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Osteoblast: Definition, Function & Differentiation

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  • 0:00 Definition of Osteoblasts
  • 1:51 How Osteoblasts Form
  • 4:08 Balance
  • 4:39 Lesson Summary
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Lesson Transcript
Instructor: Catherine Konopka

Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.

There are three types of cells in the human body that together build, maintain, and remodel bone. Learn about the function and differentiation of one type - osteoblasts.

Definition of Osteoblasts

Bones in the human body are constantly remodeled. In fact, during fetal development, your bones were just cartilage molds, into which bone material was laid. With changes in age, exercise, lifestyle, and eating habits, there are changes in the strength and shape of your bones. There are three types of cells in your body that bring about changes in our bones: osteoblasts, osteocytes, and osteoclasts. In this lesson, we're going to discuss osteoblasts.

Osteoblasts are bone forming cells. Of the three types of bone cells, they are the ones that produce the matrix that makes up bone. The matrix, or organic material, includes molecules such as collagen protein fibers, which give bone its flexibility, and calcium (Ca2+) and phosphate (PO4-) ions, which give bone its rigidity. Osteoblasts make and package the matrix molecules for release into the extracellular environment. Once released, the molecules in the matrix react with each other to form a rigid yet flexible bone tissue called osteoid that eventually hardens to form bone.

Osteoblasts are typically found on top of or next to existing bone. The matrix they produce becomes a new layer of bone tissue, making the existing bone stronger and thicker. Think of osteoblasts as a child making a snowman. In order to make the snowman stronger, the child packs more snow on the outside of the snowman in specific places. However, in the case of bones, osteoblasts eventually become encased in the matrix that they themselves produce. Once this happens, we call the cells osteocytes. One osteoblast cannot make a bone all by itself. Instead, groups of osteoblasts work together in communities called osteons.

How Osteoblasts Form

All cells of the human body come from existing cells. Some cells of your body are stem cells; think of them as the queen of an ant colony. Their job is to reproduce. You have different types of stem cells--some for making blood cells, some for making muscle cells, others for bone cells, and so on. As for osteoblasts and chondrocytes (cells that make cartilage), the name tells it all. They are derived from osteochondral progenitor cells (OPCs) (osteo = bone; chondr = cartilage). OPCs in turn are derived from mesenchymal stem cells (MSCs). Remember, all cells come from other cells!

Think of it as a family tree. MSCs are made early in fetal development. When an MSC divides, two cells are formed. One cell remains an MSC; the other changes (differentiates) a little and could become an OPC. When an OPC divides, one cell remains an OPC and the other differentiates a little and could become an osteoblast or a chondrocyte.

The timing and location of osteoblast differentiation is complex and under the control of many signaling molecules. After load-bearing exercises, such as walking, running, or weight training, osteoblast formation and activity increase, and bones become thicker and stronger. Alternatively, after periods of inactivity due to paralysis or sickness, the activity of osteoblasts decreases, and bones become weaker. This is a large concern for astronauts who experience many months with no gravity. How do you perform load-bearing activities without gravity!?!?

Let's take a look at this image:

Osteoblast differentiation from mesenchymal stem cells

1.) When a mesenchymal stem cell divides into two cells, one remains an MSC

2) The other cell can become many types of cells. Its fate depends on different signaling molecules and growth factors in the immediate area at the time

3.) If an osteochondral progenitor cell is made, it will divide. When it does, one of the daughter cells remains an osteochondral progenitor cell

4.) The other daughter cell can either become an osteoblast or a chondrocyte

5.) The osteoblast then becomes an osteocyte once it is imbedded in bone material

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