Bone – Structure and Composition

by Dr. Vinaya Kunjir

Bone is a specialised connective tissue that makes the framework of our skeleton. Bone has three functions.

  1. It forms the skeleton framework and gives support to the body. It is the site of muscle attachment.
  2. It protects the internal organs and the bone marrow.
  3. It acts as a reservoir for minerals such as Calcium and Phosphate.
The main constituents of bone are the cells and the extra cellular matrix. 30% of the bone is made up of cells. There are 2 types of cells – Osteoblasts (bone forming cells) and Osteoclasts (cells responsible for bone resorption).
70% of bone is made of extra cellular matrix, matrix which contains collagen fibres and non collagenous proteins. Collagen fibres give elasticity and flexibility to the bones. Non collagenous proteins contain minerals – calcium and phosphates which provide rigidity and strength to the bones.

Structure of long bone:
A long bone shows two broad ends called the epiphysis and a cylindrical tube in the middle called the diaphysis. There is a developmental zone between them called the metaphysis. In a growing long bone, the epiphysis and metaphysis are separated by a layer of cartilage called the growth plate. This growth plate consists of proliferative cells of expanding cartilage which is responsible for the growth of the bone. By the end of the growth period, this growth plate becomes calcified and is replaced by bone.
The outer part of the bone is thick and forms calcified (calcium containing) tissue called Cortex. In the diaphysis, the cortex is thick and dense and hence called Compact or Cortical bone which contains the bone marrow.
Towards the metaphysis and diaphysis, the cortex becomes thinner and spongy and is called Cancellous bone or trabecular bone. It contains bone marrow, blood vessels and connective tissue.
80%-90% of cortical or compact bone is calcified and thus has a mechanical or protective function. Only 15%-25% of cancellous or trabecular bone is calcified. Rest is occupied by blood vessels, bone marrow and connective tissue. Thus trabecular bone has metabolic functions. There are 2 bone surfaces – the outer, periosteal surface and the inner endosteal surface.


Schematic view of a longitudinal section through a growing long bone.
( From : Jee WSS. The skeletal tissues In : Weiss L,ed, Histology, cell and tissue biology. new York Elsevier Biomedical.1983: 200-255.)

Composition of Bone:
Bone must be stiff and strong so that it can carry load. Bone must be flexible and light so that it can be compressed while carrying load. If the bone is too stiff or too flexible it could develop micro-cracks which later lead to fracture of the bone.
Bone is composed of collagen fibres which are strengthened or mineralised with calcium hydroxyapatite crystals. Human bone is 60% mineralised. If the mineral density of bone is increased, then the bone becomes stiffer but looses flexibility, which may lead to fracture of the bone. The internal structure of bone is similar to a woven cloth fabric in which the mineral containing collagen fibres are woven together in alternating loose and tight pattern and contain cells of the bone in between.
There are two types of bone cells.
  1. Osteoblasts – bone forming cells.
  2. Osteoclasts – cells which reabsorb bone
causing bone loss.

Bone Modeling and Re-modeling:
The purpose of bone modeling and remodeling is to maintain strength of the bone so that it can bear heavy load.
Bone modeling is bone construction when new bone is deposited without any previous bone loss. Osteoblasts are responsible for new bone formation.
During bone re-modeling, the bone loss or bone resorption by osteoclasts is more than bone formation by osteoblasts. Bone modeling and remodeling change the size and shape of the bone by deposition or removal of bone from the surface of bone which causes bone thickening during growth and thinning during ageing.
During ageing especially in post menopausal women, the rate of bone loss by osteoclasts in more than new bone deposition. Thus, there is a negative balance in the bone. The rate of re-modeling is high. Abnormalities in the balance and rate of re-modeling lead to decrease in the bone strength so that it can no longer bear heavy load. There are many factors which lead to abnormal rate of bone re-modeling and damage to bone like advancing age, hormonal deficiency, deficiency or excess of growth factors, nutritional deficiency or decrease in muscle mass or mobility. Decreased bone mass and high re-modeling rate leads to osteoporosis and increase in fracture risk.
Treatment with anti-resorptive agents and parathyroid hormone reduces bone loss and fracture risk.

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