7 Characteristics of epithelial cells

Epithelial cells are cells grouped closely together in layers. Among its functions are protection, secretion, absorption, reception of sensory stimuli and delimitation. The most important types are glandular epithelial cells, sensory epithelial cells and lining epithelial cells, although they can also be classified according to the shape, additional accessories they have or by where they are located.

Living beings are made of millions of cells, but these are not simply arranged, but there are different types according to the degrees of specialization that exist between them in order to fulfill the particular functions of each organ. One of these types of cells are epithelial cells. They are so important for living organisms because of the functions they carry out, which are the first to form during embryonic development. In this BIOencyclopedia article we will be explaining what epithelial cells are, their functions and types.

What are epithelial cells

Epithelial cells are closely packed cells that form sheets, and are at the ends of tissues.

They have the characteristic of having great cohesion between them, which is called epithelium. These epithelial cells have structures that help attach the cytoskeletons of each individual cell to each other, while at the same time anchoring it to the extracellular matrix so that it does not move around.

They are found on the surfaces of organs, glands, cavities or ducts, always in the outermost part and moving inward.

These cells have a polar structure, where an apical or upper pole is distinguished, which is the external part, and a lower basal pole, where they are held by a membrane or basal lamina.

The cellular epithelium can be found in a single layer, or made up of several layers of cells.

In epithelial tissue there are no blood vessels, and there is a constant turnover of cells to produce new cells and eliminate old ones.

Under the epithelial cell is the basal lamina, and the fibroreticular lamina.

Characteristics of epithelial cells

Epithelial cells are one of the main types of cells found in the body. They line the surfaces of organs, cavities, and blood vessels. Here are some key characteristics of epithelial cells:

  1. Cell Shape: Epithelial cells can have different shapes, including squamous (flat and thin), cuboidal (cube-shaped), and columnar (elongated and tall). The shape of epithelial cells is related to their function and location in the body.
  2. Cell Layers: Epithelial cells can form single layers (simple epithelium) or multiple layers (stratified epithelium). Single-layered epithelia are involved in absorption, secretion, and diffusion, while stratified epithelia provide protection.
  3. Cell Junctions: Epithelial cells are tightly joined together by specialized structures called cell junctions. These junctions provide strength and prevent substances from leaking between cells. There are different types of cell junctions, including tight junctions, adherens junctions, desmosomes, and gap junctions.
  4. Apical Surface: The apical surface of epithelial cells is the side that faces the external environment or a body cavity. This surface may have microvilli or cilia, which increase surface area or facilitate movement, respectively. For example, the epithelial cells lining the small intestine have microvilli that help with absorption.
  5. Basal Surface: The basal surface of epithelial cells is the side that faces the underlying connective tissue. This surface is anchored to the basement membrane, a specialized extracellular matrix. The basement membrane provides support and acts as a barrier between epithelial cells and underlying tissues.
  6. Specializations: Epithelial cells can have various specializations depending on their location and function. For example, some epithelial cells secrete mucus, such as the goblet cells in the respiratory tract. Others have cilia that help move substances, like the ciliated cells in the trachea.
  7. Regeneration: Epithelial cells have a high regenerative capacity. They can rapidly divide and replace damaged or lost cells. This ability ensures the maintenance and repair of epithelial tissues throughout the body.

In summary, epithelial cells are specialized cells that line the surfaces of organs, cavities, and blood vessels. They can have different shapes and form single or multiple layers. Epithelial cells are tightly joined together by cell junctions and have distinct apical and basal surfaces. They can also have specializations and possess a high regenerative capacity.

Functions of epithelial cells

  • They delimit one area from another, differentiating between exterior and interior.
  • They protect the organ, or cavity.
  • Some can absorb substances from the outside, as occurs in the intestine.
  • Others have a secretory function, such as the salivary tissue that secretes saliva into the oral cavity.
  • They may have sensory capacity to connect with the nervous system to react to external stimuli.

On the other hand, the specificity of the organs is determined by that of the epithelial cells. That is, an organ is capable of carrying out a function due to the specialization of epithelial cells. Not all epithelial cells will function the same, as we can see in the comparison between lung epithelial cells, whose function is to capture oxygen, or that of the mammary glands, which is to secrete milk.

Types of epithelial cells

  • Glandular epithelial cells: are functional units of the glands. They synthesize a secretion, which is a particular product produced in granules, distributed to the surface of the cell so that it can find an outlet. Examples are hormones, or milk.
  • Lining epithelial cells: they are those that have a barrier function, located on internal and external surfaces. Examples of these cells are those located on the surface of the intestine, lung, or liver. Under this category there are transitional cells that are only in the urinary tract, polystratified cells with several layers, or monostratified cells with a single layer.
  • Sensory epithelial cells: these are cells that receive stimuli from the outside. As an example, there is the olfactory mucosa or the gustatory epithelium of the tongue.

On the other hand, we can classify them according to the shape of the epithelial cells. So we have:

  • Flat or scaly: they can be in the form of a simple layer, or stratified in several layers.
  • Cubic: they are only grouped in a simple way.
  • Cylindrical: they can be found simple, stratified or pseudostratified.

Epithelial cells are also classified according to the additional structures they have on the surface. These serve as adaptations for their particular functions. Such are:

  • Flagellated epithelial cells: they are sensory, or they can serve to transport liquids such as mucus.
  • Ciliated epithelial cells: their purpose is to transport liquids.
  • Epithelial cells with microvilli: their function is absorption.
  • Epithelial cells with stereocilia: they are similar to microvilli, but longer.

Finally, there are types of epithelial cells depending on where they are found. There are quite a few, and we will mention only a few to give a general idea:

  • Respiratory epithelial cells
  • Renal epithelial cells
  • Intestinal epithelial cells

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