In this video, we're going to begin our introduction to epithelial tissue. Recall from our previous lesson videos that epithelial tissue is one of just four primary types of tissue found in the human body. Epithelial tissue is commonly abbreviated as epithelia, without the "l" and the word "tissue." Epithelia or epithelial tissue covers body surfaces, lines internal body cavities and ducts, and makes up glands or cellular structures specialized for secretion or releasing products. Covering, lining, and glands are key terms often associated with epithelial tissue in your textbooks. One of the key defining features of most epithelial tissues is that it consists of a single sheet or multiple sheets of tightly packed cells on a boundary immediately adjacent to open space.
If we take a look at our illustration below, you'll notice that we're highlighting a few examples where epithelial tissue can be found in the human body. In each of the micrographs for these examples, we can clearly see that epithelial tissue consists of sheets of tightly packed cells on a boundary adjacent to open space. We can highlight the open space in each of these micrographs. Notice on the far left, you can see the open space in this region; in the second micrograph, the open space is over here. In the third micrograph, you can see there are small bits of open space in these highlighted small regions. Then, in the final micrograph, you can see small bits of open space in these highlighted regions right there.
In the micrograph on the far left, notice that it indicates that the outermost layers of our skin are epithelial tissue. Our skin covers our entire body surface, making the outermost layers a classic example of epithelial tissue. Epithelial tissue is also important for lining internal body cavities, including the lining of our airways. Notice that this second example shows the lining of our airways containing epithelial tissue. Additionally, epithelial tissue is crucial for forming glands, including salivary glands. Salivary glands, found in our mouth, secrete or produce saliva. Lastly, epithelial tissue is vital for lining ducts and forming tubules, as shown in the last example where the ducts and tubules in our kidney contain epithelial tissue.
This brief introduction to epithelial tissue is just the beginning, and as we move forward in our course, we will continue to learn a lot more about epithelial tissue. This concludes this introduction, and I'll see you in the next video where we'll talk more about the functions of epithelial tissue.