Epithelial Tissue: Characteristics, Types, and Functions
Epithelial tissue is a really important tissue in our bodies. Epithelial tissue lines all our body surfaces both inside and out and it also forms glands. We can divide epithelial tissue into two types: membranous epithelium and glandular epithelium.
Membranous epithelium covers and lines the external surface of our body as well as lining all internal cavities, organs, tubes and passageways. Glandular epithelium however is specialized epithelial tissue that produces and secretes substances such as sweat, stomach acid, mucus and hormones. Despite there being many different types of epithelial tissue all epithelial tissue have just five characteristics, these are cellularity, polarity, attachment, vascularity, and regeneration.
Cellularity as the name suggests means that the epithelium is made up almost entirely of cells. In other types of tissue the cells are spread far apart and are separated by lots of extracellular material. Another key characteristic of epithelial tissue is polarity. Polarity means that the cell looks different at the top than at the bottom. Basal means base or bottom and the part of the cell is the sitting on a basement membrane, apical means apex or top and it's the part of the cell which faces the lumen such as in the intestine or faces the external body such as the skin, this intestinal epithelial cell shows polarity. It has a brush border on its apical surface to increase the surface area available for absorption of nutrients whilst its basal surface is flat. Epithelial cells display attachment which means that the basal surface of the cells are attached to a special layer called the basal lamina or basement membrane. The basal lamina or membrane supports the epithelium and anchors it firmly to the underlying connective tissue.
Epithelial tissue is also a vascular which means, the tissue doesn't have its own blood supply. This means that the epithelial tissue must get its nutrients by diffusion from capillaries which are found in the underlying connective tissue. The final characteristic of epithelial tissue is repair and regeneration. Epithelial tissue can repair and regenerate itself much better than most other tissues of the body and this is really important because we often cut, scratch or damage our epithelial surface.
Epithelial tissue provides four key functions, they provide protection, they control permeability, they provide sensation, and they produce secretions. Our skin is a great example of an epithelial tissue that protects our body, It is made up of lots of layers of cells. For greater protection epithelial tissue controls permeability, it tightly regulates what can and can't pass through the body. For example, our skin is relatively impermeable meaning most substances can't easily pass through into our bodies but this is not the case in the epithelium that lines our intestines. This epithelium is much thinner and it allows for water and nutrients to pass easily through while keeping bacteria out. Epithelial tissue has a really rich nerve supply, this is really important because these nerves gather really important sensory information such as pressure, pain and temperature, by being able to detect all of these sensations it can actually stop us from hurting ourselves.
Some epithelial cells are highly specialized and capable of producing secretions. This could be single cells producing secretions or more often these secretory cells form epithelial glands. These secretions are able to be released into ducts and then discharged onto the surface of the epithelium. Now we call these glands exocrine glands and common examples include our sweat glands and saliva glands. Secretions can also be released into surrounding tissue and blood, and we call these glands endocrine glands and an example of an endocrine gland is the thyroid gland.
So, in summary, we've learned that although there are many different types of epithelial tissue all epithelial tissue have the same 5 characteristics they are cellularity. polarity, attachment, vascularity and regeneration. In addition we've also learned that although there are many different types of epithelial tissue that had the same basic 4 functions they are, to provide protection for the body, to control permeability for the body, regulating what comes in and what stays out, to provide us with sensation, to enable us to detect temperature and pain and also to produce secretions for the body.
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