Epithelia

"Discuss the functional and structural differentiations which may be found among epithelial cells."

Epithelia cover the outside of the body and line all the hollow structures (except blood vessels and lymphatics). It is derived from the embryonic ectoderm and endoderm. Although all epithelia have this common root in its early development, as time goes on the cells differentiate into a wide variety of function. This is seen in the protective coating of your skin, the secretion of mucus in the nasal passages and the absorption of food waste in the gut. Generally, epithelia are of polygonal shape. They have many intercellular junctions. This gives them the ability to form tightly held together sheets. Although all epithelia attach to a basal lamina of extracellular protein in the region of the cell bodies there is very little extracellular material (c.f. connective tissue). Epithelia are also able to regenerate themselves in most case. For example, areas where there is a great deal of abrasion. Classification of epithelia is by the forms they take which are in turn related to the function of the epithelia in question. There are two major classes of epithelia: unilaminar and multilaminar (single or multiple layers of cells). These can be further classified by the shape of the cells in them. The basic structure of epithelium is shown by simple squamous epithelium in such places as parietal layer of Bowmans capsule or the thin segment of the loop of Henle. These are formed from flat cells, which are often very well tessellated, spread across and firmly attached to a basal lamina of extracellular material. The basal lamina is mainly of collagen fibres. Because of the shape of the cells they allow rapid diffusion of water and gases (where appropriate) across the plasma membrane and hence into the tissues beyond. There are tight junctions between these epithelial cells and these prevent the passage of other molecules. The structure of these junctions is as yet uncertain but they are believed to be due to the combining of a fine network of sealing strand proteins that exist passing through and running along the intracellular side of the extracellular half of the lipid bilayer. This network of protein has been seen using freeze fracture techniques of electron microscopy. This seal occurs in nearly all epithelia. It is not absolute or invariable. In the gut the tight junctions can be about 10,000 times more permeable to Na+ ions than that of the epithelium in the urinary tract. The other forms of unilaminar epithelium are cuboidal and columnar, which show roughly square and rectangular shapes respectively in cross section. The cuboidal epithelia occurs in many glands The luminar surface of these epithelial cells is often covered by microvilli (brush border) or by cilia. The microvilli help to increase the surface area of the cell. This may help in secretory processes where vesicles are binding to the membrane allowing a faster response to stimuli. An extension to the microvilli is the longer and mobile cilia. Examples of ciliated epithelia are in the linings of the respiratory tract where the cilia sweep mucus up towards the pharynx. In areas of the body the are either open to mechanical stresses (including abrasion) or in danger of drying out or both multilaminar epithelia are of much useful design. In areas where there is danger of mechanical stress and of drying out a form of epithelia known as keratinized stratified squamous epithelium occurs. Areas where this occurs include the entire epidermis, mucocutaneous junction of lips and nasal passages, distal anal canal, outer surface of tympanic membranes and parts of the oral lining. The epithelial cells here are formed by the mitosis of stem cells. It is the stem cells that are firmly attached to the basal lamina in the case. In the epidermis this is the stratum basale. At this point the epithelial cells contain many ribosomes and mitochondria and have an extensive cytoskeleton. This enables them to produce a large quantity of keratin fibres to add to the extracellular matrix around them. As the cells move away from the base the surface membrane forms a number of junctions, known as desmosomes between adjacent cells. In histological section, the slight shrinkage of the cells due to the fixing techniques causes the cells to take on a spiny appearance (prickle cell). This enables the cells to interlock tightly greatly increasing the strength of the epithelial tissue. This is the layer known as the stratum spinosum. More superficially still, the cells flatten again. They also begin to synthesise a dense, basophilic protein, keratohyalin. This mingles with the keratin fibres surrounding the cells for extra strength. The production of keratohyalin cause the cells to take on a granular appearance when stained. This region is known as the stratum granulosum. As the cells pass beyond this region they begin to synthesis a glycolipid which is secreted onto their outer surface. This forms a thick, adhesive, waterproof lipidic cement and allows the epithelia to do its task of keeping the body's moisture inside. Without this lipidic cement the cell would simply lose water by evaporation through the cell membrane. With this final synthesis complete the cells become anucleate and along with keratin in the extracellular matrix they form squames in a thin layer known as the stratum lucidum. In the final layer, the stratum corneum the cells flake away. This final layer allows the cells underneath to come up through hence the epithelial is in a constant state of renewal, this also makes it resistant to abrasion, because although the outer skin layers are removed without major difficulty there is always more strongly held tissues underneath to replace them. In areas where the surface is subject to abrasion but not to drying out for example some parts of the buccal cavity, oesophagus, parts of the anal canal. This develops in the same stages as keratinizing epithelium except that the cells are not involved to any great extent in the production of keratin fibres or the glycolipid excretion. They also retain their nuclei until the cells become desquamate. In surfaces subject to considerable abrasion an amount of keratin will be produced by the epithelium. The epithelium can then be classified as parakeritanized. Very different to keritanized epithelium are stratified columnar or stratified cuboidal epithelia. These are multiple layers of the appropriately shaped epithelial cells. (Either columnar in cross-section or square). These show no progression of cells except that which is needed for repair of the tissue. Statified columnar and cuboidal epithelia occur in the ducts of some exocrine glands such pancreatic and salivary glands and the ducts of sweat glands. The multiple layers are presumed to provide the glands with more strength than a single layer would provide. A final type of protective epithelium known either as urothelium, urinary epithelium or by the descriptive title of transitional epithelium. This occurs as the name suggests in the urinary tract, running from the collecting ducts of the kidneys through the ureters and bladder to the urethra. In the male is stops at the ejaculatory ducts where it breaks up and changes into a stratified columnar epithelium. In the female the urothelium runs through to the