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Epithelium Study Guide

Epithelial tissue comprises one of the four basic tissue types.  The others are connective tissue (support cells, immune cells, blood cells), muscle tissue (contractile cells), and nervous tissue.  Organs represent various combinations of these four basic tissue types, which thus comprise the entire body.  Each tissue type retains its fundamental character wherever it occurs.  

For additional introductory exercises, see Ed (The Path Guy)'s Basic Histology Gallery.


OVERVIEW

The boundary between you and your environment is marked by a continuous surface, or epithelium, of contiguous cells.

The shape of this boundary is complex, continuing unbroken from skin through various orifices and including the many invaginations into the internal organs of the respiratory, urinary, digestive, and gastrointestinal tracts.  

Several of the body's organs are primarily epithelial tissue, with each cell communicating with the surface via a duct or tube.  Examples include lung, kidney, and liver.

Throughout these organs (with very few exceptions) the surface remains uninterrupted by any gap between adjacent cells.  All exchange of materials and information (nutrients, gases, wastes, sensation, heat) between the body and the environment must take place across this boundary.  

Epithelial tissue thus serves both as a protective barrier for the body and as an active interface with the environment.  The structural and functional integrity of this epithelium is vital for normal health.

Epithelial tissue is one of the four basic tissue types.  

Several features characterize epithelial tissue and distinguish it from connective tissue, muscle tissue, and nervous tissue.

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CHARACTERISTICS OF EPITHELIAL TISSUES

Nearly all epithelial tissues share some common features:

  • Epithelial tissue comprises an uninterrupted layer of cells.  Epithelium covers nearly all external and internal body surfaces.  

    Even when an epithelial surface seems to be penetrated by a hole (such as a the pore of a gland) that hole is really just an invagination of the epithelium (i.e., the glandular duct is itself lined by epithelium, and the secretory portion of the gland is also epithelial tissue).  [Example]

  • Epithelial cells are attached to one another.  Special devices (intercellular junctions, tonofilaments) provide for structural integrity of the epithelium.  There are several types of cell junctions.
     
    • Adhering junctions (e.g., desmosomes) provide mechanical attachment.  Keratinocytes are joined to one another by many adhering junctions.
    • Tight (occluding) junctions block diffusion; they typically form a seal or gasket around the apical end of cells comprising simple epithelia (i.e., epithelia comprising only a single layer of cells).  This junction helps assure adequate separation between different fluid compartments (i.e., between the contents of the intestine and the interstitial fluid of the body).  
    • Gap junctions provide direct intercytoplasmic communication between joined cells.  That is, ions or small molecules can pass through gap junctions directly from the cytoplasm of one cell into the cytoplasm of an adjacent cell, without passing into intercellular space.
  • Intercellular spaces in epithelium are small.  Within the epithelium, the narrow spaces between cells are sealed off from the external environment by cell junctions near the free surface of the cells.  Compared to ordinary connective tissue, epithelial tissue has very little interstitial fluid.  
  • Epithelial tissue is polarized.  An epithelium has a free surface, the apical surface, exposed to the outside, and an attached surface, the basal surface, resting on the underlying connective tissue.
     
    • In a simple (single-layered) epithelium, each cell is polarized.  The base of each cell is attached to an underlying basement membrane while the apical end faces free space.  Lateral surfaces are attached to neighboring epithelial cells.
    • Polarization of epithelial cell structure is particularly evident in secretory and absorptive cells where the arrangement of intracellular organelles reflects the directional transport of material.
    • Less visible but equally important is the localization of membrane proteins, which confer special properties to the membrane (i.e., for cell junctions or for active or passive transport of ions and molecules in and out of the cell), to apical, lateral or basal surfaces of the epithelial cell.
       
  • Epithelial cells are separated from the underlying tissue by a basement membrane.  The basement membrane is a thin sheet of collagen and glycoproteins produced in part by the epithelial cells themselves and in part by underlying connective tissue cells (specifically, fibroblasts).  
     
    • The basement membrane serves to regulate cell behavior and can limit the spread of some neoplasms.
    • In certain places, the basement membrane assumes major significance.  (For example, in glomeruli of the kidney it serves as a filter for blood plasma on its way to becoming urine.)  In other places, the absence of a basement membrane is notable and functionally significant.  (For example, the absence of a basement membrane in the liver permits blood plasma to contact hepatocytes directly.)
    • Basement membranes are at best inconspicuous in H&E stained preparations, but they can be demonstrated with PAS stain or certain silver stains.

These common features confer a characteristic appearance upon most epithelial tissues, no matter how various their cells may be.  In nearly all epithelia, cells are attached to one another and thus evenly spaced, in distinct contrast with connective tissue cells which are scattered and not attached.

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CONTRAST WITH CONNECTIVE TISSUE

The characteristics of epithelial cells (above) contrast with those of connective tissue cells, which are not attached to one another and so tend to be randomly distributed, often with conspicuous spaces containing extracellular matrix in between neighboring cells.

Many support cells and immune cells are typically unpolarized and quiescent, with little cytoplasm and with nuclei containing condensed chromatin.  

In contrast, because many epithelial cells are actively manufacturing, secreting, absorbing or dividing, epithelial cells usually have conspicuous cytoplasm and large, relatively euchromatic nuclei (with finely granular chromatin) and with prominent nucleoli.

Each of the following examples illustrates the distinct difference between epithelial tissue and connective tissue.  Click on a thumbnail image for an annotated enlargement.

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DESCRIPTIVE TERMINOLOGY

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TYPES OF EPITHELIAL TISSUE

The most common types of epithelial tissue are regularly associated with particular functions and locations.


