Epithelial tissues

Introduction

The epithelia (singular: epithelium ) are a diverse group of tissues that include both surface epithelia and solid organs . Surface epithelia cover or line all body surfaces, cavities and tubes and form the interface between different biological compartments. For instance, the epidermis of the skin is exposed to the external environment and the epithelial lining of the gastrointestinal tract is exposed to partially digested food and bacteria in the lumen of the gut. Functions of epithelia include: forming a protective barrier, regulation of the exchange of molecules between compartments (selective diffusion and absorption) and synthesis and secretion of glandular products. Many of these major functions may be exhibited at a single epithelial surface. For example, the epithelial lining of the small intestine is primarily involved in absorption of the products of digestion, but the epithelium also protects itself from noxious intestinal contents by secreting a surface coating of mucus. Epithelial cells are characterised by the production of keratin intermediate filaments (see Ch. 1 ), and this can be used to recognise epithelial cells using immunohistochemistry, a technique often used in diagnostic histopathology to classify difficult malignant tumours (see Appendix 2 ).

Surface epithelia form continuous sheets comprising one or more layers of cells. Epithelial cells are bound to adjacent cells by a variety of cell junctions that provide physical strength and mediate exchange of information and metabolites. All epithelia are supported by a basement membrane (see Ch. 4 ) which separates the epithelium from underlying supporting tissues. Thus epithelial cells are polarised , with one side facing the basement membrane and underlying supporting tissues (the basal surface) and the other facing outwards (the apical surface).

Blood vessels never cross epithelial basement membranes, so epithelia depend on the diffusion of oxygen and metabolites from adjacent supporting tissues.

Classification of Surface Epithelia

Surface epithelia are traditionally classified according to three morphological characteristics: number of cell layers, type of cell (profile perpendicular to basement membrane) and special features. Special features include adaptations such as cilia or goblet cells that may be characteristic of particular sites (e.g. the epithelium of the upper respiratory tract is a ciliated pseudostratified columnar epithelium).

Glandular Epithelia

Epithelium that is primarily involved in secretion is often arranged into structures called glands . Glands are merely invaginations of epithelial surfaces which are formed during embryonic development by proliferation of epithelium into the underlying tissues. For example, glandular epithelium is characteristic of the lining of much of the gastrointestinal tract.

However, some solid organs are composed largely of epithelial cells with a supporting tissue framework. Some of these organs are connected to the surface epithelium of the gastrointestinal tract by a branching system of ducts and belong to the category of exocrine glands (e.g. salivary glands). Endocrine glands on the other hand have lost their connection to the epithelial surface from which they developed and release their secretions directly into the blood (e.g. thyroid gland). Most of the solid epithelial organs such as liver, pancreas and thyroid are described in detail in the relevant organ system chapter and only a few examples are described here.

Simple Epithelia

Simple epithelia are defined as surface epithelia consisting of a single layer of cells. Simple epithelia are almost always found at interfaces involved in selective diffusion, absorption and/or secretion. They provide little protection against mechanical abrasion and thus are not found on surfaces subject to such stresses. The cells comprising simple epithelia range in shape from flattened to tall columnar, depending on their function. For example, flattened simple epithelia are ideally suited to diffusion and are therefore found in the air sacs of the lung ( alveoli ), the lining of blood vessels ( endothelium ) and lining body cavities ( mesothelium ). In contrast, highly active epithelial cells, such as the cells lining the small intestine, are generally tall since they must accommodate the appropriate organelles. Simple epithelia may exhibit a variety of surface specialisations, such as microvilli and cilia , which facilitate their specific surface functions.

Stratified Epithelia

Stratified epithelium is defined as epithelium consisting of two or more layers of cells. Stratified epithelia have mainly a protective function and the degree and nature of the stratification are related to the kinds of physical stresses to which the surface is exposed. In general, stratified epithelia are poorly suited for absorption and secretion by virtue of their thickness, although some stratified surfaces are moderately permeable to water and other small molecules. The classification of stratified epithelia is based on the shape and structure of the surface cells, since cells of the basal layer are usually cuboidal in shape. Transitional epithelium is a stratified epithelium found only in the urinary outflow tract, with special features to make it waterproof as well as expansile.

