Even in the school course on the anatomy of children, they learn a simple biological regularity in the structure of living multicellular creatures: the basis of everything is the cell. Their group gives rise to tissues, which, in turn, form organs. The latter are combined into systems that carry out vital functions, metabolic processes, and so on.
Therefore, what are tissues, their structure and functions, is studied from the middle level of the school curriculum. Consider what types of tissues are found in the human body, what is the epithelial variety of these structures and what is its significance.
Animal tissue: classification
Tissues, their structure and functions, features of development and functioning are of great importance in the life of all living creatures that are capable of their formation. They perform a protective function, secretory, organo-educational, nutritious, heat-insulating and many others.
In total, there are 4 types of tissues characteristic of the structure of the human body and highly organized animals.
- Different types of epithelial tissue or integument (skin).
- Connective tissue, represented by several main varieties: bone, blood, fat and others.
- Nervous, formed by a kind of branched cells.
- Muscle tissue, which forms, together with the skeleton, the musculoskeletal system of the whole organism.
Each of these tissues has its own localization place, the method of formation and performs certain functions.
General characteristics of epithelial tissue
If we characterize the types of epithelial tissues in general terms, we should highlight several basic features that all of them possess, each to a greater or lesser extent. For example:
- the absence of a substance located between the cells, which makes the structures tightly adjacent to each other;
- a unique method of nutrition, which consists not in the absorption of oxygen from blood vessels, but in diffusion through the basement membrane from the connective tissue;
- unique ability to restore, that is, regeneration of the structure;
- cells of this tissue are called epithelial cells;
- each epithelial cell has polar ends, so all tissue ultimately has polarity;
- under any type of epithelium is the basement membrane, which is important;
- localization of this tissue is carried out in the body by layers or bands in certain places.
Thus, it turns out that the varieties of epithelial tissue are united by general patterns in location and structural organization.
Types of Epithelial Tissue
They can be distinguished three main.
- Superficial epithelial tissue. The epithelium of its structure is especially dense, because it performs primarily a protective function. It forms a barrier between the outside world and the internal part of the body (skin, external integument). In turn, this view includes several more components, which we will consider further.
- Glandular epithelial tissue. The glands whose ducts open outward, that is, exogenous. These include lacrimal, sweat, milky, sebaceous genitalia.
- Secretory varieties of epithelial tissue. Some scientists believe that part of the nerve cells eventually passes into epithelial cells and forms this type of structure. The main function of such an epithelium is to perceive irritations, both mechanical and chemical, transmitting a signal about this to the appropriate authorities of the body.
These are the main types of epithelial tissue that are secreted in the human body. Now consider a detailed classification of each of them.
Classification of Epithelial Tissues
It is quite capacious and complex, since the structure of each epithelium is multifaceted, and the functions performed are very different and specific. In general, all existing epithelium species can be combined into the following system. The entire integument epithelium is divided as follows.
1. Single layer. The cells are located in one layer and directly contact with the basement membrane, in contact with it. His hierarchy is like that.
A) Single row, divided into:
Each of these types can be limbless and limbless.
B) Multirow, including:
- prismatic ciliary (ciliated);
- prismatic unreserved.
2. Multilayer. Cells are arranged in several rows, so contact with the basement membrane occurs only at the deepest layer.
A) Transitional.
B) Keratinizing flat.
C) Non-keratinized, subdivided into:
The glandular epithelium also has its own classification. It is divided into:
- unicellular;
- multicellular epithelium.
In this case, the glands themselves can be endocrine, excreting secretion into the blood, and exocrine, having ducts in the epithelium in question.
The sensory tissue does not have structural units. It consists of nerve cells that form it converted into epithelial cells.
Single layer squamous epithelium
Got its name for the structure of cells. Its epithelial cells are thin and flattened structures that are tightly interconnected. The main task of such an epithelium is to provide good permeability to molecules. Therefore, the main places of localization:
- lung alveoli;
- walls of blood vessels and capillaries;
- lining the cavity of the inner side of the peritoneum;
- covers serous membranes;
- forms some ducts of the kidneys and renal bodies.
The epithelial cells themselves are of mesothelial or endothelial origin and are characterized by the presence of a large oval nucleus in the center of the cell.
Cubic epithelium
Such types of epithelial tissue as a single-layer and multilayer cubic epithelium have a somewhat special cell structure in shape. For which, in fact, they got their name. They are cubes of slightly irregular shape.
A single-layer cubic is localized in the tubules of the kidneys and serves there as a permeable membrane. The nuclei in such cells are rounded, displaced to the cell wall.
