Tissues

Animal tissues can be grouped into four basic types:

1. epithelial
2. connective
3. muscle
4. nervous

The epithelial tissues provide a barrier between the external environment and our organism. You can find this tissue on the surface of all organs that are in contact with the outside environment, such as: the skin, the airways, the inner lining of the digestive tract, and the reproductive tract.

Epithelial tissue helps to protect our organism from microorganisms, injury, and fluid loss. In addition to this protective function, epithelial tissue may also be specialized to function in secretion and absorption.


As the name implies, connective tissue serves a "connecting" function. It supports and binds other tissues, it gives shape to our organs and holds them in place. Examples of this tissue include: bone, cartilage, tendons, ligaments, adipose tissue, blood, lymphatic tissue, and others.

Connective tissue has three main components: cells, fibers, and extracellular matrix.


Muscle tissue is the active contractile tissue of our body, which produces force and causes motion.

There are three distinct classes of muscle tissue:

• smooth muscle, which is found in the inner linings of organs;
• skeletal muscle, which is found attached to bones and enables us to move;
• cardiac muscle, which is found in the heart, allowing it to pump blood throughout our body.

Nervous tissue is the main component of the central (the brain and spinal cord) and peripheral nervous systems (the cranial and spinal nerves). The nervous system functions include sensory input, integration, control of muscles and glands, homeostasis, and mental activity.

Nervous tissue is composed of neurons, which transmit impulses, and the neuroglia (supporting cells), which assist in propagation of the nerve impulse and provide nutrients to the neurons.


Last week you learned how to recognize different types of epithelial tissues. Your task for this week is to learn how to recognize different types of muscles and connective tissues. You should work with the pictures in your text and the blackboard (you can download from there slides of all the tissues you saw in the lab). You can also come to the open lab on Monday evening and look at all these slides on our projectors. These slides will be used for the practical exam.


I'm going to give you a few "quick and dirty" tips on how to recognize different types of connective tissue, and how to determine what kind of cells and fibers are present there.

Connective tissues can be classified into four main types: blood, bone, cartilage, and connective tissue proper. I'm sure you have no problem recognizing "blood" and "bone," so I'm going to focus on cartilage and connective tissue proper. Just remember that the key cell type present in bone is osteocyte/osteoblast, whereas in blood you can find erythrocyte, white blood cells, and thrombocytes.

1. After you decided that what you see is not blood, bone, muscle, nervous, or epithelial tissue, now you have to determine if it is cartilage or connective tissue proper. One way of telling them apart is by the cell shape: since chondrocytes (the cells present in the cartilage tissue) lie in spaces called lacunae, they usually have trapezoidal or semi-circular shape. If you see cells like this, likely you're dealing with cartilage.
2. There are three types of cartilage tissue: hyaline, elastic, and fibro-cartilage. You can quite easily tell them apart by looking at the matrix: (1) the hyaline cartilage matrix is glassy in appearence and there are no visible fibers; (2) the elastic cartilage has dark thin elastic fibers around the lacunae; and (3) the fibro-cartilage is named like this because there are lots of thick purple collagen fibers in the matrix.
3. Remember, in all cartilage tissues the dominant cell type is chondrocyte (which lies in lacuna).
4. As the name suggests, the dominant fiber type in the elastic cartilage is the elastic fiber, whereas in the two other types of cartilage it's collagen.
5. If the cell shape is not trapezoidal or semicircular, likely you're looking at the connective tissue proper. The first step is to decide if it's connective tissue proper "dense" or "loose." If you look carefully at the histological sections you'll see that the "loose" tissues have some empty, white spaces between cells, whereas in the "dense" tissues cells are a bit squished and their nuclei are also squished and look more like discs than circles.
6. Remember, in all connective tissues proper, except for the adipose tissue, the dominant cell type is fibroblast. There are fibroblasts in the adipose tissue too, but they're in the minority compared to fat cells.
7. If you see the squished cells and nuclei, you're looking at the connective tissue proper dense. Now you have to decide if it's "regular," "irregular," or "elastic."
8. The connective tissue proper dense elastic has characteristic dark elastic fibers, whereas the connective tissues proper regular and irregular have collagen fibers, which are lighter in color (usually pinkish) and thicker. If those fibers are nicely organized and all align in one direction, it's connective tissue proper regular. If the fibers look messy, it's connective tissue proper irregular.
9. Again, remember the dominant fiber type in the connective tissue proper elastic is elastic (it makes sense, doesn't it?), whereas in the two other types of connective tissues proper it's collagen. The dominant cell type in all those tissue is fibroblast.
10. Now, there are three types of connective tissue proper loose. I think you can recognize the adipose tissue quite easily? Remember that those round cells are adipocytes/fat cells and they're filled with fat. In fact there is so much fat there, that all other organelles are pushed to the site. Over 90% of this tissue is fat cells, but there are some fibroblasts there too. In the end, it's the connective tissue proper.
11. The last two tissues that we need to learn to distinguish from each other are the connective tissues proper loose "reticular" and "areolar." The connective tissue proper loose reticular will have many dark reticular fibers (which often align to form a mesh/net), whereas in the connective tissue proper loose areolar you can see all three types of fibers: the thick purple collagen, the thinner elastic fiber, and the thinnest, darkest reticular fiber.
12. In summary, remember that in all connective tissues proper you can find fibroblasts, and in all cartilage tissues you can find chondrocytes. In most of these tissues the dominant fiber type is collagen (except for the ones that have "elastic" or "reticular" in their names). So if you're asked about the fiber type and you're not sure what's the right answer, the safe bet is to say "collagen." (Again, unless the tissue name suggests that it has "elastic" or "reticular" fibers. Also, areolar tissue has all three types of fibers clearly visible.)
13. Hope that helps! Don't forget also to learn where each of these tissues is located. The best strategy is to think how the tissue's properties (cell type, type and arrangement of fibers) influence its function/location in the body.

Below is the summary of slides I showed you yesterday in the lab. You can download and print them if you want.