In this section, we are going to discuss one type of tissue in the human body--the epithelial tissue. Tissues are made of individual cells that are organized in a specific way to work together for a particular function. The study of tissue structure and function is called histology.
The human body contains many kinds of tissues and there are many varieties within these types. Because tissue organization is such a broad topic, we will be focusing on epithelial tissue, the tissue that covers exposed surfaces and lines internal passageways and chambers. Like all tissues, epithelial cells fit together in a specific way for a particular function. For instance, one epithelial tissue is the skin, which forms a protective barrier that keeps your internal organs from dehydrating. If you think of tissues as the interwoven fabrics of the body, the epithelial tissues could be the outer shell and the inner lining of a winter coat.
It is important to understand the structure of your body's tissues, because it is changes to the tissues that lead to growth, aging, trauma, and disease. Our talk will be divided into three parts: the characteristics common to all epithelial tissues; the function of epithelial tissues; the way epithelial cells are classified; and the structure and function of glandular epithelium, which is responsible for forming glands. Let's start by discussing the general characteristics of epithelial tissues.
Epithelial tissue, also called epithelium or epithelia for plural, covers the internal and external surfaces of the body. It forms a boundary between your body and the outside environment, or between one part of your body and another. As we just mentioned, skin is an example of epithelial tissue. So is the lining of the intestines. All epithelial tissues have the same four characteristics. These characteristics are polarity, attachment, avascularity, and regeneration. Let's start off with a discussion about polarity.
Polarity means that the epithelium has an up side and a down side. The up side is called the apical surface. The apical surface faces the open side of the intestines, mouth, lungs, or the outside of the skin. The down side is called the basal surface. This side keeps the cells attached to the other tissues. This attachment is the second important characteristic of epithelia.
How do epithelial cells stay attached to the underlying tissues so well? Between the epithelial tissues and the connective tissues is a layer called the basal lamina or the basement membrane. Think of it this way: There are two villages making a wall between them. Each side is contributing to the wall. One side uses mortar and bricks and the other uses wood and leather bindings. As each side adds to it, the wall gets thicker and stronger.
Well, the basal lamina is like the wall being built. One side is made by epithelial cells secreting carbohydrates and proteins from their basal surface that mix together and form a kind of organic glue. This organic glue is like the mortar used to bind the bricks. Except in this case, it is used to attach the epithelial cells to the underlying connective tissues. The other side of the wall is made from wood and leather bindings that represent the connective tissues that secrete collagen protein fibers. This is called the reticular layer. This wall or basement membrane marks the epithelial border with the underlying connective tissue and helps it resist over-stretching and tearing, which could occur at the surface.
Since the epithelial tissues are found at a surface and are exposed to the environment, they serve the function of protecting the underlying tissues from exposure. Avascularity is one way they do this.
Avascularity means that the tissues lack blood vessels. This avascular nature helps keep potentially dangerous environmental factors from getting into the blood stream and affecting the rest of the body. But this also means that epithelial layers are usually thin because they rely on diffusion to carry nutrients through the cell layers.
Another consideration for epithelium is wear and tear. Epithelium gets roughed up a lot because of its exposure to the surface. Whether you are playing beach volleyball or eating nachos with tons of jalapenos, your epithelium is subject to frequent abrasion. To compensate for the loss of cells through abrasion, epithelium can replace itself. When cells are worn off by friction or damaged by rough substances, the epithelium can regenerate, because there is a stem cell population near the basal surface that will divide to replace the lost or damaged cells. Regeneration allows the epithelial linings to be replaced very quickly in order to maintain a protective barrier for the deep tissues.
So, to recap, epithelial tissues are polar, avascular tissues that attach to other tissues and can regenerate and renew themselves.
Now that we've discussed the four common characteristics of epithelial tissue, let's talk about its functions. The four functions are physical protection, permeability, sensation, and specialized secretions. Let's take the epithelium that lines your digestive system as a general example. This lining forms a physical barrier that protects your body from dehydration, abrasion, and harmful substances. It is selectively permeable. So molecules of a specific size, charge, or solubility can pass through the cell membrane. This is how your body can absorb nutrients from your diet. The taste buds on our tongue are modified epithelial cells that give you the sensation of taste and your intestinal lining secretes digestive enzymes to help break down the food for absorption. These four functions—protection, permeability, sensation, and specialization—are common to all epithelial tissues.
Epithelial tissue is classified based on two criteria: we can classify it by the number of cell layers involved or by the cell shape. Let's talk about the number of layers first: simple epithelium has only a single layer of cells. Stratified epithelium has more than one layer. Whether the epithelium is single or stratified is very closely associated with the cell shape, the next classification scheme that we will discuss.