Stratified squamous epithelium consists of flattened (squamous) cells on the surface overlying multiple layers of cells that usually are more cuboidal toward the base of the epithelium.

Stratified squamous epithelium is usually protective.  The multiple layers are too thick for efficient transport of materials (neither secretory or absorptive).  The innermost layer continually produces cells (via mitosis) to replace those lost from the outer surface.  


Simple columnar epithelium consists of a single layer of tall (columnar cells).

Simple columnar epithelium is usually involved in active secretion and/or absorption of material across the single cell layer, or (if ciliated) in movement along the surface.  Simple columnar epithelium lines the digestive tract and the female reproductive tract (as well as numerous other surfaces).


 

© Blue Histology

How many cell layers appear in a section depends on the angle between the section plane and the surface of the epithelium.  A single surface is usually not lined by several types of epithelia, so the number of epithelial cell layers will normally be the smallest number of layers visible anywhere along the surface lined by the epithelium.


A pseudostratified columnar epithelium appears stratified, typically with nuclei located in at least two more-or-less distinct levels.  But in fact every cell rests on the basement membrane, so the epithelium is technically "simple", in spite of appearances.

A pseudostratified columnar epithelium is characteristic of the respiratory tract and of the ducts in the male reproductive system.


Cuboidal epithelium consists of boxy (cuboidal) cells on the surface.  If stratified, the deeper layers are usually also cuboidal.

Cuboidal epithelium is commonly encountered in glandular ducts.  Cuboidal epithelial cells may be active (pumping material into or out of the lumen) or passive, depending on location and cellular specialization.  Small ducts typically have a simple cuboidal epithelium.  Larger ducts may have a stratified cuboidal epithelium.


Simple squamous epithelium consists of a single, very thin layer flattened (squamous) cells.

Simple squamous epithelium may be located at sites of rapid diffusion, such as the lining of lung alveoli, the lining of blood vessels (called endothelium), and at sites where very little activity is occuring, such as Bowman's capsule in the kidney and the lining of major body cavities (called mesothelium).


Transitional epithelium, also called urothelium, is a stratified epithelium lining the distensible walls of the urinary tract.  The name "transitional" derives from this tissue's ability to change its shape from cuboidal to squamous when stretched.  


Endothelium and mesothelium are special names given to the lining of certain internal surfaces.  

The entire circulatory system (heart, arteries, veins, capillaries, sinusoids and lymphatics) is lined by a simple squamous epithelium that is called endothelium.  (The inner lining of the cornea is also called "endothelium".)

The major body cavities (peritoneal, pleural, pericardial) are lined by a mesodermally derived simple squamous epithelium that is called mesothelium.  


Glandular epithelium is specialized for secretion  Glandular epithelium forms more-or-less complex invaginations of epithelial tissue that lie deep to the surface epithelium lining obvious external and internal body surfaces.  

Glandular epithelium may be differentiated into secretory units (which can be specialized for various products) and ducts (which typically appear less specialized, although ducts may function actively to reabsorb water and thus concentrate the secretory product).

Although large exocrine glands like the liver and pancreas may appear solid, each secretory cell has an apical surface exposed to a lumen.  This lumenal space, in turn, leads through ducts to the outside of the body.

For more on glands, click here.

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REPLACEMENT OF EPITHELIAL TISSUE

Certain epithelia, notably epidermis and intestinal epithelium, are continually recycled, with with new cells being created by mitotic activity while old cells are sloughed off (from the surface of the epidermis or the tips of intestinal villi).  Many additional epithelial cells (not just those of skin and intestine) have the ability to respond to the stimulus of injury with mitotic activity and cell migration, to regenerate tissue following damage.  This confers on most epithelia an "automatic" ability to deal effectively with injury by replacing lost tissue with new growth from undamaged edges.

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EPITHELIAL TISSUE PATHOLOGY

The surface location of many epithelial tissues exposes them to a variety of insults, ranging from mechanical damage (cuts, scrapes) and active penetration (mosquitoes, parasites, hypodermics) to bacterial and fungal attack and poisoning by toxic chemicals.  

In simple clean wounds of the skin, one of the earliest healing accomplishments may be proliferation and spread of epithelial keratinocytes, re-establishing epidermal continuity in as little as 24 hours.  Deep (third degree) burns are so serious largely because they destroy the many hair follicles and sweat glands that invaginate deep into the dermis and serve as efficient sources of epithelial regrowth after more superficial injury.  

The importance of epithelial cell regeneration is dramatically illustrated by recovery from cholera.  A toxin from the cholera vibrio kills the intestinal epithelium.  Resulting loss of bodily fluid from the uncovered mucosa leads to copious diarrhea, massive dehydration, and death within a few days.  However, if patients can be kept hydrated for those few days, epithelial replacement by stem cell division will restore normal function.

When an epithelial cells' ability to divide is stimulated inappropriately, it can result in the formation of a tumor.  Cells in epithelial tumors often retain their basic epithelial character, remaining attached to one another and differentiating to form layered structures.  As long as the neoplastic cells respect the basement membrane, the tumor will remain localized.  But once cells break through this boundary they can enter circulation and metastasize.  

The name carcinoma is applied to any cancer (malignant neoplasm) of epithelial origin; adenocarcinoma names a cancer of glandular origin.  (Cancers of mesenchymal origin are called sarcomas.)

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Comments and questions: dgking@siu.edu

SIUC / School of Medicine / Anatomy / David King

http://www.siumed.edu/~dking2/intro/epith.htm
Last updated:  13 April 2010 / dgk