Surface Modification of Epithelial Cell Membrane and Trans-Epithelial Potential

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Image: “Illustration of tight junction, gap junction, and Desmosomal junction (desmosome)” by Boumphreyfr – Own work. License: CC BY-SA 3.0

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Types of Epithelium

There are three basic types of epithelial cells. Each type is further categorized into simple type, if it is a single layer of cells, or stratified, if arranged in multiple layers.

Squamous epithelium is made of flattened cells. Simple squamous epithelium is present in areas of the body where diffusion, absorption or filtration takes place as the flattened structure of the cells decreases the diffusion pathway. Stratified squamous epithelium being multilayered protects organs from mechanical or chemical insult.

Cuboidal epithelium is made of cuboid shaped cells with a central nucleus. They are present in areas of the body where secretory function is required such as the pancreatic ducts.Columnar epithelium consists of elongated cells. These cells may have cilia present on their surface and form pseudo-stratified ciliated columnar epithelium. The cilia entrap particles and move them towards the external orifice, for example, ciliated columnar epithelium in the air ways.

Surfaces of Epithelial Cells

An epithelial cell typically has four surfaces:

• Apical surface, which faces the lumen. It is also known as luminal border.
• Two lateral surfaces, through which one epithelial cell communicates with the epithelial cells on each side.
• Basolateral surface, which is opposite to the apical surface and faces the basement membrane.

Modification of Epithelial Cells

Cilia, villi and microvilli

As mentioned above, thecilia are present on columnar epithelium and produce to and fro motion to entrap particles and move them towards a specific direction parrel to the surface of the epithelium. They are, therefore, included in the primary defense of the body, in addition to other mechanical barriers. Villi and microvilli are present on absorptive surfaces as they increase the surface area for absorption without increasing the size of epithelium.

Channel proteins and carrier proteins

In order to move molecules from the lumen into the cells, from cells into the blood or vice versa, channel proteins and carrier proteins are present on the apical and baso-lateral surfaces of the epithelial cells.

The channel proteins allow small ions to move across the cell membrane. These are often gated to open and close in response to a certain stimulus.

Carrier proteins move large molecules across the cell membrane. In case the molecules are moved against the concentration gradient, these carrier proteins utilize an ATP molecule. They can carry two or more molecules at a time. When these molecules travel in the same direction, the action is called co-transport. When they move in opposite directions, it is termed as counter-transport.

Sodium-potassium (Na/K) pump
The sodium-potassium pump is a carrier protein that moves three sodium ions out and two potassium ions into the cell, against the concentration gradient. An ATP molecule is broken to give the energy required for the transport of ions.

Aquaporins
Aquaporins are specialized channel proteins present on the apical and basolateral surface of the epithelial cells. These are present in areas where excessive water reabsorption is required, for example, the collecting duct of a nephron. Aquaporins allow transcellular flow of water molecules.

Epithelial cell junctions

A cell junction is a multi-protein complex that provides contact between neighboring cells or between cell and extracellular matrix. There are three types of cell junctions present in an epithelial cell:

1. Tight junctions:

They act as a barrier to regulate the movement of molecules from one cell to another through extracellular spaces by diffusion or active transport. These are present on the lateral surfaces of the epithelial cells. They can be tighter with high electrical resistance or leakier with low electrical resistance.

Gap junctions:

These are also known as communicating junctions. They allow cytoplasmic connection between two neighboring cells without the involvement of extracellular fluid. The connexin proteins form a cylindrical structure between the lateral surfaces of two adjacent epithelial cells. This cylindrical structure with pore is called the connexon.Their main role is to allow the free movement between cells of ions and small molecules.

Anchoring junctions:

Anchoring junctions are made of anchoring proteins, which link the cytoskeleton of one cell to the cytoskeleton of another cell or to the extracellular matrix. Adhering junctions and desmosomes are present on the lateral surface of two epithelial cells, while hemi-desmosomes are present on the basolateral surfaces.

Adhering junctions

Consist of actin and cadherin protein filaments. Since actin is a contractile protein, adhering junctions are also responsible for the change in shape of sheets of cells.These junctions are meant to hold epithelial cells together.

Desmosomes are made of intermediate filaments composed of keratin and desmin. Cadherin acts as a trans-membrane linker.They are specialized to hold cells tightly together.

Hemi-desmosomes form a bridge between the epithelial cells and extracellular matrix, on the basolateral surface. They have intermediate filaments as cytoskeletal anchor and Integrin as trans-membrane linker.

Trans-Epithelial Transport

Trans-epithelial transport is the transport of a molecule from lumen into the blood. In this case, the molecule, which needs to be absorbed, has to travel through more than one membrane surface.

Trans-epithelial membrane potential difference is therefore measured, which is the voltage across an epithelium and the sum of the membrane potentials for the outer and inner cell membranes.

It is also a useful diagnostic tool for the diagnosis of cystic fibrosis, in which the trans-epithelial membrane potential is more negative. In cystic fibrosis, the impaired cystic fibrosis trans-membrane regulator (CFTR) causes increased secretion of chloride ions into the lumen and increased reabsorption of sodium ions into the epithelial cells. This results in thick mucus secretions.