartilage is calcified, and the matrix breaks down. Blood vessels enter the spaces. Calcified cartilage forms at the juncture of the diaphysis and the epiphyses.
- At the ossification zone, marrow from the medullary cavity enters the calcified cartilage. Osteoclasts break down the cartilage, which is then covered with new bone by the osteoblasts. This gets replaced by spongy bone.
This type of growth also always involves the reforming of the ends of the epiphyses in order to maintain the proper ratio of size and surface area between the epiphyses and diaphysis.
Growth ends when the epiphyses and diaphysis fuse together, causing the epiphyseal plate to close. This is usually at age 21 for men and age 18 for women.
In order to accommodate for increases in length, bones also need to increase in thickness. This type of growth, called appositional growth, happens when osteoblasts in the periosteum deposit new bone matrix layers onto already-formed layers of the outer surface of bone. At the same time, osteoclasts on the endosteum break bone tissue down. This results in a greater concentration of bone being built than being destroyed, which produces thicker and stronger bones.