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Epithelial Layer: Definition & Explanation

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  • 0:00 Tissues Types in the…
  • 0:59 What Is Epithelial Tissue?
  • 1:24 General Characteristics
  • 2:23 Functions
  • 6:08 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Epithelial tissue is a diverse anatomical feature crucial in all body processes. Explore the major characteristics of epithelia and the primary functions of this layer.

Tissue Types in the Human Body

Pop quiz: What is the largest organ of your body?

Answer: Your skin!

It would be understandable if you got caught up wondering whether your liver was larger than your intestines and overlooked that your skin is considered an organ as well. It is your largest organ, covering every external surface of your body and forming a complex layer of tissues.

Throughout your body you have four primary tissue types:

  1. Muscle generates force and movement.
  2. Nervous tissue detects bodily changes and relays messages.
  3. Connective tissue protects and supports organs and other tissues.
  4. Epithelial tissue covers surfaces and lines cavities.

Based on these definitions, you probably deduced that your skin is an example of epithelial tissue. Your body is full of additional examples of epithelial tissues. Let's explore some of these examples, starting with the definition of an epithelial layer.

What Is Epithelial Tissue?

Epithelium (plural: epithelia) is a continuous layer of cells packed tightly together that covers anatomical structures or lines cavities. These cells can be either a single layer or a stack of multiple sheets. Epithelial cells can be flat (like a fried egg), cubes, tall pillars, or a combination of shapes depending on the purpose of the specific layer.

General characteristics

Epithelial cells cover every surface of the body. Your skin, the membranes of your reproductive, respiratory, and digestive tracts, and the surfaces of your individual organs are all covered by an epithelial layer. This surface location subjects these tissues to a lot of abuse and damage from environmental contact. As a result, these cells divide rapidly to replace damaged surface cells that are continuously sloughed off.

If your skin cells are constantly lost, why don't they bleed? Another characteristic of epithelial cells is that they are avascular; 'a-' means without, and '-vascular' means vessels. This means they don't have a direct blood supply. Epithelial cells have to get the nutrients and oxygen they need to survive by absorbing it from the cells beneath them.

Some epithelial cells are modified with cilia, microvilli, or glands to help them perform their individual functions. So, what do epithelial cells do?

Functions

Your many epithelial layers have very diverse functions, depending on where they're located. There are six main functions. Keep in mind that individual epithelial layers perform a combination of one or more of the following functions:

  • Protection
  • Filtration
  • Absorption
  • Sensory
  • Secretion
  • Excretion

Let's examine each function in more detail.

Protection

Your skin is the best example of epithelia meant for protection. Your skin forms a tight barrier that keeps the hostile outside environment and the delicate internal body structures apart. Your skin is crucial to your immune system by providing the barrier that keeps disease-causing germs and toxic substances from getting in. These cells divide very rapidly to maintain the unbroken barrier. As another example of protection, the epithelia of your bladder protect your internal environment by containing your urine, preventing it from contaminating your abdominal cavity.

Filtration

Your kidneys are lined with epithelial cells responsible for filtration. The blood in the kidney flows through vessels with a very thin epithelial layer. Waste products are selectively filtered out into the urine while other substances, like nutrients, remain in the blood.

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