When one speaks of this type of tissue, one recalls primarily the skin, although epithelial tissue is, in principle, a kind of “border guard”. It separates organs from each other, it is also contained in the mucous membranes. The epithelial tissue is heterogeneous, the structure and functions of which are interconnected in a special way for each of the types of this tissue. What types of speech? Read on.
A characteristic common feature of all species is cells that fit tightly together, often located in layers, which is explained by the barrier function of epithelial tissue. Also, all types of epithelial tissue are united by the fact that they are always separated from neighboring structures by connective tissue - this formation is called the basement membrane.
In general, the epithelium is simple (in one layer) and layered (in several). Accordingly, the epithelial tissue structure can have very different.
Simple epithelium may be slightly flattened in shape. This kind is called squamous. It lines the lungs, blood vessels, and cavities of the heart. Also, such cells are located in the upper layers of the skin and oral mucosa.
It is easy to guess about the shape of the cells of the cubic epithelium. Such cells are found in the glands, ducts of the glands, and renal tubules. In addition, they constitute the primordial epithelium in the ovaries of women and male testicles.
Prismatic epithelium is simple and layered. The cells are in the form of elongated columns. The nuclei in such cells are “hidden” closer to the basement membrane and away from the surface. Well, in the stomach, whose surface they line, it is very rational - the environment is too aggressive there. Some of these cells are part of the sensory organs - smell, taste and hearing.
The ciliated prismatic epithelium has outgrowths of a special shape, which are able to advance what is next to them with quick wavy movements. They "live" in the nose and ... in the fallopian tubes in women, which allows them to move the egg to the uterus.
Glandular epithelium is called prismatic, containing secreting cells. Sometimes there are more secreting cells than columnar cells. Where do cells capable of creating a secret come from? These are epithelial tissue cells that have passed the path of specialization. And “specialists” they can be very different. Engaged in the production of enzymes, hormones, milk, mucus, sweat, earwax and saliva. Of course, all of these substances are unable to produce unicellular glands; it is the work of glands consisting of many epithelial cells. And unicellular cells are called goblet cells. Initially, they were called enterocytes, because most of these cells are in the small intestine. But then it turned out that they are present in the conjunctiva of the eye, and in the respiratory system, and in the ducts of the glands.
Where the processes of friction are great and injuries are likely, the body builds a complex epithelium - the one that always consists of several layers. It is about the skin and mucous membranes of the mouth. On the surface of the skin there are cells in which there is a lot of keratin protein, there is not enough keratin in the mouth of the cells, therefore, the skin and oral mucosa are significantly different to the touch.
Epithelial tissue functions have very different. The list below is not completely complete.
Protective function . The epithelium prevents mechanical damage, chemical burns, puts a barrier against infection and protects the underlying cells from water loss.
Sensory function : specialized cells of the epithelial tissue form the sensory organs.
Secretory function : described in detail in the section on glandular epithelium.
Absorbent : nutrients are absorbed through the epithelium in the small intestine.
Excretion : sweat glands secrete unnecessary substances, and the epithelium in the kidneys is able to participate in filtration, and in reabsorption too.
Diffusion : through the thin epithelium in the respiratory system, gases penetrate into and out of the body.
Purification function : in the respiratory tract, the epithelium cleans the air of dust.
Friction reduction : Vascular epithelium reduces friction between the vessel wall and blood cells.
As you can see, there are many functions, because there are many types. You will read more about this in good textbooks on histology.