Table of Contents
Types of Epithelium
There are three basic types of epithelial cells. Each type is further categorized into a simple type if it is a single layer of cells or stratified if arranged in multiple layers.
Squamous epithelium is made up of flattened cells. Simple squamous epithelium is present in areas of the body where diffusion, absorption or filtration takes place. 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 up of cuboid shaped cells with central nucleus. They are present in areas of the body where secretory function is required such as pancreatic ducts.
Columnar epithelium consists of elongated cells. These cells may have cilia present on them and form pseudo-stratified ciliated columnar epithelium. The cilia entrap particles and move them towards the external orifice for example ciliated columnar epithelium in air way.
Surfaces of Epithelial Cells
An epithelial cell typically has four surfaces:
Apical surface that faces the lumen. It is also known as luminal border.
Two lateral surfaces through which one epithelial cell communicates with other epithelial cells on each side.
Basolateral surface is opposite to the apical surface and faces the basement membrane.
Modification of Epithelial Cells
Cilia, villi and microvilli
As mentioned above, the cilia are present on columnar epithelium and produce to and fro motion to entrap particles and move them towards a specific direction. 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 and increase the surface area for absorption.
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 also carry two or more molecules at the same time. When these molecules travel in the same direction, it 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 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 nephron. Aquaporins allow transcellular flow of water molecules.
Epithelial cell junctions
A cell junction is a multi-protein complex that provides contact between neighbouring cells or between cell and extracellular matrix. There are three types of cell junctions present in an epithelial cell:
They act as barrier to regulate movement of molecules from one cell to another. 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.
These are also known as communicating junctions. They allow cytoplasmic connection between two neighbouring cells without an 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 connexon.
Anchoring junctions are made up 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 are made up of actin and cadherin protein filaments. As actin is a contractile protein so adhering junctions are also responsible for the change in shape of sheets of cells.
- Desmosomes are made up of intermediate filaments, composed of keratin and desmin. Cadherin acts as trans-membrane linker.
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 is the transport of a molecule from lumen into the blood. In this case, the molecule which needs to be absorbed has to travel 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.