Epithelial tissue: sheets that cover and line

From a cell to a tissue

An animal body is made of trillions of cells, but they are not all alike. Cells that look the same and do the same job, and that lie together with the material between them, make a tissue. The word comes from a Latin word that means to weave.

You can already count four kinds of work in your own body. Something covers you and lines your tubes. Something holds parts together and fills gaps. Something pulls. Something carries messages. These give the four basic tissues of an animal.

In this course we study the first two. The other two come in the next course.

Epithelial tissue

The epithelial tissue, or epithelium, has a free surface that faces a fluid or the outside. It covers the skin and lines the mouth, the blood vessels, the air sacs of the lungs and the tubes of the kidney.

Look at three features that hold for all of it.

• The cells lie very close together. There is very little matrix, which is the material between cells.
• It rests on a thin basement membrane that fixes it to the tissue below.
• It has no blood vessels of its own. It receives food and oxygen by diffusion from the connective tissue beneath.

Naming an epithelium

Two questions give the name. How many layers are there? What shape are the cells?

• A simple epithelium has one layer of cells. It rests on the basement membrane and lines surfaces where absorption or secretion happens.
• A compound epithelium has more than one layer. NCERT calls it compound. Other books call it stratified. The two words mean the same thing.

The cell shapes are squamous (flat, like paving stones), cuboidal (like small cubes) and columnar (tall and slender, like pillars).

The kinds, with place and work

• Simple squamous: one thin layer of flat cells. Found in the walls of blood vessels and the air sacs of the lungs. It forms a thin boundary for diffusion.
• Simple cuboidal: one layer of cubes. Found in the ducts of glands and the tubules of the kidney. It helps in secretion and absorption.
• Simple columnar: one layer of tall cells with the nuclei near the base. Found in the lining of the stomach and the intestine. It helps in secretion and absorption.
• Ciliated: columnar or cuboidal cells with cilia. Found in the bronchioles and the fallopian tubes. It moves mucus or particles in one direction.
• Compound: many layers of cells. Found in the skin, the mouth, the pharynx and the ducts of the salivary glands and the pancreas. It protects.

In the kidney tubule, the cuboidal cells of the first part carry tiny finger-like microvilli on the free surface. Microvilli increase the area for absorption. The columnar cells of the intestine also have them.

A cilium is a hair-like beating projection. It is different from a microvillus, which does not beat.

Why the air sac is so thin

Why does the air sac have a simple squamous lining? A gas moves by diffusion, and the rate of diffusion across a layer is inversely proportional to its thickness. So a thin layer lets gas through quickly. A skin of many layers would be a poor place for oxygen to cross.

Notice what happens as you push the thickness to the left. The curve rises fast. This is why a single flat layer is the right design for an air sac, and a many-layered one is the right design for the skin, where protection matters more than speed.

Try it in Python

The program names the epithelium from the shape of its cells and from the number of layers.

Practise with new numbers

Common mistakes

• Saying that epithelium has blood vessels. It has none. It is fed from the tissue below it.
• Mixing up compound and stratified. They are two names for the same thing.
• Confusing cilia and microvilli. Cilia beat and move mucus. Microvilli are short and fixed, and they increase the area.
• Calling every flat tissue squamous epithelium. The name needs the shape of the cells and the layers together.
• Forgetting the basement membrane. Every epithelium rests on one.

Remember these

• Four basic tissues: epithelial, connective, muscular and neural.
• Epithelium: tightly packed cells, little matrix, basement membrane, no blood vessels.
• Simple means one layer. Compound (stratified) means more.
• Squamous: air sacs and blood vessels. Cuboidal: ducts and tubules. Columnar: stomach and intestine. Ciliated: bronchioles and fallopian tubes. Compound: skin.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four basic tissues of an animal.

2. Give three features of epithelial tissue.

3. Where do you find simple squamous epithelium, and what is its work?

4. What is the difference between simple and compound epithelium?

5. A gas crosses a layer in a given time. The layer becomes 3 times thicker. What happens to the rate of diffusion?

References

• OpenStax Biology 2e, chapter 33, The Animal Body: Basic Form and Function, section 33.2 Animal Primary Tissues.
• NCERT (2023). Biology, Textbook for Class XI, chapter 7, Structural Organisation in Animals.