Question

Discuss the structure and function of epithelial tissue including their membrane proteins and explain why this tissue is so important to the whole organism.

Answer 1

Functions of the Epithelium

Epithelia tissue forms boundaries between different environments, and nearly all substances must pass through the epithelium. In its role as an interface tissue, epithelium accomplishes many functions, including:

1.            Protection for the underlying tissues from radiation, desiccation, toxins, and physical trauma.

2.            Absorption of substances in the digestive tract lining with distinct modifications.

3.            Regulation and excretion of chemicals between the underlying tissues and the body cavity.

4.            The secretion of hormones into the blood vascular system. The secretion of sweat, mucus, enzymes, and other products that are delivered by ducts come from the glandular epithelium.

5.            The detection of sensation.

Characteristics of Epithelial Layers

Epithelial tissue is composed of cells laid out in sheets with strong cell-to-cell attachments. These protein connections hold the cells together to form a tightly connected layer that is avascular but innervated in nature.

The epithelial cells are nourished by substances diffusing from blood vessels in the underlying connective tissue. One side of the epithelial cell is oriented towards the surface of the tissue, body cavity, or external environment and the other surface is joined to a basement membrane. The basement layer is non-cellular in nature and helps to cement the epithelial tissue to the underlying structures.

Types of Epithelial Tissue

Epithelial tissues are identified by both the number of layers and the shape of the cells in the upper layers. There are eight basic types of epithelium: six of them are identified based on both the number of cells and their shape; two of them are named by the type of cell (squamous) found in them. Epithelial tissue is classified based on the number of cells, the shape of those cells, and the types of those cells.

Simple Epithelia

Simple epithelium consists of a single layer of cells. They are typically where absorption, secretion and filtration occur. The thinness of the epithelial barrier facilitates these processes.

Simple epithelial tissues are generally classified by the shape of their cells. The four major classes of simple epithelium are: 1) simple squamous; 2) simple cuboidal; 3) simple columnar; and 4) pseudostratified.

Simple Squamous

Simple squamous epithelium cells are flat in shape and arranged in a single layer. This single layer is thin enough to form a membrane that compounds can move through via passive diffusion. This epithelial type is found in the walls of capillaries, linings of the pericardium, and the linings of the alveoli of the lungs.

Simple Cuboidal

Simple cuboidal epithelium consists of a single layer cells that are as tall as they are wide. The important functions of the simple cuboidal epithelium are secretion and absorption. This epithelial type is found in the small collecting ducts of the kidneys, pancreas, and salivary glands.

Simple Columnar

Simple columnar epithelium is a single row of tall, closely packed cells, aligned in a row. These cells are found in areas with high secretory function (such as the wall of the stomach), or absorptive areas (as in small intestine ). They possess cellular extensions (e.g., microvilli in the small intestine, or the cilia found almost exclusively in the female reproductive tract).

Pseudostratified

These are simple columnar epithelial cells whose nuclei appear at different heights, giving the misleading (hence pseudo) impression that the epithelium is stratified when the cells are viewed in cross section.

Pseudostratified epithelium can also possess fine hair-like extensions of their apical (luminal) membrane called cilia. In this case, the epithelium is described as ciliated pseudostratified epithelium. Ciliated epithelium is found in the airways (nose, bronchi), but is also found in the uterus and fallopian tubes of females, where the cilia propel the ovum to the uterus.

Stratified Epithelium

Stratified epithelium differs from simple epithelium by being multilayered. It is therefore found where body linings have to withstand mechanical or chemical insults.

Stratified epithelia are more durable and protection is one their major functions. Since stratified epithelium consists of two or more layers, the basal cells divide and push towards the apex, and in the process flatten the apical cells.

Stratified epithelia can be columnar, cuboidal, or squamous type. However, it can also have the following specializations:

Keratinized Epithelia

In keratinized epithelia, the most apical layers (exterior) of cells are dead and lose their nucleus and cytoplasm. They contain a tough, resistant protein called keratin. This specialization makes the epithelium waterproof, and it is abundant in mammalian skin. The lining of the esophagus is an example of a non-keratinized or moist stratified epithelium.

