Multicellular organisms need organized cells that can form tissues and work together. Those tissues can make organs and organ systems, so the organism can function.

One of the basic types of tissues in multicellular living things is epithelial tissue. It consists of epithelial cells, which line the surfaces of the body.

Epithelial cells are tightly packed in various organ systems, such as your skin. You can also find these cells lining the airways and respiratory system, blood vessels, urinary tract, digestive tract and kidneys. Epithelial cells make up the lining of many tissues in the human body. Packed tightly in sheets, they create a barrier to the outside world and protect you.

Epithelial cells create the covering layer for your body surfaces. They cover organs and body cavities. In addition, they are in glands. Epithelial cells have many roles in an organism, such as playing a part in secretion, absorption, sensation, protection and transport.

For example, they offer protection for the skin and stop pathogens from entering.

Epithelial cells form a barrier that keeps you safe. They act like gatekeepers. This keeps out environmental problems like dirt, bacteria and viruses. Additionally, epithelial cells can help you stay cool by allowing you to sweat in hot conditions. Their ability to stretch allows your skin to move and stay flexible.

Some epithelial cells have sensors that are receptors. They are capable of picking up signals and transporting them.

For instance, when you touch a soft piece of bread, the sensors detect corresponding signals from your hands. Then, they can send the signal to the brain. If you eat the bread, the epithelial cells that line your digestive system can absorb nutrients that your body needs to function. To break up the food, the epithelial cells can secrete enzymes that aid digestion.

The epithelial cells in the female reproductive tract play many important roles, including secreting hormones and growth factors. You can find epithelia, the plural for epithelium tissue, in a woman's ovaries, uterus and oviduct.

The cells can secrete different hormones, cytokines and other substances that influence reproduction and health. However, when something goes wrong with the epithelia, it can cause problems ranging from infertility to cancer.

Although there are different types of epithelial cells, they all share some basic structural elements. First, these cells are polarized. The top or apical side is the one that faces the cell surface, while the bottom or basal side faces the underlying tissue.

Since epithelial cells are packed so tightly together, there is little to no room between them. This means that there is almost no intercellular matrix between them, and they can create an effective barrier. However, the cells do have one area of their surface that is not squished with other cells. This is the free surface that can be exposed to air or fluids.

For nutrients to enter the epithelial cells, they must use diffusion or absorption. The epithelial cells do not have a blood supply like other cells in the human body. Moreover, these cell types can replace damaged or injured cells quickly.

You can categorize the epithelial layer based on the shape of its cells and the number of layers. The most common types of layers are:

1. Simple
2. Stratified
3. Transitional
4. Pseudostratified 

Simple means one layer, while stratified means many layers. Transitional means the layers can vary based on stretchiness. Pseudostratified means one layer that looks like it is two.

The most common cell shapes are squamous, cuboidal and columnar. Squamous cells are flat and thin, while cuboidal cells are boxy. Columnar cells are rectangular.

Epithelial cells can secrete the basal lamina, which is a layer that can provide support and help separate cells while acting like a filter. This is also called the basement membrane. It is a special form of extracellular matrix that you can find surrounding cells, under sheets of cells or between sheets of cells.

The function of the basal lamina varies based on its location. For example, the basement membrane in a kidney works like a filter. Sometimes, epithelial cells become cancerous and go through the basal lamina to grow into other tissues.

Some epithelial cells are specialized to serve different functions in your body. Microvilli are fingerlike projections that help with absorption. You can find them in the intestines. Cilia are projections that can move and sweep things. Although cilia look a little like microvilli, they are longer and thicker.

You can find cilia in the lungs as they move dust and other particles by using rhythmic actions. Microtubules make up cilia. When the cilia beat, they can move mucus or other substances along. Microvilli have actin filaments.

Goblet cells are a special type of epithelial cells that secrete things. They often secrete mucus in glands. You can find them in the intestines and respiratory system. Their mucus can protect the membranes. In addition, they can make antimicrobial proteins, cytokines and other substances that keep you healthy and contribute to the immune system.

