Learning about the major cells and tissues of the body is a central part of any biology course. And whether you're taking general biology, anatomy or physiology classes, chances are you'll come across epithelial tissue in at least one of courses.

Why is that? Well, epithelial tissue is one of the must abundant tissue types in the body. It's one of the human body's four classifications of tissue. The others are connective, muscle and nervous tissue.

You'll find epithelial tissue in every organ in your body.

One of the first things you'll learn is that epithelial tissue is sorted into two major types. There's stratified epithelium, which is made up of several layers of epithelial cells. Then there's simple epithelium, which is composed of a single layer of epithelial cells.

The basic structure of simple epithelial tissue, is, well, simple. You have a single layer of cells attached to a membrane of connective tissue, called the basement membrane.

Simple epithelium is a polar tissue, which means it has a defined top and bottom. The basal surface is the bottom side of the cells, or the side that's attached to the basement membrane. The apical surface is the top of the cells, or the side that faces the environmental space, sometimes called the lumen.

Simple epithelial cells also have lateral sides. The faces of the cells are loaded with adhesion proteins, which allows the epithelial cells to bind strongly to their neighbors. That keeps the tissue strong and prevents any tears or gaps.

All simple epithelial tissue share that same basic structure. The difference between simple epithelial tissues is the shape of the cells you'll find in that single layer. There are four major types, each with their own functions and locations in the body.

The thinnest and simplest of the epithelial tissues are squamous epithelial cells. Squamous cells have a flattened shape and form a thin and tightly-packed layer of cells – kind of like cobblestones on a street or scales on a fish. Each cell has an oblong nucleus, which rests in the center of the cell. The tissue is made up of a single layer of squamous cells, attached to a basement membrane.

Because simple squamous tissue is so thin, it's not a great layer of protection. Its tissue-thin surface could tear easily and doesn't shield the tissue underneath. However, the squamous cells' thin structure means simple squamous epithelial tissue is great for helping to absorb, diffuse and release substances.

So why is that important? Picture the simple squamous tissue that makes up the air sacs of your lungs. Those air sacs are surrounded by blood vessels, and they're constantly pumping blood through your lungs.

The thin squamous cells in the air sacs help oxygen move easily from the air you just inhaled, through the squamous epithelium and finally into the underlying blood vessels. The squamous tissue helps ensure that your blood is enriched with oxygen when it passes through your lungs, so you have more oxygen circulating throughout your body and release that oxygen into the tissues where it's needed.

Squamous epithelial tissue plays a role in that oxygen release, too. It makes up the lining of your capillaries. So when that oxygen-rich blood finally moves to oxygen-poor tissues, that oxygen can diffuse through your blood vessel cell linings and into the tissues that need it most.

You'll find simple squamous tissue in other organs, as well. It's also found in your kidneys where it helps move substances out of your body, so they can be removed via your urine. And, finally, you'll find simple squamous epithelial tissue in your mesothelium, which is the lining for your internal organs and body cavities.

The second type of epithelial tissue you'll need to know is simple cuboidal epithelium. While simple squamous epithelium is flat, cuboidal tissue is taller.

Each cell has a cube-like shape, which is what gives this tissue its name. Each cuboidal cell has a large, round nucleus, which rests in the middle of the cell.

While cuboidal epithelium is slightly thicker than squamous epithelium, it's still not a great source of protection for the underlying tissue, though it does provide more protection than squamous tissue.

But, thankfully, it's still thin enough to work well for secretion and absorption: taking substances from the environment and drawing them into the cell or releasing substances into environment.

Given that cuboidal epithelial tissue works best at secretion and absorption, it's no surprise that it's the major epithelial tissue found within your glands. Simple cuboidal tissue is in mammary glands, for example, and plays a key role in lactation.

The tissue helps produce milk proteins and fats, then releases them into an open space, called the lumen, so they can travel down the duct and to the nipple to allow for breastfeeding.

Simple cuboidal tissue is found in other glandular tissues, too. It's a key part of your thyroid gland, which releases thyroid hormones that control your metabolism, development and more. And it's also found on the surface of the ovaries, which secrete hormones like estrogen into the body.