Transitional Epithelia

Transitional epithelia are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched, or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called the urothelium since it is almost exclusively found in the bladder, ureters, and urethra.

Tissue function depends on more than cell type and proper rates of death and division: It is also a function of cellular arrangement. Both cell junctions and cytoskeletal networks help stabilize tissue architecture. For instance, the cells that make up human epithelial tissue attach to one another through several types of adhesive junctions. Characteristic transmembrane proteins provide the basis for each of the different types of junctions. At these junctions, transmembrane proteins on one cell interact with similar transmembrane proteins on adjacent cells. Special adaptor proteins then connect the resulting assembly to the cytoskeleton of each cell. The many connections formed between junctions and cytoskeletal proteins effectively produces a network that extends over many cells, providing mechanical strength to the epithelium.

The gut endothelium — actually an epithelium that lines the inner surface of the digestive tract — is an excellent example of these structures at work. Here, tight junctions between cells form a seal that prevents even small molecules and ions from moving across the endothelium. As a result, the endothelial cells themselves are responsible for determining which molecules pass from the gut lumen into the surrounding tissues. Meanwhile, adherens junctions based on transmembrane cadherin proteins provide mechanical support to the endothelium. These junctions are reinforced by attachment to an extensive array of actin filaments that underlie the apical — or lumen-facing — membrane. These organized collections of actin filaments also extend into the microvilli, which are the tiny fingerlike projections that protrude from the apical membrane into the gut lumen and increase the surface area available for nutrient absorption. Additional mechanical support comes from desmosomes, which appear as plaque-like structures under the cell membrane, attached to intermediate filaments. In fact, desmosome-intermediate filament networks extend across multiple cells, giving the endothelium sheetlike properties. In addition, within the gut there are stem cells that guarantee a steady supply of new cells that contribute to the multiple cell types necessary for this complex structure to function properly (Figure 2).

How Does the Extracellular Matrix Support Tissue Structure?

The extracellular matrix (ECM) is also critical to tissue structure, because it provides attachment sites for cells and relays information about the spatial position of a cell. The ECM consists of a mixture of proteins and polysaccharides produced by the endoplasmic reticula and Golgi apparatuses of nearby cells. Once synthesized, these molecules move to the appropriate side of the cell — such as the basal or apical face — where they are secreted. Final organization of the ECM then takes place outside the cell.

To understand how the ECM works, consider the two very different sides of the gut endothelium. One side of this tissue faces the lumen, where it comes in contact with digested food. The other side attaches to a specialized ECM support structure called the basal lamina. The basal lamina is composed of collagen and laminin proteins, as well as various other macromolecules. On this side of the endothelium, adhesive junctions attach cells to the ECM. Transmembrane integrin proteins in the junctions bind components of the ECM and recruit signaling proteins to their cytoplasmic sides. From there, the signals travel to the nucleus of each cell.

Epithelial tissue provides four key functions, they provide protection, they control permeability, they provide sensation, and they produce secretions. Our skin is a great example of an epithelial tissue that protects our body, It is made up of lots of layers of cells. For greater protection epithelial tissue controls permeability, it tightly regulates what can and can't pass through the body. For example, our skin is relatively impermeable meaning most substances can't easily pass through into our bodies but this is not the case in the epithelium that lines our intestines. This epithelium is much thinner and it allows for water and nutrients to pass easily through while keeping bacteria out. Epithelial tissue has a really rich nerve supply, this is really important because these nerves gather really important sensory information such as pressure, pain and temperature, by being able to detect all of these sensations it can actually stop us from hurting ourselves.

Some epithelial cells are highly specialized and capable of producing secretions. This could be single cells producing secretions or more often these secretory cells form epithelial glands. These secretions are able to be released into ducts and then discharged onto the surface of the epithelium. Now we call these glands exocrine glands and common examples include our sweat glands and saliva glands. Secretions can also be released into surrounding tissue and blood, and we call these glands endocrine glands and an example of an endocrine gland is the thyroid gland.