Epithelial tissues

5


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.




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.



image


image


image


FIG. 5.1 Simple squamous epithelium
(a) Diagram (b) H&E (HP) (c) Spread preparation, silver method/neutral red (HP)
Simple squamous epithelium is composed of flattened, irregularly shaped cells forming a continuous surface that is sometimes called pavemented epithelium; the term squamous derives from the comparison of the cells to the scales of a fish. Like all epithelia, this delicate lining is supported by an underlying basement membrane BM as shown diagrammatically.
Simple squamous epithelium is found lining surfaces involved in passive transport (diffusion) of either gases (as in the lungs) or fluids (as in the walls of blood capillaries). Simple squamous epithelium also forms the delicate lining of the pleural, pericardial and peritoneal cavities where it allows passage of tissue fluid into and out of these cavities. Although these cells appear simple in form they have a wide variety of important roles.
Micrograph (b) shows a mesothelium (peritoneum) covering the surface of the appendix and illustrates the typical appearance of simple squamous epithelium in section. The mesothelial lining cells M are so flattened that they can only be recognised by their nuclei, which bulge on the surface. The supporting basement membrane is thin and, in H&E stained preparations, has similar staining properties to the underlying collagenous supporting tissue C; hence it cannot be seen in this micrograph. Deeper in the wall of the appendix, the smooth muscle SM of the muscularis propria can be identified.
In the preparation used in micrograph (c), the mesothelial lining of the peritoneal cavity has been stripped from the underlying tissues and spread onto a slide, thus permitting a surface view of simple squamous epithelium. The intercellular substance has been stained with silver thereby outlining the closely interdigitating and highly irregular cell boundaries. The nuclei N are stained a slightly darker pink.







image


image


FIG. 5.4 Simple columnar ciliated epithelium
(a) Diagram (b) H&E (HP)
Some simple columnar epithelia have surface cilia C on the majority of the cells (see also Fig. 5.13). Among the ciliated cells are scattered non-ciliated cells that usually have a secretory function.
Cilia are much larger than microvilli (see Fig. 5.14) and are readily visible with the light microscope. Each cilium consists of a finger-like projection of the plasma membrane, its cytoplasm containing modified microtubules. Each cell may have up to 300 cilia that beat in a wave-like manner, synchronised with the adjacent cells. The waving motion of the cilia propels fluid or minute particles over the epithelial surface. Simple columnar ciliated epithelium is found mainly in the female reproductive tract. Micrograph (b), taken from the Fallopian tube (oviduct), shows one of its numerous folds covered by simple columnar ciliated epithelium. The predominant cell type in this epithelium is tall columnar and ciliated, the nuclei being located towards the midzone of the cells. The less numerous blue-stained cells with basally located nuclei are not ciliated and have a secretory function. Ciliary action facilitates transport of the ovum from the ovary towards the uterus.



image


image


FIG. 5.5 Pseudostratified columnar ciliated epithelium
(a) Diagram (b) H&E (MP)
Another variant of simple columnar epithelium is described in which the majority of cells are also usually ciliated C. The term pseudostratified is derived from the appearance of this epithelium in section, which conveys the erroneous impression that there is more than one layer of cells. In fact, this is a true simple epithelium, since all the cells rest on the basement membrane. The nuclei of these cells, however, are disposed at different levels, thus creating the illusion of cellular stratification. Scattered stem cells (see Ch. 2) are found throughout the epithelium; these generally are devoid of cilia (i.e. less differentiated) and do not extend to the luminal surface.
Pseudostratified columnar ciliated epithelium may be distinguished from true stratified epithelia by two characteristics. Firstly, the individual cells of the pseudostratified epithelium exhibit polarity, with nuclei being mainly confined to the basal two-thirds of the epithelium. Secondly, cilia are never present on true stratified epithelia.
Pseudostratified epithelium is almost exclusively confined to the airways of the respiratory system in mammals and is therefore often referred to as respiratory epithelium. Micrograph (b) illustrates the lining of a bronchus. In the respiratory tract, the cilia propel a surface layer of mucus containing entrapped particles towards the pharynx in what is often described as the mucociliary escalator. The mucus is secreted by nonciliated goblet cells found amongst the ciliated cells (not seen in this micrograph, see Figs 5.16 and 5.17).



