In this lesson, you will learn what makes up compact bone. You’ll also specifically explore the structure and location of compact bone, the densest of the two bone types.
What is Compact Bone?
Bone is actually a type of connective tissue (a tissue type responsible for supporting, connecting, or separating tissues). It composes the structural support system of our bodies, which accounts for fourteen percent of your total body weight. Twenty percent of your skeleton is composed of a less rigid bone called trabecular (spongy bone), while the other eighty percent is dense, strong cortical (compact bone). Spongy bone gets its name because, you guessed it, it looks like a sponge and has many porous spaces, whereas compact bone is dense, appearing as if it were ‘compacted’ together.
Compact bone has a sturdy calcified matrix with very few spaces. This layer not only forms a protective shell around the spongy bone tissue, but it also gives our bones their rigidity, strength, and resistance. Can you imagine going to the gym or even trying to support your own body weight relying solely on porous spongy bone? I doubt we’d survive long enough to make it out of our house!
Structure and Function
Under magnification you can clearly see the system of concentric circles that forms compact bone. You can think of compact bone as being very similar to a bundle of cut trees or logs. Each ‘log,’ or long cylindrical unit, is called an osteon and, just like trees, osteons have rings of growth. Now, unlike trees, you can’t figure out a person’s age by counting their growth rings, called lamellae, but they do grow like the rings of a tree.
As bone grows, it creates new lamellae around older lamellae until different osteons meet and fuse together. These osteons, when fused, create a unit that becomes stronger than any one osteon could be on its own. Just like wires can be bundled to create those giant cords that support huge suspension bridges, osteons, when bundled, form amazingly strong pillars of support, structural strength, and rigidity.
Inside each osteon is a space called a central canal, which houses blood vessels and nerves within the bone. These vessels supply blood to the interior spongy bone as well as the living cells housed within the compact bone. That’s right;bone is actually a living tissue!
Between the growth rings of lamellae are spaces, called lacunae, that contain osteocytes. Osteocytes are cells that deposit bone. Osteocytes are proactive cells, too; they respond to repeated stress on your bones (such as consistent exercise) and can actually make your bones stronger so that they can withstand the additional stress applied onto your bones by your muscles. Each lacunae is connected to the others through a fine network of canals, called canaliculi. The canaliculi supply nutrients to the osteocytes, remove cellular wastes, and enable communication between cells.
Finally, compact bone is covered by a double-layered membrane called a periosteum. The inner layer of the periosteum contains osteoblasts, which are cells that secrete collagen as well as mineral salts, which increase bone’s resistance to breakage. You could think of osteoblasts as cells whose job it is to ‘blast’ bone with hardening and flexibility agents.
Lastly, the exterior layer of the periosteum contains the blood vessels and nerve fibers that pass into the bone and weave throughout the large central canals, into the little canaliculi, and to the osteocytes sitting within the lacunae of the bone tissue.
Let’s review. Compact bone (also called cortical bone) is one of two types of bone connective tissue, the other being spongy bone (also called trabecular bone). Compact bone forms a protective shell around spongy bone that offers our skeleton a much higher degree of strength, rigidity, and resistance to compressive forces than spongy tissue would have on its own.
Compact bone is composed of cylindrical units, called osteons, which are formed from layers of concentric circles, called lamellae, around a central canal containing blood vessels and nerve fibers. Each osteon has many little spaces between its rings. These spaces are called lacunae, and they house bone-producing cells, called osteocytes, which are united through a network of canals, called canaliculi. The canaliculi supply nutrients via blood vessels, remove cellular wastes, and provide a means of communication between osteocytes.
Composition of Compact Bone
- Compact bone, as opposed to spongy bone, is made of cylindrical units, called osteons, that are tightly formed together.
- As compact bone grows, osteons begin to fuse together.
- In the center of the osteon is a central canal with blood vessels and nerve fibers.
- Bone-producing cells are found in small spaces called lacunae.
Once the lesson on compact bone ends, you can pursue these goals:
- Compare compact bone and spongy bone
- Recognize the way in which compact bone is able to provide rigid support to the skeleton
- Describe the structure of compact bone