He Epithelial lining Is one that covers the body surface of animals. Epithelial tissues, or epithelia, are those formed by one or more layers of Cells Which cover all the surfaces of the organism.
Epithelia are sets of cells with a lot of binding between them through intercellular nexuses. These close joints prevent the free circulation of substances through the formation of protective and impermeable barriers. Epithelia are in continuous regeneration, as they are subject to great wear.
Each stem cell Divides and survives one of the divisions, which in turn divide again, thus continuing the life cycle of the epithelia.
Epithelial tissues fulfill several functions, protection, segregation, absorption, sensory reception, excretion and transport functions. In the protective function is the epithelial lining tissue, which controls the entry and exit of substances.
Epithelia of segregation are able to synthesize and secrete molecules, depending on which part of the body is found. Absorption epithelia, as its name indicates, have the functionality of absorbing molecules through microvilli.
Epithelia responsible for sensory reception, have nerve endings in the sense organs. Through the epithelia of excretion toxins and debris are released.
Transport epithelia move the cilia to carry substances. You may also be interested in reading about Epithelial cells: characteristics and diseases .
Characteristics of lining epithelial tissue
The lining epithelial tissue is that covering the body with cells closely attached to one another. It has little intercellular space, and to prevent the flow of molecules, it has an extracellular matrix.
The cells that make up the lining epithelial tissue age very soon, since they are subjected to greater wear, than cells of other parts of the body. These cells are worn more by the free part that is in contact with the outside, and to regenerate it does it through its deep part, that has less wear.
These cells form an extracellular matrix, also known as basal lamina or lamina propria. This sheet separates the lining fabric from the connective tissue. The connective tissue is what provides the lining tissue with nutrients and oxygen, since the epithelial tissue has no blood or lymphatic vessels.
To provide the nutrients, the connective tissue transports them through capillary beds, through transudation through the extracellular matrix. Transudate is basically a filtration of extravascular fluid, which is not found in the capillaries. The coating fabric depends on this trasudado to be able to maintain its metabolism.
The basal lamina is a dense membrane formed mostly by electrodensing material. Electrodense structures are easier to distinguish in a microscope, as they are darker. This depends on the amount of lipids and water, the more lipids it contains, the less electrodens it will be and the membrane will become clearer in a microscope .
A distinction is made between the cells depending on their position in the lining fabric. Those that are most in contact with the surface or the outside, are called apical pole. Those that are in the interior, or in contact with the basal lamina are known by basal pole.
Within the apical pole, which is in contact with the outside, we can find microvellosities, stereocilia, cilia and flagella. Microvilli are extensions of cylindrical shape that increase the absorption surface.
Stereocilies, which are pear shaped, promote transport and nutrient absorption. On the other hand, the cilia, resemble microvilli, although they have greater width. The flagella, similar to the cilia, have an even larger size.
In the basal pole, the part closest to the membrane, we find invaginations and hemidesmosomes. Invaginations are folds of the membrane, whereas hemidesmosomes are desmosomes that bind the epithelium with the membrane.
Desmosomes are cellular structures that maintain cohesion between adjoining cells.
Classification of epithelial tissue
To classify the different types of epithelial tissue, we are based on the arrangement, body part where they are, and morphology, ie the number of layers between the surface and the sheet.
Simple or mono-stained epithelium
This tissue is found in areas of low wear, formed only by a layer of cells, and participates in processes of diffusion, osmosis, filtration and absorption. We can in turn classify it into several categories.
- Simple or pavement scaly epithelium
- Simple cuboidal or cuboidal epithelium
- Simple cubic epithelium with microvilli
- Simple cylindrical epithelium or simple columnar
- Secretory simple cylindrical epithelium
- Simple cylindrical epithelium with absorbent cells
- Simple cylindrical epithelium with hair cells
Stratified epithelium
It is found in areas with wear or friction, and is formed by more than one layer of cells. It is perpendicular to the membrane. The classification of stratified epithelium focuses only on cell morphology and upper layer, and may be:
- Non-keratinized stratified squamous epithelium
- Stratified keratinized squamous epithelium
- Stratified cuboidal epithelium
- Laminated cylindrical epithelium
- Transitional epithelium
- Pseudostratified
Transitional or polymorphic epithelium
The transitional epithelium is formed by several layers of cells and was originally thought to be a transition between the cylindrical stratified and the stratified squamous. But after several investigations, he is considered a different type.
This is normally found in the Urinary tract . The surface of this epithelium is shaped like domes, and for example, when the bladder is distended, these domes flatten, causing a narrowing of the epithelium.
Pseudostratified cylindrical epithelium
It resembles the stratified epithelium, but only has a layer of cells, where the nuclei of these are at different levels, making it appear stratified.
Only some of the cells that make up this epithelium come to touch the outside. Within this we find the following distinctions:
- Ciliated Pseudostratified cylindrical epithelium
- Ciliated pseudostratified cylindrical epithelium
- Pseudostratified cylindrical epithelium with stereocilia
References
- S. Beckett (1976) Biology, A modern Introduction. Oxford University Press.
- Johnstone (2001) Biology. Oxford University Press.
- Byrum (2005) Cells. Popular Prakashan.
- Lewin (2007) Cells. Jones & Bartlett Learning.
- Ian Freshney, Mary G. Freshney (2002) Culture of Epithelial Cells. Wiley Publishers.
- Andrew J. Shaw (1996) Epithelial Cell Culture. Oxford University Press.
- Ashton Acton (2013) Epithelial Cells. Scholarly Editions.