Epithelial Structure & Function: Types, Adhesion, Basement Membrane, Study notes of Cell Biology

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Epithelial Structure

Peter Takizawa

Department of Cell Biology

• Type of epithelia

• Adhesion between epithelial cells

• Basement membrane

• Epithelial cell polarity

• Cell renewal

Epithelia perform a variety of functions.

Protection

Absorption

Diffusion

Secretion

Epithelia serve a number of functions. They offer protection to organs such as the skin. The absorb material in the intestine. They secrete material in glands. And they allow for passive diffusion of gases in the lung and blood vessels.

Layers and shape of cells in epithelium facilitate its

functions.

To accomplish these different functions, epithelia come in a variety of structures. Most epithelia are classified based on two criteria: shape and layers of cells. A single layer of cells is called simple whereas a epithelium with two or more layers of cells is called stratified. If the most superficial layer of cells is flat, the epithelium is referred to as squamous. Squamous epithelia facilitate diffusion of gases and other small molecules. Epithelia where the cells are about as tall as they are wide are called cuboidal. If the cells are taller than they are wide, then they are called columnar. Cuboidal and columnar cells are usually involved in secretion and/or absorption and need more cytoplasmic volume to accommodate the organelles needed for these activities. Two special types of epithelia are pseudostratified and transitional.

Epithelial cells are held together by three junctional

complexes.

Adhering junctions

Desmosomes

Gap junctions

Integrins

Tight Junctions

Epithelial cells are held together by three junctional complexes. All epithelia will have adhering junctions, but only some will have desmosomes. Epithelia also contain tight junctions which control the diffusion of material between epithelia cells.

Adhering junctions form a belt-like adhesion zone

around epithelial cells.

Adhering junctions have characteristic arrangement in epithelial cells. They form continuous belt around the circumference of epithelial cells. The adherins junction are linked to bundles of actin filaments that also wrap around the cell. Myosin filaments can pull on the actin filaments to contract the cell. Because the adherins junctions are located closer to the apical surface, contraction causes the apical surface to shrink. This allows epithelia to form a tube from a sheet of cells.

Tight junctions: controlling paracellular diffusion

Tight junctions prevent diffusion of molecules

between epithelial cells.

Lumen Epithelia usually separate two compartments. Often one compartment is an external space or lumen of a tube and the other is the rest of a tissue or organ. The apical side of the epithelial cells faces the external space or lumen and the basal side faces the rest of the organ. One role for epithelia is to control the mixing of material between the two compartments. To accomplish this, epithelia must control the ability of small molecules and ions to diffuse between cells or paracellular diffusion. Tight junctions that link adjacent epithelial cells determine how permeable an epithelia is to ions and small molecules. Some epithelia are very permeable (e.g. intestine) where as others are restrictive (e.g. bladder).

Claudins are the primary functional component of

tight junctions.

Cell 1 Cell 2 Tight junctions contain more than 50 different types of proteins. Claudins are the proteins that primarily determine the permeability of the tight junction. Claudins contain four transmembrane domains and interact with cluadins in adjacent cells. The intracellular domains of claudins are linked indirectly to actin filaments by a set of proteins called ZO proteins. The interaction with actin filaments help stabilize the claudins in the cell membrane. Also, myosin filaments can generate tension on the actin filaments to loosen the tight junctions.

Claudins are the primary functional component of

tight junctions.

Tight junctions contain more than 50 different types of proteins. Claudins are the proteins that primarily determine the permeability of the tight junction. Claudins contain four transmembrane domains and interact with cluadins in adjacent cells. The intracellular domains of claudins are linked indirectly to actin filaments by a set of proteins called ZO proteins. The interaction with actin filaments help stabilize the claudins in the cell membrane. Also, myosin filaments can generate tension on the actin filaments to loosen the tight junctions.

Interactions between claudins generates size

restrictive pores.

The extracellular domains of claudins interact to form size-restrictive pores. The permeability of an ion or molecule is inversely related to size.

Leakiness of tight junctions varies by type claudin

expressed in cells.

claudin- claudin- claudin- claudin-

Stomach

Small

Intestine

Large

Intestine

Bladder

The permeability of epithelia varies between organs up to 10,000 fold difference. The epithelia of the small intestine is more permeable that the epithelia of the bladder. There are 24 different claudin genes and each encodes a protein with different permeability properties. Some claudins are selective for certain ions. For example, mutations in claudin 16 lead to magnesium wasting because the kidney epithelia becomes permeable to magnesium. Epithelia generate different degrees of permeability by expressing different combinations of claudin genes.

All epithelia rest on a basement membrane.

All epithelial cells are attached on their basal surface to a basement membrane. The basement membrane provides some mechanical support as it tethers together a sheet of epithelial cells. It also supports the growth and survival of the epithelia as it controls the access of epithelia to nutrients, ions, proteins and oxygen. Epithelia lack their own blood supply and rely on the capillaries in the underlying tissues. All the nutrients from the blood must cross the basement membrane to reach the epithelial cells. The basement membrane also regulates the growth and division of epithelial cells.

Basement membrane separates epithelial cells

from surrounding tissue.

Basement membrane

Epithelial Cells