What is histology?

An introduction to histology means we’re looking at cells and tissues on slides through a microscope. Seeing and learning about the body at this level is enlightening! Did you know that everything in your body is made of just 4 basic tissue types? Read below to learn about each one and start exploring as an introduction to histology!

Four basic tissues

Yes! There are FOUR basic tissues in the body and everything is made of a combination of these four:

1. Epithelial tissue

   • Consists of sheets of cells that form the linings of body tubes and cavities, as well as surfaces (like skin!).

   • Also forms the secretory and duct portion of glands.

2. Connective tissue

   • This tissue binds and supports other tissues – it “connects” them, if you will.

   • CT comes in a wide variety of presentations. A good rule of thumb is that if something isn’t one of the other basic tissue types, it is probably connective tissue.

3. Muscle tissue

   • Muscle makes things move, secrete, squeeze, contract – you name it – if it involves action, muscle tissue is part of it.

4. Nervous tissue

   • This tissue is responsible for communication and coordination between structures in the body.

   • Nervous tissue sends impulses down axons to cause an effect on the other end. Supporting cells, as their name suggests, support the neurons in producing and propagating that impulse.

Epithelial tissue histology

Tubes, cavities, spaces, and surfaces of our bodies need protection and in some places, lubrication. Both endocrine and exocrine glands have to be made of something capable of secreting a product either into the bloodstream (endocrine) or a duct to the surface (exocrine). To accomplish both of these tasks, epithelial tissues form sheets of adjacent cells to either line those surfaces or form the secretory portion of glands.The outermost layer of skin, the epidermis, is a great example of epithelial tissue (stratified squamous keratinized, in that case). Another good example of epithelial tissue is the lining of your mouth all the way down to the other end (stratified squamous non-keratinized and simple columnar epithelium form these linings).Glandular epithelium is seen forming salivary glands, mammary glands, endocrine glands, mucosal and submucosal glands in the gastrointestinal and respiratory systems, and even large organs like the liver are made mostly of epithelial cells.

Connective tissue histology

Connective tissues represent a vast array of structures in our bodies – from the dermis of the skin, to bone and cartilage, fatty tissue, or blood. I call CT the “good guy” of the four basic tissues since it supports the other types of tissue – it “connects” epithelium and muscle, nervous tissues and muscle, and muscle and epithelium where they occur together. Connective tissues are made of specialized cells (depending on the type of connective tissue) and fibers (collagen, elastin, or reticular fibers) sitting within a matrix they created. The matrix varies a lot depending on the CT – it is ossified in bone but firmly squishy in cartilage.As an example, bone is ossified connective tissue – it is still made of cells (osteocytes, predominately) and fibers (lots of collagen) that become mineralized. Fat is a loose connective tissue but it still consists of cells (adipocytes) supported by fibers (collagen and reticular, especially). Blood is even a fluid connective tissue, made of red and white blood cells and plasma, but lacking fibers. The dermis of the skin is a lot of collagen fibers with scattered fibroblasts (and many other skin structures!).

Muscle tissue histology

Muscle tissue consists of cells that contain contractile proteins within them, allowing them to shorten or lengthen during contraction and relaxation. This powerful tissue comes in three pretty different types: cardiac muscle found in the heart walls, smooth muscle found as the walls of tubular organs including vessels, supporting glands, and even in the dermis of the skin (arrector pili muscles are the coolest!), and finally skeletal muscle tissue which moves the skeleton, of course.

Nervous tissue histology

Nervous tissue consists of cells (neurons) and their processes (dendrites and axons) working together with supporting (glial) cells to allow for communication and coordination of processes in the body. Collections of neurons are found in the gray matter of the cerebrum and cerebellum, in the gray matter of the spinal cord, and scattered throughout the body in clusters called ganglia. Connecting those neurons are extensions called axons, and many, many dendrites forming lots of connections between adjacent and far-off cells.

Four basic tissues

You know what? Organs are just various combinations of these four basic tissue types. Each tissue has it’s own job to do, it’s own peculiarities, and comes in a variety of forms.

Image sources:

Micrographs on this page come from a variety of CC sources. Unless listed below or noted on the image itself, the images are from: Berkshire Community College Bioscience Image Library on Flickr. Any images noted as “Magscope” come from: the Magscope Histology Slide Bank.