Osteoblasts, Osteoclasts, Calcium, and Bone Remodeling
Impressive Structures With Multiple Functions
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. Cells known as osteoblasts deposit calcium into bone, remaking it. The process of replacing old bone with new bone is known as remodeling.
Microscopic Bone Structure
Types of Bone Tissue
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 tissue (also called cancellous or trabecular tissue) forms the inner part of bones. It's made of a network of solid bone enclosing many pores. Marrow is located in these pores.
Bone marrow is red or yellow in color. The red type makes blood cells and the yellow type stores lipids (fats). Bones in different areas of the body have different proportions of compact and spongy tissue as well as different types of marrow.
Compact or Cortical Bone
The unit or building block of compact bone is a cylindrical structure called an osteon. The name comes from the Greek word for bone.
- An osteon contains a central canal called the Haversian canal. Blood vessels and nerves run through this structure.
- A 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 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 dark 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.
Inside the Skeleton
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. The matrix contains lacunae and canaliculi, as well as osteocytes, osteoblasts, and osteoclasts. Nutrients move from the marrow in the pores of the lattice into the trabeculae. The nutrients nourish the cells in the bone.
When the amount of bone manufacture equals the amount of disintegration, the mass of a bone remains the same. At certain stages of our lives or under certain conditions, however, the amounts are different. In these cases a bone's mass will change.
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 its cells, and the signaling processes that occur are complex activities. Scientists have discovered that osteoblasts make a protein hormone, which is known as osteocalcin. The functions of this hormone and the activities that take place in bone are still being investigated.
Remodeling the Skeleton
Osteoblasts work as a group to form new bone. They move over the 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 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 ones. 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. This increases the surface area for absorption of minerals. The minerals (in their ionic form) are absorbed into the osteoclast, which later releases them into the tissue fluid 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 (also known as 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.
Bone Production and Resorption
In general, when someone performs regular weight-bearing exercise, the amount of bone production exceeds the amount of resorption and bones increase in size. On the other hand, if someone is bedridden, 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 resorption tends to predominate as we age. Researchers have found that the amount of resorption becomes larger than the amount of production in our mid thirties, although the difference doesn't become significant until our forties or fifties. A nutritious diet, moderate exercise of the correct type, and a healthy lifestyle can slow resorption and stimulate the production of new bone as we age.
What Is 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 as well. 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.
Worldwide, osteoporosis causes more than 8.9 million fractures annually, resulting in an osteoporotic fracture every 3 seconds— International Osteoporosis Foundation
A Doctor Describes Osteoporosis
How to Keep Bones Strong
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 does the same thing.
Someone who wants to use exercise as a tactic to prevent osteoporosis should do some research. Some types of exercise are great for maintaining general health but don't stimulate bone growth significantly. It's also important to do exercises that strengthen the high-risk bones for osteoporosis, which are 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 reduce the chance of developing osteoporosis and slow its progression if it's already developed. There are medications that can treat osteoporosis, but prevention is better than treatment.
© 2013 Linda Crampton