What Are Osteoblasts?

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Human bones are in a constant state of turnover and any shift in this balance can be a cause for concern. One of the issues with osteoporosis is that the balance between bone deposition and bone consumption is shifted the wrong way resulting in bones that are decalcified, brittle, and prone to injury. In reality, a favorable bone balance is important regardless of age and the risk for osteoporosis. The balance between bone deposition and bone consumption is regulated by a number of hormones and factors that control cells of various types including the osteoblasts and osteoclasts.

Fortunately, for much of adult life, the balance of burn turnover is in a favorable equilibrium, that is bone is able to adequately release calcium from bone when needed and use calcium and other substances to form new bone when that is needed. Although this equilibrium may seem strange, it is part of the way that bone adapts to stresses that are normally encountered in the environment. For example, the bowlegged stance that cowboys develop is just the normal result of the bones reshaping due to the stress of riding in the saddle. Here we will review the important role that osteoblasts play in bone formation and overall bone health.

What Are Osteoblasts?

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The term osteo refers to bone, while blast refers to growth. There are many different cell-designated blasts in different tissues in the human body and osteoblasts are the cells for growth within bone. The other cells that are important in the normal cycle of bone creation and consumption are the osteophytes and the osteoclasts. The osteocytes are merely the cells that represent the basic component of bone tissue, so to speak. The osteoclasts are the cells responsible for bone consumption, with –clast referring to breaking or break down.

We have already mentioned that osteoblasts are important because they are the cells responsible for bone deposition. As you can imagine, the creation of bone is a long, involved process that involves a number of nutrients, hormones, factors, and not to mention, a good state of health. This is why fractured bones can take so long to heal, sometimes a year or more; there are just so many different components involved.

In terms of the way that osteoblasts work, they are present on the surface of the bone, essentially existing in the form of a layer of bone-developing cells. Osteoblasts have tentacle-like parts that protrude into the developing bone. We will get into the details of growth and the importance of bone growth in a little while, but suffice it to say that osteoblasts are the primary foot soldier in the trenches leading to bone growth. Osteoblasts themselves differentiate from osteogenic progenitor cells that become specialized in different types of bone cells that perform different functions.

The differentiation of osteogenic progenitor cells into cells such as osteoblasts and others occurs in the periosteum and the endosteum. The periosteum covers the outer surface of bone while the endosteum exists deep inside the bone as well as in the bone marrow. Osteoblasts help to regulate bone by releasing growth factors, enzymes, and connective tissue. Osteoblasts are themselves regulated by a number of factors and hormones that serve to keep the bone in homeostasis while also priming the bone to respond to environmental stressors and trauma.

The formation of bone is known as ossification and, as mentioned, osteoblasts play an important role in this both directly as bone formers and as regulators of other agents involved in the formation of bone. Dedicated ossification does not begin until after birth, although some skeletal bone does begin to form after conception of the fetus. At the time of birth, a pattern of skeletal structure has been formed by connective tissue and cartilage. Ossification of these early structures occurs throughout early life and the body maintains the cells it needs to build and repair bone.

We have mentioned that bone formation is called ossification, and it is important to note that there are two types of ossification. The two types of ossification are:

  • Intramembranous ossification
  • Endochondral ossification

The difference between these two types of ossification is primarily centered around what the precursor structures are. As we mentioned, a skeletal pattern consisting of cartilage and connective tissue, but not bone, exists at the time of birth. Intramembranous ossification involves bone formation on the base of connective tissue membranes, while endochondral ossification involves formation of bone on hyaline cartilage in areas like the long bones.

Why Are Osteoblasts Important For Growing?

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At this point, it should be clear why osteoblasts are critical for growth, even if it is just in a general sense. We have distinguished between the two types of ossification, and osteoblasts are important in both types. It is perhaps easier to explain the role of these cells in growth by focusing on endochondral ossification, which is responsible for most of the bone growth that leads to height. Intramembranous ossification is responsible for the bones of the skull, the shoulder blades, and the jaw.

Growth of the long bones occurs at an area at the end of the bones called the epiphyseal growth plates. Growth ceases when the epiphyseal growth plates close, but while they are still open bone growth at this site leads to a great increase in length of the bones. As mentioned, this area originally contains cartilage and cells nearby called chondrocytes. Osteoblasts migrate to this area and form bone. As you can imagine, this process does not happen overnight but all throughout childhood, adolescence, and early adulthood.

During the periods of growth, the cartilage is growing, too, and the osteoblasts are forming bone matrix on top of the degraded chondrocytes. When this growth stops and the growth plates close, there are still osteoblasts around to help with maintenance and repair of bone. In other words, though the bones may not be getting longer and denser through the tandem efforts of cartilage, chondrocytes, and osteoblasts, these latter cells are still needed to help maintain a healthy balance of bone. As we shall see, any incident or factor that reduces the number or action of the blasts will shift the bone in favor of resorption, or reduction, in bone density.

Why Are Osteoblasts Important For Stronger Bones?

The lengthening of bones, or what we think of as growth, usually ends in the early 20s. That is the point at which the growth plate closes by being ossified. With the growth plates closed, bone growth is no longer possible. This does not mean that bones cannot change in shape or mass after this event. Bones can still become thicker, which is called appositional growth. The osteoblasts are responsible for this growth, as well – primarily as the result of factors like weight gain. As mentioned, the cells of the bone are designed to respond to environmental factors and stressors, and increased weight is a stress that will lead to a need for denser bones.

The reality is that the blasts do not shape bone alone but require the action of the other major cells of the bone, the osteoclasts. Although osteoclasts may be thought of as problematic in bone because they are responsible for bone resorption or breakdown – resulting in the release of calcium – osteoclasts also play a role that cannot be overlooked. In order for bone to be reshaped in response to stress, bone must be broken down and then reformed in a way that accommodates for the stress. The osteoclasts are the cells that break down the bone and allow them to be reformed by the blasts into bone that best matches the needs of the person.

Therefore, a healthy state of bone requires a careful balance between the blasts and the clasts. Although too much action of the osteoclasts may lead to brittle bones and osteoporosis, if there were no osteoclasts, then the bones would not be able to reshape themselves in response to trauma or stress. Without osteoclasts, the bones would merely get larger in response to stress without removing areas of bone that are problematic or that no longer fit the needs of the individual. In truth, many tissues in the human body have this interplay between growth and destruction, and the bone is merely one of them.

Although the osteoblasts may be essential for the creation of stronger bones, they receive much important help in the form of the osteoclasts which help them achieve bone with the desired shape.

Conclusion

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The study of human tissues can be fascinating because of the constantly changing state of growth and resorption that is a normal part of achieving homeostasis in the tissue. The osteoblasts are an important part of this homeostasis because of their role in bone growth and bone density, but they are not alone. In reality, the other cells of the bone, including the osteoclasts, are just as important, even if they are better known for the potentially dysfunctional aspect of their roles. These cells work in concert to create bone that is healthy, strong, and capable of responding to stressors in the environment.