This lesson explores the structure and function of two types of connective tissue: adipose tissue and loose connective tissue. You'll learn about the cells and fibers that make up each tissue and explore how these structures help our body function.
Connective Tissue Cell Types
Connective tissue does just what its name implies - it connects things in our body together. This general tissue ranges from anything to bone to blood to fat. Recall that connective tissue is defined as a cellular matrix with living cells embedded within it. The cells that make up connective tissue are separated into two main categories - fixed cells and wandering cells.
These cells are aptly named - fixed cells don't move. The most common type is called a fibrocyte. Fixed cells create fibrous material, or the matrix, of some connective tissues. Wandering cells move through the cellular matrix. Our blood is a connective tissue, and the individual components that make up blood (red blood cells, white blood cells, and platelets) are perfect examples of wandering cells.
Connective Tissue Fibers
Let's focus on the fibrous material created by fixed cells in connective tissue. Tissue fibers are classified based upon the type and 'weave' the fibers create. There are different types of weave, just like we have different types of fabric which make up our clothes. The three main fiber types are collagenous fibers, elastic fibers, and reticular fibers.
Collagenous fibers are made of collagen, a very important material. It provides structural support and can be quite literally described as the glue which holds our body together. We find collagen in everything from our skin to our bones to the lining of our gut. In fact, the ancient Egyptians used to boil the skin and sinew of horses to make glue from collagen!
Elastic fibers are made up of a material called elastin and are 'stretchable.' These fibers work just like the elastic waistband in athletic shorts; the fibers stretch when needed and then contract back to their original length.
Reticular fibers are made by specialized cells, called reticular cells, which secrete materials that make a very thin fiber. These fibers form a 'mesh' which helps support our organs such as our liver. These fibers also form an organ on their own - reticular connective tissue is what makes up our spleen!
Areolar Connective Tissue
All three of these fibers types are present in one type of connective tissue, also known as areolar (loose) connective tissue. This tissue is important because it has several jobs in our bodies. Areolar connective tissue is strong enough that it can bind different tissue types together (such as our skin to our muscles) but also soft enough to provide a cushion for our organs.
Areolar connective tissue provides cushioning for the organs.
The amount of the three fiber types varies in this tissue depending on where it is located and the job it performs. Therefore, the appearance of areolar connective tissue varies depending on where it is found. On the slides shown above, note that the image on the left has a lot of elastic fibers in the tissue. The image on the right has many more collagenous fibers, but they are both areolar connective tissue.
The Basement Membrane
Another important feature of connective tissue is the basement membrane. While I'm sure you're used to hearing about the basement membrane in relation to epithelial tissue, it is also an important structure for connective tissue. While connective tissues do not have their own basement membrane, this is where connective tissues attach to epithelial tissues. If you look at the slide below of the trachea, you will clearly see the basement membrane of the tracheal epithelial tissue is connected to the connective tissue right underneath it.
Connective tissues attach to epithelial tissues through the basement membrane.
Adipose Connective Tissue
Although it is a bit confusing, loose connective tissue can be considered a parent category of specialized connective tissues. In this case, instead of being labeled as loose connective tissue, areolar connective tissue would fall under this category, along with adipose (fat) connective tissue.
We find adipose connective tissue everywhere. It surrounds organs, cushions and insulates our body, and provides an energy source. The next time you're looking at a histology slide, look a bit closer; I'll bet you see some adipose tissue! There are actually two types of adipose tissue: white adipose tissue and brown adipose tissue. White adipose tissue accounts for about 20-25% of a healthy, non-overweight human's body weight and is used as an energy source. White adipose tissue consists of a single fat droplet.
Brown adipose tissue is found in newborns and in hibernating mammals. This type of fat is used primarily to generate body heat so we don't shiver. Unlike white adipose tissue, brown adipose tissue has multiple fat droplets and a higher number of mitochondria, which give it its signature brown color.
As we grow, the mitochondria in our brown fat disappear, and this tissue behaves more like other tissues in our body. This is how we 'lose baby fat' as we get older - we get rid of the brown adipose tissue.
So, now that we've covered fixed cells, let's move on to wandering cells. The most important wandering cell in connective tissue is called a macrophage. 'Macrophage' is Greek for 'big eater.' Macrophages use a process called phagocytosis, which means 'cell eating.' These cells will move throughout connective tissues to engulf and digest cellular debris and pathogens to get rid of them. These cells help keep our connective tissues clean and healthy. In the slide below, on the right, you will clearly see the macrophages, labeled by arrows, with cellular debris inside them.
Macrophages digest debris within cells.
There are two main types of cells in connective tissue: fixed and wandering. Fixed cells do not move and create a fibrous material that helps make up our connective tissue. There are three main types of fiber created by fixed cells, which are collagenous, elastic, and reticular. These fibers help make up the matrix of several connective tissues, such as areolar, or loose connective tissue, and fat, or adipose connective tissue.
Wandering cells move throughout the connective tissue matrix. Macrophages, which eat dead and harmful cells, as well as the cells which make up blood connective tissue are examples of wandering cells.
At the end of this lesson, you'll be able to:
- Describe the two main categories of connective tissue cells
- Explain the composition and function of the three types of fibers created by fixed cells
- Identify the differences between adipose connective tissue and areolar connective tissue
- Distinguish between white and brown adipose tissue
- Summarize the function of macrophages, which are a type of wandering cell