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).

TABLE 5.1

Classification of epithelia

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.

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.

BM basement membrane C collagenous supporting tissue M mesothelial lining cells N nucleus SM smooth muscle

FIG. 5.2 Simple cuboidal epithelium
(a) Diagram (b) Azan (HP)
Simple cuboidal epithelium represents an intermediate form between simple squamous and simple columnar epithelium; the distinction between tall cuboidal and low columnar is often arbitrary and is of descriptive value only. In the section perpendicular to the basement membrane BM, the epithelial cells appear square, leading to its traditional description as cuboidal epithelium; on surface view, however, the cells are actually polygonal in shape. The nucleus is usually round and located in the centre of the cell.
Simple cuboidal epithelium usually lines small ducts and tubules that may have excretory, secretory or absorptive functions; examples are the collecting tubules of the kidney and the small excretory ducts of the salivary glands and pancreas.
Micrograph (b) shows the cells lining a collecting tubule in the kidney. Although the boundaries between individual cells are indistinct, the nuclear shape provides an approximate indication of the cell size and shape. The underlying basement membrane BM appears as a prominent blue line with the Azan staining method, in contrast to basement membranes stained with the standard H&E stain (see Fig. 5.3b) that are generally indistinguishable.

BM basement membrane C cilia

FIG. 5.3 Simple columnar epithelium
(a) Diagram (b) H&E (HP)
Simple columnar epithelium is similar to simple cuboidal epithelium except that the cells are taller and appear columnar in sections perpendicular to the basement membrane. The height of the cells may vary from low to tall columnar, depending on the site and/or degree of functional activity. The nuclei are elongated and may be located towards the base, the centre or occasionally the apex of the cytoplasm; this is known as the polarity of the nucleus. Simple columnar epithelium is found on absorptive surfaces such as in the small intestine, as well as at secretory surfaces such as that of the stomach.
Micrograph (b) shows simple columnar epithelium taken from the endocervix where it has the function of secreting mucus. Note the typically basally located nuclei.

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.