Osteoblasts, Osteoclasts, Calcium and Bone Remodeling
Bones are impressive structures that are even more amazing than many people realize. They provide attachment sites for muscles and enable us to move. Some, such as the skull and ribs, protect vital organs. They also make our blood cells, store minerals such as calcium and release them when necessary, and store lipids, which are an energy reserve.
One very important function of bone is to send calcium into the bloodstream when the body needs it. Calcium is a vital chemical in our bodies. It's necessary for muscle contraction, blood clotting, nerve conduction and other functions. It also provides strength to bones and teeth.
Specialized cells called osteoclasts break down bone to free the calcium, while cells known as osteoblasts deposit calcium into bone, remaking it. The process of replacing old bone with new bone is known as bone remodeling.
When the amount of bone manufacture equals the amount of bone disintegration, the mass of a bone remains the same. At certain stages of our lives or under certain conditions, however, the amount of bone production and the amount of bone breakdown are different. In these cases the bone's mass will change.
Compact and Spongy Bone
There are two types of bone tissue. The outer layer of a bone is composed of compact (or cortical) tissue. This is a dense material with low porosity. Spongy bone tissue (also called cancellous or trabecular bone) forms the inner part of bones. It's made of a network of bone enclosing many pores. Bone marrow is located in these pores. Red bone marrow makes blood cells and yellow bone marrow stores lipids (fats). Bones in different areas of the body have different proportions of compact bone and spongy bone and different types of bone marrow.
Microscopic Bone Structure
Structure of Bone
Compact or Cortical Bone
The unit or building block of compact bone is a cylindrical structure called an osteon (which is the Greek word for bone).
- The osteon contains a central canal called the Haversian canal. Blood vessels and nerves run through this canal.
- The Haversian canal is surrounded by circular, concentric layers of tissue called lamellae. The lamellae are made of a material called bone matrix.
- Bone matrix is made of a mineral called hydroxyapatite. This mineral contains both calcium and phosphorus, as well as a protein called collagen.
- Extending from the Haversian canal and through the lamellae are small horizontal canals called canaliculi.
- Lacunae are small cavities or chambers located between one lamella and the next. (The purple structures in the diagram above are the lacunae.) The osteocytes or mature bone cells are located in the lacunae.
- Osteocytes are star-shaped cells. They have long extensions that project into the canaliculi.
- The membrane that covers the outer surface of the bone is called the periosteum.
Spongy, Cancellous or Trabecular Bone
Spongy bone looks like a honeycomb or latticework. Each rod of bone is called a trabecula or a spicule. Trabeculae don't contain osteons or Haversian canals. They do contain lamellae, or layers of bone matrix, but the lamellae are parallel to each other. Spongy bone contains lacunae and canaliculi, as well as osteocytes, osteoblasts and osteoclasts. Nutrients diffuse from the marrow in the spongy bone spaces into the trabeculae to nourish their cells.
Functions of Osteoblasts and Osteoclasts
Osteoblasts build new bone matrix and osteoclasts break it down. (I remember the difference in the words' meanings by the fact that the letter b in "osteoblast" is also the first letter of the word "build".)
The creation and destruction of bone, the communication between bone cells and the signaling processes in bone are complex activities. Some of the details still need to be discovered or clarified.
Osteoblasts work as a group to form new bone. They move over the bone matrix and make and deposit a protein mixture called osteoid, which contains collagen as its major protein. Then the osteoblasts deposit minerals - including calcium - into the osteoid to make bone. The new bone material fills in the cavity formed by osteoclasts.
Some osteoblasts become trapped in the bone matrix and are transformed into osteocytes inside lacunae. Osteocytes are thought to be sensory cells that are involved in signaling processes inside the bone. They connect to other osteocytes through their projections, which extend through the canaliculi. Other osteoblasts become flattened and turn into lining cells that cover the surface of the matrix.
Unlike osteoblasts, osteoclasts contain more than one nucleus. They are large cells produced by the fusion of several smaller cells. Osteoclasts travel over the surface of the bone matrix and secrete acids and enzymes to disintegrate it, forming a little pit on the surface of the bone.
As an osteoclast becomes active, the surface that is contact with bone becomes ruffled, increasing the surface area for absorptions of minerals. These minerals (in their ionic form) are absorbed into the osteoclast, which later releases them into the tissue fluid which is located between cells. From there the ions enter the blood. The process of bone breakdown and mineral uptake by the osteoclasts is known as resorption.
Chemical Signaling Molecules and Osteoclasts
Hormonal Control of Calcium Deposition and Release
The parathyroid glands make a hormone called parathyroid hormone (PTH or parathormone) which stimulates the action of osteoclasts when the amount of calcium in the blood falls. The hormone causes the transfer of calcium from bone to blood. On the other hand, the thyroid gland makes a hormone called calcitonin which slows the activity of osteoclasts, decreasing bone breakdown. Parathyroid hormone seems to be the more significant of the two hormones.
Estrogen in females and testosterone in males help to maintain bone strength. Other hormones that have an influence on bone mass are growth hormone, which is made by the pituitary gland, and cortisol, which is made by the adrenal gland. Growth hormone increases bone mass, while excess cortisol decreases it.
What is Osteoporosis?
Bone Production and Resorption
In general, when someone performs regular weight-bearing exercise, the amount of bone production exceeds the amount of bone resorption and bones increase in size. On the other hand, if someone is bedridden, the amount of bone production falls and the net effect is bone loss.
Our stage of life also influences the behavior of our bones. Bone production predominates during growth, while bone resorption tends to predominate as we age.
Researchers have found that the amount of bone resorption becomes larger than the amount of bone production in our mid thirties, although the difference doesn't become significant until our forties or fifties. A nutritious diet, moderate exercise and a healthy lifestyle can slow bone resorption and stimulate the production of new bone as we age.
How to Strengthen Bones and Prevent Osteoporosis
Osteoporosis is a disorder in which the bones become unusually porous and brittle and bone density decreases. The condition generally appears in older people, although it occurs in young people too. In osteoporosis the amount of bone resorption is much higher than the amount of bone production.
Osteoporosis can occur in both males and females, but it's most common in post-menopausal women. After menopause the amount of estrogen in a women's body decreases significantly, increasing the risk of weakened bones.
In order to strengthen bones and prevent osteoporosis, good nutrition, exercise (which is moderate but not extreme) and a healthy lifestyle are very important. An adequate intake of both calcium and vitamin D is necessary. Vitamin D is needed for calcium absorption through the lining of the small intestine. Other nutrients are also needed for bone health, so a varied diet with lots of nutritious foods should be followed. Smoking should be avoided, since plenty of research shows that it weakens bones. Excessive alcohol intake also weakens bones.
Someone who wants to use exercise as a tactic to prevent osteoporosis should do some research. Some types of exercise - although great for maintaining general health - don't stimulate bone growth significantly. It's also important to do exercises that strengthen the high-risk bones for osteoporosis - the hips, spine and wrists.
With care and effort, we can fight our body's tendency to reduce bone mass as we age. We can also reduce the chance of developing osteoporosis, and we can slow its progression if it's already developed. There are medications that can treat osteoporosis, but prevention is better than treatment.
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