There are three basic shapes to epithelial cells—squamous, cuboidal, and columnar. With these shapes come specialized features. Squamous cells are flat. Since they are flat, they can be stacked, so they can be simple or stratified. An example of simple squamous epithelium is the cells that line your capillaries. Stratified squamous cells are found in your skin. These cells specialize in controlling the permeability of fluid movement using intercellular connections. Squamous cells have tight junctions, which contain a type of intercellular cement that makes them impermeable. This is important in the digestive system, for instance, because it keeps the digestive enzymes and acids from coming into contact with the deep tissues and damaging them. They also have desmosomes, which are areas of dense protein plaques called proteoglycans mixed with extracellular fibers. Squamous cells secrete cell adhesion molecules, or CAMs, that interact with the desmosome to link adjacent cell membranes. Since desmosomes are very strong sites of adherence and resist twisting and stretching, they can maintain structural integrity. Desmosomes are why your skin peels off in thin sheets when you get sunburned.
Like, squamous cells, the next type of epithelial cell can also be simple of stratified. These are known as cuboidal cells. As the name implies, cuboidal cells are shaped like cubes. They are freely permeable for correctly sized molecules, which makes them important in secretion and absorption. You find cuboidal epithelium lining the tubules to the kidneys or the ducts of many types of glands. The last type of epithelial cell is also important in secretion and absorption. It is known as columnar epithelium. These cells are tall and thin. Because of this, they don't layer. You find columnar epithelium lining the digestive tract. Since cuboidal and columnar epithelia are found in areas where absorption is important, both have specialized features to enhance their absorptive abilities. For example, cuboidal and columnar epithelia have microvilli on their apical surfaces. Microvilli are small fingerlike projections that greatly increase the surface area for absorption.
Now that we've talked about the three basic shapes of epithelial cells, we need to talk about a unique type of epithelium calls transitional epithelium. Transitional epithelium is stratified. But it has a unique ability to stretch and then return to its original shape. You find this kind of epithelium in the urinary bladder. When the cells are not stretched, there are five or more layers of cuboidal cells. As they stretch (because the bladder fills with urine) the cells flatten and the number of layers decreases.
OK. So far we have discussed the common characteristics of epithelial tissues and how they are classified. Now let's discuss the last major type of epithelial tissue. This is called glandular epithelium.
Glandular epithelium is a collection of epithelial cells that produce secretions. This is the tissue that forms glands. Glandular epithelium is classified by where the secreted product goes. Is the product released onto a surface, like sweat onto the skin, or is it dumped into circulation, like insulin into the bloodstream? Glandular epithelium that secretes its product onto a surface is called exocrine. In contrast, endocrine glands release their secretions into the interstitial fluids and blood.
Exocrine glands discharge their products onto skin or other epithelial surfaces through tubular structures called ducts. Examples of exocrine secretions are mucus, digestive enzymes, perspiration, and milk. These may all seem like very different types of glands to you. That is because exocrine glands have three different ways to secrete their products.
The first method of product secretion is by exocytosis. Merocrine glands produce and package the product in the cytoplasm and then transport it to the cell membrane where it is then expelled. Sweat glands are a type of merocrine gland. They produce mucin, which mixes with water to produce a watery perspiration. You can remember this because merely the product is released in merocrine glands.
The next method of secretion is to pack the apical portion of the cell with vesicles containing the secretory product. This apical portion, the vesicles with product and the surrounding cytoplasm, all get shed. This occurs in apocrine glands. You can remember this by thinking that A Piece of the cell is lost with the products in APocrine glands. Both "a piece" and "apocrine" start with the letters A P. Organelles in apocrine glands are highly polar and orient themselves to face the apical side, but stay in the basal portion so they won't be lost. Milk production is an example of apocrine glands in action.
The last type of exocrine gland is called Holocrine. In holocrine glands the whole cell is destroyed when the product is released. Stem cell populations replace these cells. The sebaceous glands, associated with hair follicles, are an example of a holocrine gland. When you are studying this, remember that holocrine starts with "whole."
In contrast to exocrine glands, endocrine glands secrete their products into the interstitial fluid or the bloodstream. One well-known endocrine product is hormones. Hormones regulate and coordinate the activities of other tissues, organs, and organ systems. The endocrine glands that produce hormones are ductless. This means the product is secreted directly into the extracellular fluid by exocytosis.
So far we have talked about the different types of glands and their methods of secretion. Now let's focus on the types of products secreted by exocrine glands. Exocrine glands produce three types of products: serous, which is a watery substance that contains enzymes; mucous, which is viscous and contains mucins; and mixed products, which comes from a gland that secretes both types of products. Examples of serous products are the salivary enzymes produced by the parotid glands. Mucous is produced by goblet cells in the digestive and respiratory systems. The submandibular salivary gland produces a mixed secretion.
To recap, we have talked about the epithelial tissues of the body. In our discussion, we talked about their common characteristics, how they are classified, the types of products they produce, and how those products are released into the body.
That's the end of this section.