Epithelial polarity
Cell polarity is a fundamental feature of many types of cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s. Epithelial cells are one example of a polarized cell type, featuring distinct 'apical', 'lateral' and 'basal' plasma membrane domains. Epithelial cells connect to one another via their lateral membranes to form epithelial sheets that line cavities and surfaces throughout the animal body. Each plasma membrane domain has a distinct protein composition, giving them distinct properties and allowing directional transport of molecules across the epithelial sheet. How epithelial cells generate and maintain polarity remains unclear, but certain molecules have been found to play a key role.

Apical membranes

In the context of renal tubule physiology, the term apical membrane refers to the membrane which forms the outside barrier of the lumen of the tubule. The principal function of this membrane is to excrete metabolic waste products from the cytoplasm of the epithelial cell into the lumen of the tubule. From there, it is transported out of the body as urine. A secondary role of the apical membrane is to absorb desirable substrates, such as glucose, for recycling.

A variety of molecules are located at the apical membrane, but only a few key molecules act as determinants that are required to maintain the identity of the apical membrane and, thus, epithelial polarity. These molecules are the proteins Cdc42
Cell division control protein 42 homolog also known as CDC42 is a protein involved in regulation of the cell cycle. In humans, CDC42 is encoded by the CDC42 gene.- Function :...

, atypical protein kinase C
Protein kinase C
Protein kinase C also known as PKC is a family of enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins. PKC enzymes in turn are activated by signals such as increases in...

 (aPKC), Par6
Partitioning defective 6 homolog alpha is a protein that in humans is encoded by the PARD6A gene.-Interactions:PARD6A has been shown to interact with CDC42, RAC1, ECT2 and Protein kinase Mζ.-Further reading:...

, Par3
Partitioning defective 3 homolog is a protein that in humans is encoded by the PARD3 gene.-Interactions:PARD3 has been shown to interact with JAM3, JAM2, PVRL3 and PRKCI.-Further reading:...

/Bazooka, Crumbs, "Stardust" and protein at tight junctions (PATJ). These molecules appear to form two distinct complexes: an aPKC-Par3-Par6 "aPKC" (or "Par") complex that also interacts with Cdc42; and a Crumbs-Stardust-PATJ "Crumbs" complex. Of these two complexes, the aPKC complex is the most important for epithelial polarity, being required even when the Crumbs complex is not. Crumbs is the only transmembrane protein in this list and the Crumbs complex serves as an apical cue to keep the aPKC complex apical during complex cellular shape changes.

Basolateral membranes

In the context of renal tubule physiology, the term basolateral membrane refers to the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

 which is oriented away from the lumen
Lumen (anatomy)
A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...

 of the tubule. The principal function of this membrane is to take up metabolic waste products into the epithelial cell for disposal into the lumen where it is transported out of the body as urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

. A secondary role of the basolateral membrane is to allow the recycling of desirable substrates, such as glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, that have been rescued from the lumen of the tubule to be secreted into the interstitial fluid
Interstitial fluid
Interstitial fluid is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid...


Basal and lateral membranes share common determinants, the proteins Lethal Giant Larvae (Lgl), Discs Large (Dlg) and Scribble (Scrib). These three proteins all localize to the basolateral domain and are essential for basolateral identity and for epithelial polarity.

Mechanisms of polarity

How epithelial cells polarize is still not fully understood. Some key principles have been proposed to maintain polarity, but the mechanisms behind these principles remain to be discovered.

The first principle is positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...

. In computer models, a molecule that can be either membrane-associated or cytoplasmic can polarize when its association with the membrane is subject to positive feedback: that membrane localization occurs most strongly where the molecule is already most highly concentrated. In the yeast saccharomyces cerevisiae
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...

, there is genetic evidence that Cdc42 is subject to positive feedback of this kind and can spontaneously polarize, even in the absence of an external cue. In the fruit fly Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

, Cdc42 is recruited by the aPKC complex and then promotes the apical localization of the aPKC complex in a probable positive feedback loop. Thus, in the absence of Cdc42 or the aPKC complex, apical determinants cannot be maintained at the apical membrane and consequently apical identity and polarity is lost.

The second principle is segregation of polarity determinants. The sharp distinction between apical and baso-lateral domains is maintained by an active mechanism that prevents mixing. The nature of this mechanism is not known, but it clearly depends on the polarity determinants. In the absence of the aPKC complex, the baso-lateral determinants spread into the former apical domain. Conversely, in the absence of any of Lgl, Dlg or Scrib, the apical determinants spread into the former baso-lateral domain. Thus, the two determinants behave as if they exert mutual repulsion upon one another.

The third principle is directed exocytosis
Exocytosis , also known as 'The peni-cytosis', is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane...

. Apical membrane proteins are trafficked from the Golgi to the apical, rather than baso-lateral, membrane because apical determinants serve to identify the correct destination for vesicle
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...

 delivery. A related mechanism is likely to operate for the baso-lateral membranes.

The fourth principle is lipid modification. A component of the lipid bilayer, phosphatidyl inositol phosphate (PIP) can be phosphorylated to form PIP2 and PIP3. In some epithelial cells, PIP2 is apically localised while PIP3 is basolaterally localised. In at least one cultured cell line, the MDCK cell, this system is required for epithelial polarity. The relationship between this system and the polarity determinants in animal tissues remains unclear.

Basal versus lateral

Since basal and lateral membranes share the same determinants, another mechanism must make the difference between the two domains. Cell shape and contacts provide the likely mechanism. Lateral membranes are the site of contact between epithelial cells, whereas basal membranes connect epithelial cells to the basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...

, an extracellular matrix layer that lies along the basal surface of the epithelium. Certain molecules, such as Integrin
Integrins are receptors that mediate attachment between a cell and the tissues surrounding it, which may be other cells or the ECM. They also play a role in cell signaling and thereby regulate cellular shape, motility, and the cell cycle....

s, localise specifically to the basal membrane and form connections with the extracellular matrix.

Epithelial cell shape

Epithelial cells come in a variety of shapes that relate to their function in development or physiology. How epithelial cells adopt particular shapes is poorly understood, but it must involve spatial control of the actin cytoskeleton
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

, which is central to cell shape in all plant cells.


All epithelial cells express the transmembrane adhesion molecule E-cadherin, which localises most prominently to the junction between the apical and lateral membranes. The extra-cellular domains of E-cadherin molecules from neighbouring cells bind to one another via a homotypic interaction. The intra-cellular domains of E-cadherin molecules bind to the actin cytoskeleton
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...

 via the adaptor proteins alpha-catenin and beta-catenin
Beta-catenin is a protein that in humans is encoded by the CTNNB1 gene. In Drosophila, the homologous protein is called armadillo...

. Thus, E-cadherin forms adherens junctions that connect the actin cytoskeletons of neighbouring cells. Adherens junctions are the primary force-bearing junctions between epithelial cells and are fundamentally important for maintaining epithelial cell shape and for dynamic changes in shape during tissue development. How E-cadherin localizes to the boundary between apical and lateral membranes is not known, but polarized membranes are essential for maintaining E-cadherin at adherens junctions.
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