The multilayer cubic epithelium is located in the form of a series of a deep layer in contact with the basement membrane. All other external structures cover it from above in the form of flat scales of epithelial cells. This type of tissue forms many organs:
- cornea of the eye;
- esophagus;
- oral cavity and others.
Prismatic epithelium monolayer
This is one of the types of tissue that is also called epithelial. Structural features, functions are explained by the shape of the cells: cylindrical, elongated. The main places of localization:
- intestines;
- small and rectum;
- stomach;
- some tubules of the kidneys.
The main function is to increase the suction surface of the working body. In addition, ducts of specialized goblet cells secreting mucus open here.
Types of epithelial tissues: single-layer multilayer
This is a type of integument epithelium. Its main task is to provide the external integument of the respiratory tract, which they are lined with. All cells are in close contact with the basement membrane, the nuclei in them are rounded, located at an uneven level.
Atrial this epithelium is called because the edges of the epithelial cells are framed by cilia. In total, 4 varieties of the structure of the cells can be distinguished:
- basal;
- atrial
- long insertion;
- goblet mucus-forming.
In addition, a single-layered multilayer epithelium is found in the reproductive ducts and the corresponding system (in the oviducts, testicles, and so on).
Stratified transitional epithelium
The main distinguishing feature of any stratified epithelium is that its cells can be stem cells, that is, those that are capable of differentiating into any other tissue species.
Specifically, transient epithelial cells are part of the bladder and corresponding ducts. They are divided into three large groups, united by a common ability - to form tissues with high extensibility.
- Basal cells are small cells with round-shaped nuclei.
- Intermediate.
- Superficial - cells of a very large size, most often in the form of a dome.
There is no contact with the membrane in these tissues, therefore, the nutrition is diffuse from the connective tissue of a loose structure located under them. Another name for this type of epithelium is urothelium.
Multilayered non-keratinizing epithelium
This type includes epithelial tissues of the body that line the inner surface of the cornea of the eye, the structure of the oral cavity and esophagus. All epithelial cells can be divided into three types:
- basal;
- prickly;
- flat cells.
In organs, they form cords of a flat structure. They are called non-keratinizing for their ability to peel off over time, that is, to be removed from the surface of an organ, being replaced by younger analogues.
Stratified keratinized epithelium
Its definition may sound as follows: this is the epithelium, the upper layers of which are capable of differentiation and the formation of solid scales - corneas. Among the entire integument epithelium, this is the only one for which such a feature is characteristic. Everyone can see it with the naked eye, because the main organ of this layer is skin. The composition includes epithelial cells of different structures, which can be combined into several main layers:
- basal;
- prickly;
- grainy;
- brilliant;
- horny.
The latter is the densest and thickest, represented by horny scales. It is their desquamation that we observe when the skin of the hands begins to peel off under the influence of adverse environmental conditions or old age. The main protein molecules of this tissue are keratin and filagrin.
Glandular epithelium
In addition to the integumentary, glandular epithelium is of great importance. It is another form that epithelial tissue has. The tissues under consideration and their classification are very important for a correct understanding of the place of their localization and the functions performed in the body.
So, the glandular epithelium is very different from the integumentary and all its varieties. Its cells are called glandulocytes, they are an integral part of various glands. In total, two main types can be distinguished:
- exogenous glands;
- endogenous.
Those that throw their secrets directly into the glandular epithelium, and not the blood, belong to the second group. These include: salivary, dairy, sebaceous, sweat, lacrimal, sexual.
There are also several options for secretion, that is, the removal of substances out.
- Eccrine - cells secrete compounds, but do not lose their integrity in the structure.
- Apocrine - after secretion is removed, they are partially destroyed.
- Holocrine - cells are completely destroyed after performing functions.
The work of the glands is very important and significant. For example, their function is protective, secretory, signal, and so on.
The basement membrane: functions
All types of epithelial tissues are in close contact with at least one layer of their own with such a structure as the basement membrane. Its structure consists of two bands - a light one, consisting of calcium ions, and a dark one - including various fibrillar compounds.
It is formed from the joint production of connective tissue and epithelium. The functions of the basement membrane are as follows:
- mechanical (hold epithelial cells together, maintaining the integrity of the structure);
- barrier - selective permeability to substances;
- trophic - nutrition;
- morphogenetic - providing high ability to regenerate.
Thus, the joint interaction of epithelial tissue and the basement membrane leads to a coordinated and orderly work of the body, the integrity of its structures.
In general, not only epithelial tissue is very important. Tissues and their classification are considered at all levels of education related to medicine and anatomy, which proves the importance of these topics.