Osteoblasts and Osteoclasts

Osteoblasts and osteoclasts are cells in your body that help your bones grow and remodel to stay strong.

Your bones are living tissue like any other part of your body. It might not seem like it, but they’re constantly growing or changing and reshaping themselves throughout your life.

You might only think of bones growing and changing in children and teens. But osteoblasts and osteoclasts play important roles in helping your bones stay healthy and strong, even after your bones reach their adult size.

Osteoblasts and osteoclasts work together to form new bone tissue and break down old or damaged bone tissue. Osteoblasts form new bone tissue. Osteoclasts break down old bone tissue to make room for new, healthier tissue to replace it.

What do osteoblasts do?

Osteoblasts are like construction crews that build new bone cells. Osteoblasts strengthen your existing bones and help form new bone tissue.

Osteoblasts have three main functions:

• Growing new bones (bone formation)
• Reshaping bones to help them change as you age (remodeling)
• Healing damaged or broken bones

Osteoblasts are triggered by chemical reactions or hormones when a bone grows or changes. They create and release a mix of proteins called bone matrix. Bone matrix is made of proteins, like collagen, mixed with calcium, phosphate and other minerals.

After they’re activated, osteoblasts move into place. They deposit bone matrix in spaces on a bone that needs to grow or be repaired and strengthened. After it’s in place, the bone matrix solidifies and hardens into new, healthy bone.

Picture a worker pouring concrete. They might create a whole new sidewalk. But they can also use that same concrete to patch cracks or broken chunks in an existing path. In this example, osteoblasts are the worker, and bone matrix minerals are the concrete they use to create new bone.

Once the osteoblasts have completed their job of laying down new bone tissue, they can die if your body doesn’t need them anymore. Or they can become part of the bone by transforming (differentiating) into osteocytes. Osteoblasts can also turn into flat bone lining cells that cover the surface of your bones.

Osteocytes are the most common type of cell in your bones. They act like a security system that monitors changes in pressure and stress. They respond to everything from normal movement and the force of you using your body to more intense changes like injuries. They send signals to the osteoclasts and osteoblasts to repair damaged bone tissue.

For example, if a bone is cracked, damaged or broken, osteocytes trigger a reaction that attracts osteoclasts to dissolve the area around the break (to resorb damaged bone tissue) and osteoblasts to lay down new bone tissue, so it can begin to heal.

What do osteoclasts do?

Osteoclasts dissolve and break down old or damaged bone tissue. They make space for osteoblasts to create new bone tissue in areas that are growing or need to be repaired.

If osteoblasts are builders, osteoclasts are your bones’ demolition crew. Think “clasts” for cleaners and “blasts” for builders. Osteoclasts release enzymes that break down old bone. They trigger chemical reactions on the surface of old bone tissue that dissolves it and creates space for newer, stronger tissue to form in its place.

Osteoclasts dissolve bone tissue. But it’s not as violent or aggressive as acid eating a hole in metal in a cartoon. Your body tightly controls the process of breaking down areas of old tissue. Osteoclasts only target specific areas that osteocytes have tagged.

The enzymes that osteoclasts release break down hardened bone matrix and reabsorb it into your body. This leaves microscopic pits and divots on the surface of your bone. Once the targeted tissue in those places is dissolved, osteoblasts move in and deposit new bone in the same spot.

What do osteoblasts and osteoclasts look like?

Osteoblasts and osteoclasts are tiny cells found along the bone lining and in the bone itself.

Osteoblasts are shaped like cubes. They’re slightly box-shaped.

Osteoclasts are bigger than osteoblasts. They’re shaped like rounded domes.

What are the common conditions and disorders that affect osteoblasts and osteoclasts?

Osteoblasts and osteoclasts are important parts of your overall bone health. Any condition that affects your bones can impact these cells, too. Some of the most common issues that affect your ability to grow and maintain strong bones include:

• Osteoporosis
• Osteopenia
• Hyperparathyroidism
• Paget’s disease of the bone (osteitis deformans)
• Brittle bone disease (osteogenesis imperfecta)

Tests you might need

Usually, you won’t need any tests done to check on your osteoblasts or osteoclasts. Healthcare providers generally use tests to check the health of your whole bone rather than looking for specific cells. But sometimes, they run labs to see how your osteoclasts are working or to help diagnose and treat osteoporosis.

Common tests for bone health include:

• Bone density test: A bone density test measures how strong your bones are with low levels of X-rays. It’s also called a DXA scan.
• Vitamin D 25-hydroxy test: This checks to see if you’re getting enough vitamin D.
• Calcium tests: Your healthcare provider can use these to check if you have the right amount of calcium in your body.
• Intact parathyroid hormone (iPTH) test: You may need this to make sure your body is making the right amount of parathyroid hormone.
• Checking bone turnover markers (CTX and PINP): Your provider will look for these markers in your blood to make sure your bones are remodeling themselves correctly.
• Creatinine test: This checks your kidney function.

How do I keep my bones healthy?

Anything you can do to keep your bones healthy will support your osteoblasts and osteoclasts, including:

• Eating the right amount of nutritious food
• Doing weight-bearing exercise regularly
• Avoiding smoking
• Limiting how much alcohol you drink
• Getting enough calcium and vitamin D
• Seeing a healthcare provider for regular checkups