Epithelial Tissue

They are thin tissue that covers all the exposed surfaces of the body. They cover various surfaces of body parts like the inner lining of the mouth, digestive tract, secretory glands, lining of hollow parts of every organ such as heart, lungs, eyes, the urogenital tract, etc. 

The cells which form epithelial tissue are closely attached to each other through a structure called tight junctions. They don't have blood vessels and nerves and also they are supported by connective tissue called the basement membrane. They show polarity with distinct basal domains facing the basement membrane and other apical surfaces of the lumen of an organ or the external environment.

Types of Epithelial Tissue

Classification of epithelial tissue is mainly based on its layers, shape, and functioning.

1. Classification Based on Their Shape:

1. Squamous Epithelium: This type of tissue has cells with a wider length than their height. They are mainly found in the lining of the mouth, esophagus, alveoli of lungs including blood vessels.

2. Cuboidal Epithelium: This type of tissue has similar width and height i.e they are mainly cube-shaped.

3. Columnar Epithelium: This type of tissue is taller in height than width i.e column-like shape. They are further classified into two types i.e ciliated columnar epithelium and glandular columnar epithelium.

2. Based on Layer, Epithelial Tissue is Classified into Two Types. They are:

Simple Epithelial/Unilayered Epithelial: Having a single layer. They are in direct contact with basement membranes that separate them from the underlying connective tissue. They are further divided into various types:

1. Simple Squamous Epithelium: Their cells appear like scale and flattened or round form. Example: walls of capillaries, linings of pericardial, pleural, and peritoneal cavities, linings of alveoli of the lungs.

2. Simple Cuboidal Epithelium: These types of cells may have secretory, absorptive, or excretory functions. Examples: collecting ducts of the kidney, pancreas, and salivary gland.

3. Simple Columnar Epithelium: These types of cells can be secretive, absorptive, or excretory in nature. Some of these tissues contain goblet cells and are referred to as simple glandular columnar epithelium. These secrete mucus and are found in the stomach, colon, and rectum.

4. Pseudostratified Epithelium: They can be either ciliated or nonciliated. An example of ciliated is the respiratory epithelium.  

3. Stratified Epithelium: Having two or more cell layers i.e they are multilayered. Further, they are of three types:

1. Stratified Squamous Epithelium: They mainly provide protection against abrasion and water loss. They are mainly found in the oral cavity, esophagus, larynx, etc. Stratified cuboidal epithelium:

2. Stratified Columnar Epithelium: Their main functions are secretion and protection. They are present in the covering of the conjunctiva of the eye.

3. Stratified Cuboidal Epithelium: Their main function is protective in nature. They mainly line the excretory ducts of the sweat glands, large ducts of excretory glands.

Structure of Epithelial Tissue

They are formed from tightly fitted continuous layers of cells. They are either exposed to the external surface or the body fluid inside the body. There are specialized junctions present between the cells of the epithelium. Some of these junctions are given below:

Tight Junctions: They mainly prevent leakage across the tissue.

Adherens Junction: They help in keeping neighboring tissues properly cemented.

Gap Junction: They help in the movement of ions and molecules across the tissue. 

Epithelial Cell Mainly has Two Types of Membrane:

1. Mucous Membrane: It is also known as mucosal cells. Goblet cells have mucosal membranes. They mainly perform lubrication, protection process, and easy movement of material across the cell. They are mainly present in digestive tracts.

2. Serous Membrane: They mainly line body cavities which mainly open inside not outsides. Example: pleural cavity, pericardial membranes. 

3. Glands: They are mainly of two types i.e exocrine or endocrine in nature. Exocrine glands secrete their secretion into a duct, for example, goblet cells, sweat glands. Whereas endocrine glands secrete their secretion directly into the blood. Example: hormones.

Functions of Epithelial Cells 

Epithelial cells play a major role in the various function of the body. Some of the crucial functions are given below:

1. Secretion Process: Most of the epithelial cells have the capability to produce secretive macromolecules. One of the best examples is the glandular epithelium. As endocrine gland is also secretive in nature i.e it secretes hormones directly into the bloodstream and some of these hormones are insulin, thyroxin, etc. Whereas exocrine glands which are also secretive in nature release their hormones into ducts, for example, sebum of the skin, digestive enzymes in the small intestine.

2. Absorption Process: The absorption process is mainly performed by surface epithelia with the help of microvilli as they increase surface area for the absorption process. Another example is the columnar epithelium in the small intestine. Their main function is the absorption of nutrients in the digestive tract.

3. Transportation: There are various pumps that are operated by epithelial tissues. Example: Hydrogen ion pump of the kidney ( H+ ion pump) as they function during urine formation. 

4. Protection Process: They act as selective barriers and protecting barriers. They produce intoxication, tearing, and infections. 

5. Receptor Function: Epithelial cells can receive receptors i.e sensory information and they convert them into neural signals. An example of such epithelial tissue is olfactory nasal mucosa, they detect the odors coming from outside and further transmit this information to the central nervous system.

Location of epithelium tissues

Epithelium does have outside and inside cavities. Among that outside cavity is skin. Lumina of bodies is also possessed. As already said, the outside layer is human skin and it is composed of :

• Keratinized cells

• Epithelial cells

• Stratified squamous cells

Non keratinized squamous is seen within the esophagus, the mouth’s inside, vagina, and rectum parts. While in the lungs and gastrointestinal tract it is the other epithelial cells are covered. There is a specialized form of epithelium which is also called by the name vascular endothelium. And it is formed is inside of heart and blood vessels. The fast-growing and regenerating epithelium cells are seen in the outer parts of the cornea. The walls of the peritoneum and pericardium are formed by mesothelium.

The Development of epithelial cells

The embryological germ layers are from which these epithelial tissues are derived. And they are:

• Ectoderm

• Mesoderm

• Endoderm

However, it is a lesser-known fact that pathologists don’t consider endothelium and mesothelium as true epithelium. And both of them mentioned now are derived from mesoderm. And students must know that pathologists call the cancers in mesothelium and endothelium sarcomas. While carcinomas for the cancers in the true epithelium. However, it is generally accepted that epithelium is derived from all three germ layers. 

Epithelial tissue is called to have the major tissue present in our body. This is because they cover almost all of our body surfaces. The surfaces include hollow organs and for lining body cavities. And it is the diversity in functions that makes the tissue more significant. The functions include:

• Excretion

• Secretion

• Absorption

• Diffusion

• Filtration

About Ciliated Epithelium

The ciliated epithelium is capable of clearing the parts inside and nearby the trachea for easy passage. It helps in moving mucus. It is possible because of the folds present in the membrane. And these folds will have appeared just as hair-like projections. And this type of tissue is found up to bronchioles which are present inside the lungs. The foreign particles trapped inside are taken out as mucus by the ciliated epithelium present there. 

Students may wonder why epithelial tissues don’t have a blood supply! It is already known that the primary function of epithelium is to protect cells. So there is this slightest possibility of losing blood as a result of the rubbing and shaking of the parts containing epithelial tissue. So it is nature’s gift to not have blood supply in those areas.