In this lesson, we will explore bone as a connective tissue. We will discuss the anatomy of bone, as well as the cell types which make up bone, including how bone is formed and broken down.
Bone Connective Tissue
Connective tissue is defined as a tissue that supports and/or connects our body together in some way. Bone connective tissue provides structural support for other tissues such as our muscles and skin. It's a little strange to think of bone as a living tissue, but that's exactly what it is.
Have you ever built a tall tower with building blocks, and then, it got knocked over so you had to start again? Bones are constantly being damaged and rebuilt throughout the course of our lives, just like a tower of blocks. Think about how much stress we put on our bodies on a daily basis - we need to have some way to repair small injuries to our bones, otherwise we would all be walking around in casts all the time. There are several different structures and types of cells that make up bone connective tissue we can examine to understand how this tissue works.
Histology of Bone
A histological slide of bone tissue
First, let's look at a histological slide and learn the major components that make up bone tissue. Interestingly, bone tissue appears almost exactly like the cross section of a tree. Each bone in our body is made up of a large number of trees, much like a forest. Each individual tree represents the functional unit of bone connective tissue, or an osteon. The Greek root 'osteo' means 'bone.'
The center circle that you see on the slide above is called the Haversian, or central, canal. The blood and nerve supply of bone (remember, it's a living tissue; therefore, oxygen is delivered to bone via red blood cells and carbon dioxide is taken away) runs through the Haversian canal.
The circles of tissues that surround the Haversian canal (or the 'rings' of the tree), are called lamellae. Lamellae are defined as plate-like structures. These rings are edged by a small space called the lacunae (singular 'lacuna'). There are several small, canal-like projections coming off the lacunae, which are called the canaliculi. The lacunae and canaliculi resemble a plant root system moving through the tissue matrix. So, we have the anatomy of bone tissue covered. So, let's examine the cells that make up bone tissue more closely.
Osteoblasts are responsible for mineralization of bone.
The two main types of cells that are responsible for bone formation and degradation are osteoblasts and osteoclasts. Osteoblasts are responsible for bone formation. Recall that all connective tissues are comprised of a matrix in which living cells are suspended; osteoblasts are responsible for building that matrix in bone tissue. These cells are also responsible for mineralization of bone. In this case, mineralization simply means to use minerals, such as calcium, to create solid bone.
The number of osteoblasts in our body decreases with age. This means that as we get older, our bones are not repaired as efficiently, so they become more brittle and can break more easily. Many people can become afflicted with osteoporosis when they get older. Osteoporosis (literally defined as 'porous bone') means that we have 'pores' or 'holes' in our bones, which makes them more apt to break. These pores occur because we no longer have enough osteoblasts to fix them.
Osteoclasts break down bone cells.
Alternatively, osteoclasts are responsible for bone degradation. These cells break down the bone matrix and the bone itself. The broken down bone cells and matrix are reabsorbed by our bodies. Osteoclasts are very important because they 'clean out' bad or damaged bone, so osteoblasts can come in and repair the damage to build new bone.
The most common type of cell in bone connective tissue is an osteocyte. This is a star-shaped cell found in mature bone. Specifically, osteocytes reside in the lacunae. These cells live a long time. They can live up to 50 years; compare that to something like a red blood cell that lives for hours or a few days! When osteoblasts are trapped in the matrix, they secrete an osteocyte if formed. These cells perform two important functions. First and foremost, they assist osteoblasts and osteoclasts in forming and breaking down bone tissue. The second important function is communication and exchange. Blood vessels and nerve cells located in the Haversian canal communicate with osteocytes using the canaliculi. The canaliculi are also used to exchange nutrients and waste throughout bone tissue.
In summary, bone connective tissue is comprised of a few different cell types. Osteoblasts form bone, and osteoclasts degrade bone. Osteocytes live within healthy bone tissue and assist osteoblasts and osteoclasts in performing their respective jobs. The functional unit of bone is an osteon. Within each osteon is the Haversian canal, through which the blood and nerve supply of bone tissue runs. Three other structures found in each osteon are the lamellae, lacunae and canaliculi. Lacunae are gaps in which osteocytes reside, and canaliculi are canal-like projections coming off each lacunae. The lamellae are plate-like structures, which help form bone connective tissue.
After viewing this video, you'll be able to:
- Describe the structures in an osteon: Haversian canal, lamellae, lacunae and canaliculi
- Explain the functions of the different types of bone cells: osteoblasts, osteoclasts and osteocytes
- Summarize why people develop osteoporosis