Epithelial cells

Introduction

Epithelial cells are sheets of specialized cells that cover and line the interior and exterior surfaces of the body, for example, the outer layers of the skin and the inner luminal surface of the gastrointestinal tract. They have diverse functions such as secretion, absorption and protection (as a barrier), made possible by many specialized surface and internal modifications, with cells attached to each other by several types of cell junctions. Cells can be adapted to manufacture and secrete a product, and secretion of large amounts of product is facilitated by the folding and invagination of sheets of epithelium to form glands. Certain solid organs are composed of an intricate arrangement of epithelial cells, for example, the pancreas, kidney and liver.

Epithelial cells form very cohesive sheets of cells, called epithelia, which function mainly as:

• A covering or lining for body surfaces, for example, skin, gut, respiratory and genitourinary tracts; blood vessels; body cavities; and ducts

• The functional units of secretory glands and solid organs, for example, salivary tissue and liver

Epithelial cells are firmly joined together by adhesion specializations. These special structures serve to anchor the cytoskeleton of each epithelial cell to its neighbours and to anchor the epithelium to extracellular matrix materials. Epithelia are separated from their support tissues by a layer of extracellular matrix, the basement membrane. Epithelia do not have blood vessels, obtaining their nutrition via diffusion from capillaries running in the support tissue to which they are attached.

Early microscopists described the cells simply by their observed shape, as function was not yet understood. This nomenclature persists to this day.

Traditionally, cells are classified into three main cell groups according to their shape. These groups are squamous (flat plate-like, Figs 3.1 a and 3.1 b); cuboidal (height and width similar, see Figs 3.1 c and 3.1 d); and columnar (height 2–5 times greater than width, see Figs 3.1 e and 3.1 f).

Epithelial cells form either a single layer, in which all of the cells contact underlying extracellular matrix ( simple epithelium ), or several layers, where only the bottom layer of cells is in contact with the extracellular matrix ( stratified epithelium , Figs 3.2 a and 3.2 b). Stratified epithelia are described by the shape of the upper layers of cells, so that a multilayered epithelium with flat, plate-like cells at the surface is named stratified squamous .

A transitional epithelium is a further special type of stratified epithelium, which is restricted mainly to the lining of the urinary tract (see Figs 3.2 c and 3.2 d) and varies between cuboidal and squamous depending on the degree of stretching. Pseudostratified epithelium (see Figs 3.1 g and 3.1 h) contains epithelial cells that appear to be arranged in layers, but which are all in contact with the extracellular matrix.

Epithelia are also grouped according to whether they occur as a surface or glandular component.

The traditional morphological classification has limitations. Although two epithelia may be described as cuboidal, their function and biology may be so different that it is misleading to equate them. However, the use of a morphological classification of epithelia is still descriptively valuable and gives some potentially useful information about function. For example, simple squamous epithelia tend to be found in areas where transport via diffusion occurs such as lining the lung gas exchange areas (alveoli) or lining capillaries, whereas the multiple layers of stratified squamous epithelium tend to be found in areas of wear and tear such as the skin and the lining of the oesophagus, anal canal and vagina. Columnar epithelial cells, with their abundant cytoplasm, tend to be associated with absorption and secretion, as both require a large complement of organelles.

The traditional terms used to describe epithelia are found throughout this book but are always qualified to give insight into their function.