What is Interstitial and Appositional Growth?

Growth in bones occurs in two ways either longitudinally or increase in the width of bone size. This increase in the length size of the bone is referred as interstitial growth whereas the increase in the preexisting diameter or width size of bones is referred as appositional growth. This growth can be commonly observed after birth of an individual but generally in the first trimester that is around fourth or fifth week the development of bones and stops growing at the age of 21 for males and comparatively in female at lesser age of 18.

How does Interstitial Growth Occur?

This growth occurs within the cavity present in the bone called lacunae where chondrocytes are present. There will be a repeated mitotic cell division and cell maturationoccurs in two zones, that is proliferate and maturation zones respectively as a result of which the thin and flexible tissue namely cartilage will develop and are replaced with the bone tissue and ultimately the longitudinal growth of bone occurs.

To explain in detail  longitudinal growth occurs in the epiphyseal plate that is our bone is divided into three region they are epiphysis, metaphysis and diaphysis so epiphyseal plate is a hyaline cartilage that is found in the metaphysis region on the long bone end epiphysis region of the bone consist of many hyaline cartilage cells, these cells are very active and they undergo mitotic division as a result of which they form a hyaline cartilage bed like cell structure and as the cartilage grows the entire bone also grows if the cell stops then bone cannot elongate. On the other side the cells in the diaphysis region of the epiphyseal plate which is also known as growth plate calcium salt is deposited and the hardening of bone occurs as a result of which the cells  disband they are replaced with bones

Zones Involved in the Epiphyseal Plate for Interstitial Growth

These zones will give us stepwise information of growth of bone ,the first zone is the reserve zone where it contains chondrocyte’s that does not show active participation in the growth of bone and found to be present inside the matrix, the second one is the proliferation zone in this region the cells are very active and undergo mitotic division of cell and helps to replace the cells that die in the diaphysis region the next zone is the hypertrophy region which contains chondrocyte which is large in size comparatively to the previous zone and enlarges its lacunae  and these cells are old cells and these together contribute the growth of the bone that is cell division and maturation will increase the bone length which is known as interstitial growth ,finally in the calcified zone death of the cell takes place due to calcification and limiting nutrient diffusion ultimately the osteoblast from the diaphysis will secrete the bone tissue on the calcified cartilage by this means the calcified zone gets linked to the diaphysis via growth plate and attaches the osseous tissue to the diaphysis and results in lengthening of bones

Note: Hormones secreted by the pituitary gland which is located below the brain secrete a growth hormone which plays a significant role in chondrocyte cells growth and contribute to the longitudinal growth of bones

How does Appositional Growth Occur?

Increase in the preexisting diameter of bone is achieved by the deposition of bone tissue on two surfaces of bone namely the endosteal and the periosteal, endosteal is the inner surface of bone which encloses cavity present in it and the periosteal is a thin layer of connective tissues that covers the outer surface of bones as a result of which new layer is formed on the bone and increase its diameter. Appositional growth continues to happen even if interstitial growth stops and this thickening of bones helps to balance the weight of an individual.

There are two types of specialized cell present in this growth process they are osteoblast and osteoclast, osteoclast is a multinucleated cell that functions to resorb the bone in medullary cavity whereas osteoblast is the cells that involve in the new bone formation thus new bone is deposited in the periosteum and  removal of old bones in medullary cavity will increase in the size of diaphysis and medullary cavity and ossification occurs this process is very helpful during the process of injury ,fracture exercise and other activities when the bone gets damaged Note: Osteoblast cells arise from bone marrow and osteoclast are the cells that arise from fusion of many cells derived from circulating monocytes in blood

Some Common Disease in Bones

Osteomalacia- some bones become very soft in nature due to deficiency in vitamin D and this condition is referred as osteomalacia that causes fracture in adults

Osteoporosis it is a condition in which new bone is not formed that is generally old bone will be replaced by new bones which does not happen as a result fracture of bone occur even to a small injury and results in fracture

Osteogenesis imperfecta it is a genetic disorder that causes the bone to become brittle in nature commonly called as brittle bone disease

Content and Applications  

• This topic is significant in the professional exams for both undergraduate and graduate courses, especially for
• Bachelors in Zoology
• Bachelors in General physiology
• Masters in Human physiology
• Masters in Anatomy and Physiology

Related Concepts

The skeletal system of human Skeletal muscles

Practice Problems

1.    Difference between interstitial and appositional growth?

Interstitial growth is increase in length of bones that ceases at a particular time. In contrast, appositional growth is the growth in the diameter of bone and continues to occur even after the interstitial growth stops.

2.    Which hormones are involved in bone regulation?

There are many hormones that play an important role in regulating the bone health. These are the parathyroid hormone, growth hormone, calcitonin and even the sex hormones testosterone and progesterone.

3.    Where does interstitial and appositional growth occurs?

Interstitial growth occurs at the epiphyseal plate within the cartilage whereas appositional growth occurs in the endosteum or periosteum.