The human skeletal system is comprised of the axial skeleton (the central parts of the skeleton, including the skull, vertebral column and rib cage) and appendicular skeleton (the peripheral parts of the skeletal system, including all of the bones in the arms, legs and the pelvic and pectoral girdles). Discover the parts of a long bone (diaphysis, epiphysis, and metaphysis), and the functions of osteoblasts and osteoclasts in bone formation.

The Functions of the Skeletal System

Your brain helps you think, your lungs help you breathe, and your heart circulates blood all over the body to give you life. But, make no bones about it; one of the unsung heroes of your body is the skeleton. You usually pay no attention to it unless something is broken or hurts; however, nothing would work without your skeleton. It's kind of like the framework of a house. That framework helps to support everything built within it or on top of it. Without the framework, the house would collapse and, frankly, so too would your body.

Your bones help support your body's organs, provide leverage to lift heavy objects and help produce red blood cells that carry oxygen gained in the lungs, circulated by the heart and used by the brain to keep you alive. So, no bones, no bone marrow and no red blood cells means no life.

The Axial and Appendicular Skeleton

As I'm sure you know, a house usually has a main framework, a central location so to speak, and some additional attachments added onto the house, like a garage, a deck or a veranda. Likewise, your skeleton has a main framework, called the axial skeleton, which includes the central parts of the skeleton, including the skull, vertebral column and rib cage.

Everything else, the additional elements added onto our main house, are known as appendages. Hence, our appendicular skeleton includes the peripheral parts of the human skeletal system, which includes all of the bones in our two arms, legs and the pelvic and pectoral girdles. For your reference, many of the bones in our appendicular skeleton, like those in our legs and arms, are called long bones, since they are longer than they are wide.

The Diaphysis, Metaphysis and Epiphysis

As you build your house's framework, that is to say, as your bones grow, they are comprised of three main parts. The main, long, central part of a long bone is called the diaphysis, while the the tip, or end, of a long bone is called the epiphysis. The epiphysis and diaphysis are separated by a part called the metaphysis, which is the growing portion of bone located in between the diaphysis and epiphysis.

The Epiphyseal Plate

The reason your bones can grow in the first place is thanks to something known as the growth plate, or the epiphyseal plate, which is located in between the metaphysis and epiphysis and helps to lengthen your long bones. The epiphyseal plate is made of hyaline cartilage, which can easily elongate, explaining the bone growth you experience during a young age. However, once this hyaline cartilage is replaced by solid bone, it fuses the metaphysis with the epiphysis, and you, in so many words, stop growing at that point.

Osteoclasts and Osteoblasts

However, just because you've stopped growing doesn't mean your bones aren't constantly changing. Just like your house initially needs to be built and then repaired with age, so too must your skeleton. The initial construction of your skeleton and its life-long breakdown and repair are accomplished thanks in part to two cells.

One of these cells is known as an osteoblast. This is a cell responsible for the formation of bone. In contrast, the cells that are responsible for the removal, or resorption, of bone tissue are called osteoclasts. Since the prefix 'osteo-' refers to bone, and the suffix '-clast' refers to the breakdown of something, we get the word osteoclast, or a cell that breaks down bone. The 'blast' in osteoblast refers to a germ, sprout or bud, something from which another thing germinates or buds. In our case, an osteoblast helps to germinate new bone and may itself develop into another cell, called an osteocyte.

Knowing that, it shouldn't be too much of radical leap of judgment to realize that the increasing amounts of osteoclast activity or the decreasing amounts of osteoblast activity can lead to a condition called osteoporosis. Osteoporosis is when your bones lose mass, become fragile and brittle and are more prone to fracture.

Lesson Summary

If you can't strengthen your bones, at the very least, you can strengthen your mind by reviewing everything with me one more time.

The axial skeleton includes the central parts of the skeleton, including the skull, vertebral column and rib cage, while the appendicular skeleton includes the peripheral parts of the skeletal system, which includes all of the bones in the arms, legs and the pelvic and pectoral girdles. Keep in mind that many bones in the appendicular skeleton are called long bones and these are broken down into three main parts. One of these parts is called the diaphysis, which is the main, long, central part of a long bone. The other part is called the epiphysis, which is the tip, or end, of a long bone.

Finally, the last part is called the metaphysis, which is the growing portion of bone located in between the diaphysis and epiphysis. Don't forget that the metaphysis will fuse with the epiphysis once the growth plate becomes solid bone. Regardless, one of the reasons why your bones elongate in the first place and can be repaired is thanks to cells called osteoblasts, which are cells responsible for the formation of bone, and cells called osteoclasts, which are the cells that are responsible for the removal, or resorption, of bone.

Learning Outcomes

After watching this video, you'll be able to:

• Differentiate between the axial and appendicular skeletons

• List the parts of a long bone

• Explain the importance of the metaphysis

• Describe the functions of osteoblasts and osteoclasts