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Understand the Facts about the Epithelial Tissue
1. What is the function of the skin in humans?
The skin is the external covering of the body. In humans its main functions are protection, perception of information from the environment, control of the body temperature and secretion of substances.
The Epithelium - Image Diversity: skin
2. What are the tissues that form the skin in vertebrates?
The skin of vertebrates is made of epidermis, an external layer of epithelial tissue, and dermis, a layer of connective tissue under the epidermis. One can cite also the hypodermis, a layer of adipose tissue under the dermis.
Skin annexes may exist in some phyla and classes, like hair, sweat glands and sebaceous glands.
The Epithelium - Image Diversity: skin histology
3. Besides the skin what are the other coverings of the body?
Besides the skin there are other covering tissues made of epithelium over other tissue layers. They are the tissues that cover the internal surfaces of hollow organs, like the organs of the digestive tube, the airway, the renal tubules, the ureters, the bladder, the urethra and the blood vessels. The glands and the serous membranes are made of epithelial tissue too.
4. What are some functions of the epithelium?
The epithelial tissues can perform covering, impermeability and protection against the environment, for example, in the skin, resorption, as in the guts and renal tubules, gas exchange, for example, the amphibian skin, thermal regulation, like sweating, secretion of substances, as in the epithelium of glands. In some animals the skin also has the important function of camouflage and mimicry .
5. What is the typical feature of the epithelia? How different is it from the connective tissue?
The typical feature of the epithelium is the absence or almost absence of space between cells. The epithelial cells are compactly positioned side-by-side with the help of specialized structures for cell adhesion like desmosomes and interdigitations. This feature relates to the fact that these tissues are generally exposed to an exterior surround and so they need more resistance and impermeability against the entrance of strange material into the body.
The connective tissue presents opposite features due to its filling function. It has much interstitial material (the matrix) and relatively large space between cells.
6. What are the specialized structures that help the adhesion between cells?
The structures responsible for the union of the epithelial cells are called cell junctions. The main cell junctions are interdigitations, desmosomes, zonula adherens (adherens junction), tight junctions (zonula occludens) and gap junctions.
The Epithelium - Image Diversity: cell junctions
7. Is the epithelium vascularized? How do nutrients and oxygen reach the epithelium? Why is this feature an important evolutionary acquisition?
Epithelia are not vascularized (capillaries do not directly reach their cells). The epithelium exchanges substances by diffusion with the connective tissue situated under it.
Since the epithelia are not vascularized minuscule skin injuries or scratches that happen all the time do not trigger bleeding and do not expose the blood to contamination from external agents. This is an important protective strategy discovered by evolution.
8. How are the epithelial tissues classified?
The epithelial tissues are classified according to the shape of the cells that form it (epithelial cells may be cuboidal, columnar, or squamous) and according to the number of layers in which those cells are placed in the tissue (into simple or stratified).
The main types of epithelial tissues are simple cuboidal, simple columnar, simple squamous, stratified squamous and pseudostratified columnar (resembling more than one layer but actually having only one). There are also stratified cuboidal and stratified columnar epithelia (rare).
The Epithelium - Image Diversity: types of epithelial tissue
9. How different is the simple cuboidal epithelium from the columnar epithelium? Where can these epithelia be found in the human body?
The simple cuboidal epithelium is made of a single layer of cuboidal epithelial cells. The simple columnar epithelium is made of a single layer of prismatic cells.
The simple cuboidal epithelium can be found, for example, in the renal tubules and in the walls of the thyroid follicles. The simple columnar is the epithelium that covers internally the intestines, the stomach and the gallbladder, for example.
10. How different is the simple squamous epithelium from the stratified squamous epithelium? Where can these epithelia be found in the human body?
The simple squamous epithelium is made of a single layer of flat (squamous) cells. The stratified squamous epithelium is made of the same type of flat cells placed in several superimposed layers.
The simple squamous epithelium is found in the pulmonary alveoli. The stratified squamous epithelium can be found in the moist mucosae, like the mucosae of the mouth, esophagus and vagina, and it is the epithelium of the skin.
11. What is the function of keratin in the epidermis?
The epidermis is the outer layer of the skin made of epithelial tissue. In the epidermis there are keratin-secreting cells (keratinocytes). Keratin is an insoluble protein that impregnates the surface of the skin providing protection and impermeability. In mammals keratin also forms the hairs.
The keratinized cells of the skin surface form the corneal layer. These cells die and are continuously replaced by others.
12. How different is the fish epidermis from the amphibian epidermis?
The fish epidermis is very thin and contains mucus-secreting cells. The fish skin does not present keratin. The mucus has a protective function and it also helps the sliding of the animal under water. (The fish scales originate from the dermis and not from the epidermis.)
In amphibians there is already a slight keratinization of the skin, probably an additional adaptation to the terrestrial environment. Amphibians have smooth and wet epidermis without scales. These features facilitate their cutaneous respiration.
13. Which are the glands present in the epidermis of mammals, birds and reptiles?
In the epidermis of birds and reptiles there are practically no glands. In mammals there are sweat glands and sebaceous glands.
The Epithelium - Image Diversity: gland histology
14. What are melanocytes?
Melanocytes are epithelial cells of the skin specialized in secretion of melanin. Melanin is a pigment that besides coloring the skin, the iris of the eye and the hair, also works as a filter against the ultraviolet radiation of the sun thus protecting the body against the harmful effects of this radiation (mainly burns and carcinogenic mutations).
Melanocytes are the cells affected in one of the more deadly skin cancers: melanoma.
The Epithelium - Image Diversity: melanocytes
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