urogenital membrane. The characteristic feature of this epithelium is that the cells flatten on stretching whilst still maintaining their relative positions. The cells are held tightly together by many desmosomes. On closer inspection it can be seen that a great many of the cells are attached to the basal lamina. It has been suggested that all cells would be found o be attached to the basal lamina in which case this is a unilaminar epithelium and not a multilaminar but this view is not widely held. At the basal lamina the cells are uninucleate diploid cells. They show themselves to be basophilic and contain a lot of ribosomes. Apically they join to form larger cells. These are either binucleate or polyploid (they have multiple sets of the genome). The surface cells are larger again and may have up to an octoploid number of chromosomes. On top of that the luminal surface is covered by a plasma membrane covered in plates of glycoprotein. When the tissues is in the relaxed state these plates can be partially taken into the cytoplasm into vacuoles or diverticulae. The urothelium acts as an effective barrier to prevent what could be incredibly toxic urine from entering the surrounding tissues. An important functional feature of the epithelium here is the inclusion of tight junctions between the cells of the epithelium. These perform the job of providing a seal around the edges of cell preventing passage of urine. In general there is a slow turnover of cells as abrasion is slight but if the urothelium is damaged it can be quickly regenerated. A major differentiation occurs in those epithelial cells that have a secretory function, either as isolated individual cells (for example goblet cells) or as a coherent secretory sheet (for example the mucous lining of the stomach). The structure and function of these is intimately tied in with the products of the secretory cells and the mechanisms by which they secrete their product. The simplest secretory mechanisms are those of exocytosis and this occurs in examples such as parietal cells in the gastric glands which secrete HCL, and the eccrine sweat glands which secrete a dilute mixture of electrolytes for which there is no major structural reorganisation. More typical secretory products are glycoproteins for example the mucin produced by goblet cells in the alimentary tract. These cells are rich in rough endoplasmic reticulum to synthesise the protein component and also endowed with a large Golgi-complex component to complete glycosylation. The mucin secretors have a greater proportion of Golgi-complex to rough ER due to the abundance of carbohydrate needed. Pancreatic acinar cells secrete a different glycoprotein and hence have a larger rough ER proportion as in this case the protein component is bigger. Because of the protein synthesis, the cytoplasm of glycoprotein secretors are rich in nucleic acids and this is shown by their basophilic appearance although the secretory vacuoles may differ. Those cells that secrete steroids are adapted to their role in that they have large quantities of smooth ER. These cells which occur in such places as the adrenal cortex, the gonads and the placenta contain many lipid droplets and these are thought to be essential in the chemical pathways of steroid synthesis. The mechanisms of release of steroids are however far from being understood. The third main structural differentiation of epithelial cells is that to allow them to be absorptive. Absorptive cells can be found primarily in the lungs and in the small intestine. Those cells in the lung contribute little to the absorption process itself and serve greater purpose in separating body fluid from lung space. Gases diffuse across the cell membranes with little difficulty. More important structural features are the long cilia that sweep mucous away up from the lungs to the pharynx. In the small intestine however the cells play a vital role, not only in absorption of the metabolites from the intestinal lumen but in their protection of the underlying tissues from the potential destructive intestinal juices. The tight junctions aid these cells in their protective role. It can be shown that the apical and basolateral surface of these cells consist of widely different proportions of the surface membrane proteins. While in other cells the proteins would be free to diffuse around all of the plasma membrane surrounding the cell, the tight junctions prevent this from occurring. Therefore transport proteins and specific receptors can be concentrated in the membrane where they need to be. This reduces the work of the cell in protein production and sets up a one way traffic of particles. In through the apical side and out through the basolateral membrane. The apical surface is also covered by a thick glycocalyx (a selection of polysaccharide chains on the external proteins). This prevents protease getting to the surface of the cell and attacking the plasma membrane. Conversely it also adsorbs a selection of these digestive enzymes into the region just off the cell surface and promotes a region of digestion so that materials are readily available to the cell. There are also many enzymes contained in the apical membrane, such as dissacharidases. For this reason it is thought that a significant proportion of intestinal digestion may occur at the apical surface of epithelia. One result of the tight junctions is that nutrients must travel through the epithelial cells where their absorption can be controlled or they can be modified for transportation. An example of this is the absorption of triglycerides, and packaging into chylomicrons, which can then be transported through to adipose tissue. However, in high concentrations some nutrients (for example glucose) will leak through the gut. In some cases, perhaps premature babies, contraction of the cell web may cause large enough gaps for antibodies to pass through. To aid in the absorptive processes intestinal epithelial cells have an apical surface covered in up to three thousand microvilli. These small projections considerably increase the surface area of the cell and hence increase the absorption of material from the lumen of the gut. A less common but still important type of absorption is that which occurs at the microfold cells in the intestine. These cells have widely spaced villi which are seen to have many endocytotic vesicles. They occur over sections of lymphoid tissue and are thought to absorb antigens and pass them through to the underlying lymphoid. Thus they act as a sampling system enabling the lymphoid to produce appropriate antibodies for secretion. Epithelia therefore form many vital areas of our bodies. From the outer surface that keeps the inner environment at exactly the right conditions of temperature and keeps everything in to the secretory organs of the body that regulate the majority of metabolic processes. They have therefore evolved an array of structural forms to enable them to carry out their allotted tasks. Protective cells include greater proportion of connective fibres, secretory cells contain more of the productive organelles and absorptive epithelia have ways of expanding their surface area and including more surface membrane proteins. And yet epithelia all derive from the same basic ectoderm that is formed within the first two weeks of embryonic development.