Junctions between epithelial cells help keep them close together. There are different types of cell junctions, including tight junctions, gap junctions and anchoring junctions.

Tight junctions are like a seal between the cells; they stop molecules and fluid from getting between the cells. They consist of proteins that fuse together to accomplish this. For instance, your bladder has tight junctions.

Gap junctions create a small opening or gap between cells. This allows ions or small molecules to pass though them. In addition, electrical signals can travel this way. Cells sometimes need space to function properly.

Anchoring junctions give cells connections that are flexible.

The desmosomes, hemidesmosomes and adherens are the main types. These junctions help keep the cells together while providing some structural support. For example, your skin has anchoring junctions because it needs to be stretchy and strong at the same time.

The structure and function of the different types of epithelial cells can vary. The common types are simple squamous cells, simple cuboidal cells, simple columnar, stratified squamous, stratified cuboidal, stratified columnar and pseudostratified columnar.

It is important to know all of these categories because they have different purposes in the body and can cause serious problems when things go wrong.

Simple squamous cells are flat and only have one layer. Since they are thin, they are useful in areas that need to move molecules quickly through absorption or filtration. They line the alveoli or air sacs in the lungs, capillary endothelium, pleural cavity, pericardium and the peritoneum.

You can also find them in the Bowman's capsule of the kidneys. The thinness and flatness of these cells make them more common in interior portions of the body because they are fragile.

Simple cuboidal cells are cubes and only have one layer. They are thicker than simple squamous cells. However, they are also common in areas that need to secrete or absorb substances.

They rely on active transport to accomplish this. You can find them in the lining of secretory ducts in the kidneys or glands.

Simple columnar cells are long and only have one layer. They are taller than simple cuboidal cells. Their main functions are to secrete mucus and enzymes or provide sensory input. These cells can absorb and secrete different substances.

You can find them in the bronchi, uterine tubes, uterus, digestive tract and bladder. In general, the digestive tract and the female reproductive tract have many simple columnar cells.

The ciliated columnar cells are also long and have one layer, but they have cilia. You can see the cilia on the apical sides. In general, these specialized cells appear in the respiratory system or reproductive system. In women, they make up the lining of the fallopian tubes and help move the eggs.

Stratified squamous cells are flat and are arranged in multiple layers. You can find these cells in different parts of the body, including the pharynx, esophagus, oral cavity, uterine cervix, vagina and skin.

It is the most common type of epithelial tissue in people. Sometimes, the top layer of the cells has keratin, a protein, on top of it for extra protection, which you can see in the skin.

Stratified cuboidal cells are cubes and are arranged in multiple layers. They are rarer in the human body.

You can find them in the ducts of sweat glands. In general, they secrete and absorb substances in the glands.

Stratified columnar cells are rectangular and are arranged in multiple layers. They are not as common in the body. You can find these cells in the large excretory duct of the salivary glands, parotid glands, submandibular glands and sublingual glands.

You can also find them in the eyes, uterus and anus.

Pseudostratified columnar cells are rectangular and have one layer, but they look like they have more layers. They allow for the secretion and absorption of substances like mucus and enzymes.

You can find them in the trachea and upper respiratory tract.

When things go wrong in the body, cancer can develop. If the cancer is in epithelial cells, it is called carcinoma. Many cancer cases are carcinomas.

Two main types of cancers are adenocarcinoma and squamous cell carcinoma.

Adenocarcinoma can happen in organs or glands. It is usually found in mucus-secreting areas of the body. Some common examples are lung cancer, prostate cancer and pancreatic cancer. Squamous cell carcinoma is in the squamous cells of the body. It is a type of skin cancer, and it can appear in the legs, arms and other parts of the body.

Epithelial cells are the perfect examples of specialization in multicellular organisms. As living beings grow, they need specialized cells that can perform different functions.

They require complex systems of tissues, organs and organ systems to live and reproduce. A single-celled organism can get away with little organization and no specialization, but a human being with trillions of cells needs order.

Epithelial cells have important roles in secretion and absorption. They help multicellular organisms maintain a stable internal environment. They also offer protection and a barrier to the outside world.