Membrane Specialisations of Epithelia

The plasma membranes of epithelial cells exhibit a variety of specialised structures that allow them to perform their function as a barrier with selective permeability. In some cases, the epithelial barrier is very impermeable, such as the transitional epithelium of the bladder, while other epithelia, such as the lining of the small intestine or the convoluted tubules of the kidney, promote movement of selected ions and molecules across the epithelium.

• Intercellular surfaces . The adjacent or lateral surfaces of epithelial cells are linked by cell junctions so that the epithelium forms a continuous, cohesive layer. Cell junctions also operate as communication channels governing such functions as growth and cell division. The various types of cell junction are composed of transmembrane proteins that interact with similar proteins on adjacent cells and are linked to intracellular structures on the cytoplasmic side. Adhering junctions and gap junctions (or communicating junctions ) are not exclusive to epithelia and are also present in cardiac and visceral muscle where they appear to serve similar functions. The main features of intercellular junctions are summarised in Table 5.2 .

  Table 5.2

  Intercellular junctions

  Intercellular junction type	Main protein components	Cytoskeleton connections	Site	Main functions
  Tight junction (occluding junction, zonula occludens)	Claudins, occludins, tricellulin	Actin microfilaments	Found at luminal end of lateral cell membrane	Control of paracellular diffusion Prevents exchange of intrinsic proteins and lipids between apical and basolateral plasma membrane
  Adhering belt (zonula adherens)	Classic cadherins, catenins	Actin microfilaments Microtubules	Lateral plasma membrane, immediately deep to tight junction	Link cytoskeletons of adjacent cells to form strong cohesive epithelium
  Desmosome (macula adherens)	Cadherins	Intermediate filaments (keratins in epithelia)	Lateral plasma membrane	Link cytoskeletons of adjacent cells to form strong cohesive epithelium
  Hemidesmosomes	Integrins Laminins of basement membrane	Intermediate filaments (keratins in epithelia)	Basal plasma membrane	Link cells to underlying basement membrane
  Gap junctions	Connexins	None	Lateral plasma membrane	Passage of ions and small molecules between cells Intercellular signalling

• Luminal surfaces . The luminal or apical surfaces of epithelial cells may incorporate three main types of specialisation: cilia, microvilli and stereocilia . Cilia are hair-like organelles that are easily resolved by light microscopy. In contrast, microvilli are shorter projections of the plasma membrane that cannot be individually resolved with the light microscope. A single cell may have thousands of microvilli or only a few. Stereocilia are extremely long microvilli usually found only singly or in small numbers.

• Basal surfaces . The interface between all epithelia and underlying supporting tissues is marked by a non- cellular structure known as the basement membrane (see Ch. 4 ) that provides structural support for the epithelium and constitutes a selective barrier to the passage of materials between epithelium and supporting tissue. Hemidesmosomes , a variant of desmosomes , bind the base of the cell to the underlying basement membrane by linking to the cell’s intermediate filament network.

  

Exocrine Glands

As discussed earlier in this chapter, epithelial cells are the major component of all the glands of the body. The simplest glands can be easily recognised as an invagination of a surface epithelium. However, increasingly complex glandular structures have evolved over time, and some of the most elaborate have lost contact with the epithelial surface completely. Thus there are two major subdivisions in the classification of glands: exocrine glands , which release their contents onto an epithelial surface either directly or via a duct, and endocrine glands , which have no duct system but by releasing their secretions into the bloodstream can act on distant tissues. Endocrine glands are dealt with briefly at the end of this chapter and in much more detail in Ch. 17 .

This section deals with exocrine glands, which vary from microscopic, such as sweat glands of the skin, to large solid organs such as the liver, weighing approximately 1.2 kg. The duct system of the liver ramifies throughout the solid gland and empties its secretions ( bile ) into the duodenum. In contrast, the simple tubular glands ( crypts ) of the large bowel (see also Ch. 14 ) consist entirely of the secretory component and empty directly onto the surface of the bowel. Indeed, the simplest exocrine glands of all are single mucus-secreting cells such as goblet cells.

Exocrine glands may be subclassified according to the morphology and the means of secretion of the gland.