You'll also find simple cuboidal tissue in your kidney tubules, where they help absorb nutrients your body wants to retain, and secrete compounds your body wants to remove through your urine.

And you'll find special, ciliated cuboidal epithelial tissues in your airways. There, they secrete a substance, called a surfactant, that helps your lungs work properly. And the cilia on the cells' surface helps distribute the surfactant across the surface of your airways, so that they can function.

The thickest of the epithelial cells are columnar cells. They have a tall column-like shape, which is where they got their name. Simple columnar epithelium is organized as a single layer of columnar cells attached to the basement membrane.

Each cell has a large, round nucleus, found at the base of each columnar cell or the side of the cell nearest the basement membrane.

Because they're the thickest simple epithelial tissue, columnar cells provide slightly more protection than simple squamous or columnar epithelial tissues.

They're divided into two categories: ciliated columnar cells, which each contain a cilium, and non-ciliated columnar cells, which have no cilia.

Ciliated simple columnar epithelial tissue is the main epithelium you'll find lining your respiratory tract. Simple columnar cells in your airways each have one cilia on the apical end of the cell, facing into the lumen of the airways.

Those cilia "row" in unison, helping to distribute surfactant and mucus throughout your airways. They also help "row" unwanted substances, like dust particles, up and out of your airways, so they don't get in the way of your breathing.

Similarly, ciliated columnar epithelial tissue makes up the lining of the fallopian tubes. There, the cilia help "row" the ovum from the ovary, down the fallopian tube and into the uterus, where it could be potentially fertilized by a sperm cell.

You'll find non-ciliated columnar epithelial cells in the lining of your digestive tract. Simple columnar tissue lines your stomach, small and large intestines, where they secrete substances that aid in digestion, and also help absorb the nutrients released from the food you eat.

Simple columnar epithelium is especially abundant in your villi, the small outgrowths in your intestines that increase surface area and allow for better digestion.

The final type of simple epithelial tissue is pseudostratified columnar epithelium. Like regular columnar epithelial tissue, pseudostratified columnar tissue is made up of a single layer of column-shaped cells.

What sets pseudostratified epithelial tissue apart, though, is the location of the nuclei. While regular columnar cells have nuclei located along the base of the cell, pseudostratified columnar tissue has its nuclei located at varying heights within the cell.

That gives the appearance of a stratified tissue, since you'll see nuclei located high, low and in the middle of the tissue, even though it's still a simple tissue made up of a single layer of cells.

You'll find pseudostratified columnar tissue among the epithelial tissue lining your upper airways. The pseudostratified columnar tissue in your airways functions similarly to ciliated columnar epithelia by helping to "row" unwanted substances up and out of your respiratory tract before they can cause problems.

You'll also find pseudostratified columnar tissue in your throat, or trachea, where it serves a similar function.

Finally, you'll find pseudostratified columnar in your reproductive tract. Pseudostratified columnar tissue lines the vas deferens, the tube that carries sperm cells away from the testes toward the urethra, and it makes up part of the endometrium, or uterine lining, in women.

Here's a summary of the key points you should remember about simple epithelial tissue:

• Simple epithelial tissue is made up of a single layer of cells, attached to a layer of connective tissue called the basement membrane.

• Each epithelial tissue has a top (apical) surface, a bottom (basal) surface and side (lateral) surfaces.

• Simple squamous epithelium is thin and flat. It's found in tissues like your lungs and capillaries, and it's important for diffusion and absorption.

• Simple cuboidal epithelium features cube-shaped cells. It's found in tissues like your glands, as well as your kidneys, and specializes in absorption and secretion.

• Simple columnar epithelium features tall, column-shaped cells, and can be found in ciliated and non-ciliated forms. Ciliated simple columnar epithelium is found in your respiratory tract, while non-ciliated columnar cells are found in your digestive tract. 

• Pseudostratified columnar epithelium contains a single layer of columnar cells but takes on a stratified appearance because of the varied locations of its nuclei. You'll find it in your respiratory and digestive tracts.