Bone Structure

Bones can be described on the basis of their overall macroscopic shape. They can also be described on the basis of their microscopic composition. These are the ways of describing bone that we will focus on in this web page. In addition, bones can be described on the basis of the way they develop embryologically; that is the subject of another web page in this unit.

Macroscopic Description of Bone

4 types of bones: long bones, short bones, flat bones, and irregular bones. These bone types are pretty simple to learn, and are nicely described on page 185 and in Figure 7.1 of your Hole textbook. There is one additional type of bone, called a sesamoid bone; sesamoid bones are round and are usually found entirely surrounded by tendons, like they are floating within the tendon.

Most of the bones that you'll be learning about in lab are long bones. Long bones have various regions within them, so you will need to get familiar with the anatomy of a long bone.

Anatomy of a long bone:

In this extremely oversimplified drawing of a long bone, you see that it has two slightly swollen ends and a middle region that connects them. The end regions (denoted by the letter "C") are called the epiphyses (singular: epiphysis), and the middle region is called the diaphysis (indicated by the letter "B"). There is always a region where the epiphysis and the diaphysis meet, and this disk-shaped region is called the epiphyseal disk (indicated by the letter "A"). Click on any picture of an ear to hear how the word is pronounced.

    It is at the epiphyses where one bone contacts another in a joint to allow for movement. Bone is so hard that it doesn't make for a smooth contact between the bones at the joint. Therefore, each epiphysis is coated at the end with articular cartilage. "Articular" because that is the cartilage that is involved in articulating with other bones in the joint. Articular cartilage is simply a coating of hyaline cartilage.

    All bones are covered in a thin membrane called a periosteum. The periosteum is made up of dense connective tissue, which contains a lot of collagenous fibers and also many cells. The cells that it contains are called osteoblasts, which are cells that can develop into osteocytes. You'll see the importance of these cells within the periosteum as we get into the embryonic development of bone and the repair of bone after injury-- both topics on other web pages within this unit.

    Finally, long bones are typically hollowed-out in the middle of their diaphysis. This "hole" or cavity is not empty, however. The bone facing the cavity is covered with a thin (simple squamous) epithelium, called the endosteum. The cavity itself is filled with marrow. Marrow can be either red marrow (where blood is made) or yellow marrow (where fat is deposited).

Terminology note: "endo-" means within or into (like you saw for endocytosis) while "peri-" means around. You will see these prefixes over and over. "-Osteum" always refers to bone. "-blast" always refers to a cell that remains ready to divide to produce mature, functional cells. Do you remember what "epi-" stood for? See the bottom of this page for a reminder. Therefore, the literal translation of the periosteum is around the bone while endosteum is within the bone. And osteoblast means a cell that will be able to make osteocytes. Got it?

Microscopic Description of Bone

There are two types of bone based on their microscopic organization: compact and spongy. These two types can also be differentiated based on their macroscopic appearance. You'll find that the epiphyses contain more spongy bone than compact bone, while the wall of the diaphysis is mainly made up of compact bone.

The type of bone that we have been looking at in lab is compact bone. So, all you have learned about the osteocytes, lacunae, canaliculi, osteons, central canals, and penetrating canals applies to compact bone. Some pretty images of compact bone are visible from the Loyola University Medical Education Network, abbreviated LUMEN. Once there, you can click on Part 9: Specialized Connective Tissue-- cartilage and bone. This is just for pretty images of osteocytes (you will have to skip past the cartilage images to see bones images).

You can find a drawing of bone structure at the Body Online. From the main page, select "Skeletal System," and then scroll down to the bottom of the page (underneath the skeleton) to find the link for "Bone Cross Section." This image, again, focusses on the detailed structure of compact bone.

In order to understand spongy bone, all you have to do is to consider connective tissue principles again. Connective tissues are made up of both cellular and matrix components. The cellular components in bone are osteocytes. The matrix components in bone are tough materials, that we will discuss more in the web page on bone function. The matrix materials are organized into rings to form osteons in compact bone. This allows blood vessels to penetrate through the hard material. However, in spongy bone, the hard material runs in a more haphazard way, leaving spaces unoccupied, providing much room for blood vessels and nerves. The strands of hard matrix material are called trabeculae, and the osteocytes lie within them. The osteocytes send little processes through tiny canaliculi (just like in compact bone), but these processes don't have far to go because the trabeculae are surrounded by spaces containing blood vessels for nourishment.

Why do our bodies need both compact and spongy bone? Well, compact bone is very strong and stores a lot of materials (see bone function page). But strength alone is not all that a bone in our bodies needs. Bones also need the ability to withstand some compression forces. For example, when we pick up something heavy, all that extra weight rests on our bones. If they were only tough, strong substances, they might crack under the extra weight. But, since they have some spongy bone, too, they can handle a tiny bit of compression instead of cracking.

"epi-" means upon. The epiphyses are upon the ends of the diaphysis.