♫ Dem bones, dem bones, dem dry bones.
…Now shake dem skeleton bones!
The foot bone’s connected to the leg bone.
The leg bone’s connected to the knee bone.
The knee bone’s connected to the thigh bone.
Now shake dem skeleton bones!♫
The Inside Scoop on Bones
If you think the bones inside your body are dry, hard and unchanging, you’d be wrong. Your bones are alive, your skeleton is an active organ: growing and transforming throughout your life.
Parts of Bones
To understand problems with bones, you have to know the parts of the bones and their function. So here is a short review.
The outside of the bone is covered with a thin layer called the periosteum. This layer contains collagen on the outside with blood vessels that nourish the bone and nerves involved in balancing and pain reception. The inner layer of the periosteum is made up of osteoblasts (see the types of cells below.)
Right under the periosteum is a layer called cortical or compact bone. This is the hard smooth layer of bone. It is made up of cells, called osteocytes, surrounding canals (Haversian Canals) filled with blood vessels and nerves. One canal and the osteocytes that ring it is called an osteon. The compact layer is made up of these osteons lying parallel to each other.
Further into the bone is trabecular or cancellous bone.
Cancellous bone has many pores or holes in it, giving the appearance of a sponge. Don’t be fooled, cancellous bone it is very hard. In the pores are more blood vessels, nerves and (in flat bones like the skull) bone marrow.. In long bones–legs, arms, jaw–like the one pictured above, the center of the bone is the bone marrow. There are 2 types of bone marrow, red and yellow. Most of the blood of the body–red blood cells, white blood cells and platelets–is made in red marrow. Yellow marrow is mostly fat cells. As an adult half of your marrow is red; the other half yellow.
Cells of the bone
There are three types of bone cells: Osteogenic, osteoblasts, osteocytes and osteoclasts.
- Osteoblasts-These are bone forming cells. Some of these cells flatten out to line the surfaces of bones. Osteogenic cells are the precursors of osteoblasts and are also called stem cells.
- Osteocytes-Osteoblasts that have become part of the compact layer of the bone. They maintain the bone.
- Osteoclasts-These are cells that resorb bone–they break down bone.
Resorption, Bone Modeling and Remodeling
During childhood until humans are around 25 years old, our skeleton grows. Bone modeling is the term for this process. Bone is added in some places and the bone is resorbed in other places. Resorption means that the bone is broken down and the minerals go into the blood stream. The minerals are recycled to make new bone or eliminated from the body. Modeling is responsible for the skeleton changing its form.
By the time we are 25, our bodies contain the largest amount of bone mass that we will have throughout our lives. During adulthood, bone remodeling occurs. Old, mature bone tissue is replaced by new bone tissue. This cycle of maintenance helps retain bone strength.
Balance is key. Throughout the process, all the bone cells communicate with each other. Since osteoclasts resorb minerals faster than osteoblast form bone, balance can be hard to maintain.
Osteopenia and Osteoporosis
Osteopenia and subsequent osteoporosis occurs when bone remodeling is out of balance. If too much bone is resorbed, osteopenia, or bone weakening occurs. Bone loss through resorption or too little bone formation makes bones become brittle and likely to fracture. This is osteoporosis. Researchers have been exploring what causes an imbalance in osteoclastic and osteoblastic activity to occur. Hormones like estrogen, testosterone and parathyroid hormone seem to play a role. Research has found receptors on osteoblasts and osteoclasts that switch them on and off. More research is needed.
Who Is At Risk For Osteoporosis?
Women (1 in 2) are more likely than men (1 in 4) to develop osteoporosis. Certain chronic diseases increase the risk of osteoporosis. For example, both Type 1 and Type 2 Diabetes increase the risk of osteoporosis but different mechanisms are involved. Prostate cancer is a risk factor as well. Certain cancer treatments increase the risk of osteoporosis. Below is a diagram (created from information on the National Osteoporosis Foundation website) detailing some of the conditions that increase risk of osteoporosis.
Having a higher bone mass as a child and young adult can help to stave off bone loss in later life. To help reduce the risk of getting osteoporosis, there are several recommendations.
1) Don’t smoke.
2) Stay physically active: do weight bearing muscle strengthening exercises
4) If you drink alcohol, do so in moderation.
5) Be sure to get enough Vitamin D.
6) If you have risk factors, get a bone density test.
7) Stay alert to signs of bone loss, for example loss of height.
8) Talk to your doctor about osteoporosis.
Bones Are Alive
So, did your mom explain all of this to you? Are you ready for the bones are alive quiz? Then someone else needs to give that to you.
What’s the take-away? May is Osteoporosis Awareness Month. After age 50, everyone is at risk for osteoporosis! So drink your milk already!