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.



image


image


image


image


FIG. 5.6 Stratified squamous epithelium
(a) Diagram (b) H&E (HP) (c) H&E (MP) (d) Papanicolaou (HP)
Stratified squamous epithelium consists of a variable number of cell layers that exhibit maturation from a cuboidal basal layer to a flattened surface layer. The basal cells which are adherent to the underlying basement membrane include continuously dividing stem cells, their offspring migrating towards the surface where they are ultimately shed as anucleate squames. Stratified squamous epithelium is adapted to withstand abrasion, with plentiful cell junctions and a prominent intermediate filament (keratin) cytoskeleton. This type of epithelium lines the oral cavity, pharynx, oesophagus, anal canal, uterine cervix and vagina, sites which are subject to mechanical abrasion but which are kept moist by glandular secretions, such as the salivary glands of the mouth.
The epithelium in micrograph (b) is from the uterine cervix. Note the cuboidal basal layer and the maturation through the large polygonal cells of the intermediate layers to the flattened superficial squamous cells. The cytoplasm in these cells often appears clear due to the glycogen content.
Keratinising stratified squamous epithelium (c) constitutes the epithelial surface of the skin (the epidermis) and is adapted to withstand the constant abrasion and desiccation to which the body surface is exposed. During maturation, the epithelial cells accumulate keratin intermediate filaments which are cross-linked with proteins such as involucrin and loricrin in a process called keratinisation (or cornification). This results in the formation of a tough, non-living surface layer (stratum corneum) consisting of a compacted cross-linked keratin matrix K interspersed with specialised lipids (see Ch. 9). The underlying granular cell layer G consists of epithelial cells with extensive tight junctions, forming a waterproof barrier. The nuclei of the maturing epithelial cells become progressively condensed (pyknotic) and eventually disappear along with the other cellular organelles. Keratinisation may be induced in normally non-keratinising stratified squamous epithelium such as that of the oral cavity when exposed to excessive abrasion (e.g. poorly-fitting false teeth).
Micrograph (d) shows a smear made from normal cells scraped from the uterine cervix as it projects into the vagina. The degenerate, scaly superficial cells stain pink with this staining method, while the living cells from deeper layers stain blue. This is the basis of the well-known ‘Pap smear’ which examines cytological preparations of cervical cells for pre-cancerous changes.






Keratins


Keratin intermediate filaments (also called cytokeratins) are the characteristic intermediate filaments of epithelial cells. Keratins may be subclassified into α- and β-keratins. α-Keratins are the only types found in mammals and may be further subdivided into acidic and basic subtypes (type I and type II, respectively). β-Keratins are typical of feathers, scales, beaks and claws of birds and reptiles and do not occur in mammals. Humans have 54 genes for keratins found on chromosomes 17 and 12. Keratins are vital for the maintenance of cell shape and polarity, and different keratin types are found in different epithelia and indeed in different layers of stratified epithelia. For instance, the basal cells of epidermis produce keratins K5 and K14, while the suprabasal layers exhibit K1 and K10, and hair is characterised by K31-40 and K81-86.


Keratins confer mechanical strength on epithelia, so it is not surprising that those epithelia subjected to the greatest mechanical stresses contain large amounts of keratins which are connected to the intercellular junctions (desmosomes), thus linking the cytoskeletons of adjacent cells and cell–basement membrane junctions (hemidesmosomes).


Simple epithelia also contain characteristic keratins, usually K8 and K18 as well as others that are sometimes restricted to particular sites. For instance, colonic mucosa characteristically contains K20 while gastric epithelium expresses K7. In diagnostic histopathology, this differential expression of cytokeratins can be used classify poorly differentiated metastatic tumours where there is no known primary site. This may be very important to enable decisions about treatment to be made, as different tumours may respond better to different types of chemotherapy and/or radiotherapy.

Aug 22, 2016 | Posted by in HISTOLOGY | Comments Off on Epithelial tissues
Premium Wordpress Themes by UFO Themes
%d bloggers like this: