There are four main basic types of tissue that are connective tissue, muscle tissue, nervous tissue, and epithelial tissue. Epithelial tissue is a large sheet of cells that covers the outside of the organs and covers the surfaces that are in contact with the outside environment. The structures that are in contact with the outside environment are not only the skin but there are also other structures such as airways and digestive tract that come in contact with the external environment and these structures are also covered by the epithelium.
Generally, the epithelium is made of three types of cells that are squamous, columnar, and cuboidal cells. Unlike other structures of the body, these cells are derived from all three principal germ layers that are ectoderm, endoderm, and mesoderm. For example, the skin developed from the ectoderm, the epithelial cells that line the digestive tract developed from the endoderm, and the epithelium that line the blood vessels are derived from the mesoderm.
The epithelial cells from which the epithelium are made are either arranged in a single layer to make simple epithelium or in multiple layers to make stratified epithelium. Mostly glands are made of up epithelial tissue.
Epithelial tissue is highly cellular with no or little extracellular material is present between their cells. Epithelial tissue exhibit polarity and are avascular. As they are avascular they are nourished through diffusion. Diffusion is a process in which substances from high concentration gradient goes to low concentration gradient.
Epithelial tissue has the capability to replace defective or dead cells.
Structures of epithelial tissue
Epithelial tissue is made of epithelial cells that are held together by a set of other cells through tight junction, adhering junctions, and desmosomes. Epithelial tissue attached to the basement membrane which separates it’s from the underlying connective tissues. The basement membrane also called a basal lamina is a specialized form of the extracellular matrix.
A typical epithelial cell has two surfaces that are the apical surface and a basal surface. The apical surface of the epithelial cell faces the lumen of a tube while the basal surface attached to the basement membrane.
Epithelial cells are held by three junctions among which tight junctions are very important because it restricts diffusion and regulates the passage of ions and metabolites.
The basement membrane is a specialized type of extracellular matrix that provides support to the epithelial tissue. The basement membrane regulates the metabolism and proliferation of the epithelial cells. There are four major components of the basal lamina that are the laminin, type IV collagen, Nidogen, and perlecan. Epithelial cells attached to the basal lamina through integrins.
Transcytosis is a process in which epithelial cells transport material from basal to the apical environment.
Functions of epithelial tissue
1. Epithelial tissue protects the tissues that lie beneath it from radiation, toxins, desiccation, pathogens, and physical trauma.
2. Another important function of epithelial tissue is to regulate the exchange of material between the underlying tissues and the cavity above.
3. Epithelial tissue provides sensation, for example, the respiratory epithelium has cilia that provide chemosensation, thermoception, and mechanosensation.
4. Glands are formed from the infolding of epithelium by a special tissue called glandular epithelium.
5. Epithelium control permeability and allows the selective absorption of substances.
These are the cells that arrange in layers and make epithelial tissues. Normally they are found in three shapes that are squamous, columnar, and cuboidal. These cells are characterized by organelles and proteins present between their basal and apical surfaces. A typical epithelial cell has an apical surface and a basal surface. Different structures are present and attached to these surfaces.
Cilia are extensions of the cell membrane of the epithelium that beats and move the fluids and particles.
Basic epithelial cells are
a). Squamous cells
These are thin flat cells that look polygonal when you see it from the above. The squamous cells make smooth and less friction surfaces. The nucleus of these cells is oval-shaped. These cells are found in the skin and alveoli. Mesothelium and endothelium also consist of squamous cells. Mesothelium line the heart while the endothelium is present in the blood vessels.
b). Cuboidal cells
These are cube-shaped cells that appear square you made a cross-section. The nucleus of these cells is spherical. These cells are found mostly in the exocrine gland, pancreas, and in the lining of the kidney tubule. Germinal epithelium of the ovary and testes are also made of cuboidal cells. Cuboidal cells may either arranged in a single layer to make simple cuboidal epithelium or in multiple layers to make stratified cuboidal epithelium.
c). Columnar cells
Columnar cells are elongated and column-shaped cells that have a height four times more than their width. The nucleus of these cells are elongated. Columnar cells are present in the stomach and intestine. Some columnar cells possess microvilli and cilia and some are specialized for sensory reception.
d). Pseudostratified cells
These are simple columnar cells that give a misleading impression of stratified appearance. Some pseudostratified cells possess cilia. Ciliated pseudostratified cells are found in airways.
Classification of epithelium
Epithelial tissue is classified on the basis of layers and on the basis of the shape of the cells.
On the basis of shape
On the basis of shape epithelial tissues are classified into
On the basis of layers
On the basis of layers epithelial tissues are classified into the simple epithelium and stratified epithelium.
This type of epithelium has only a single layer that is in contact with the basement membrane. Simple epithelium is fragile due to which it is found generally inside the body forming the lining of the blood and lymph vessels, body cavities, heart, and respiratory system. Simple epithelium also lines the digestive tract. Simple epitheliums are further classified into
a). Simple squamous epithelium
A simple squamous epithelium consists of a single layer of squamous cells. This type of epithelium is present in the area of the body where passive diffusion occurs. This type of epithelium is found in the skin, glomeruli, pleurae, and in the pericardium.
b). Simple cuboidal epithelium
this type of epithelium consists of a single layer of cube-shaped cells. The cells of this type of epithelium are almost equaled in height and width. The cytoplasm of simple cuboidal epithelium has more organelles such as mitochondria, and endoplasmic reticulum than simple squamous epithelium. These types of epitheliums are found in the ovaries, renal tubules, seminiferous tubules, pancreas, and salivary glands.
c). Simple columnar epithelium
The simple columnar epithelium consists of a single layer of columnar cells that are elongated and column-shaped cells. The main functions of these types of epithelium are absorption and protection. These types of epitheliums are found in the fallopian tube, stomach, intestine, uterus, and central canal of the spinal cord.
d.) pseudostratified columnar epithelium
These are simple columnar epitheliums but due to the arrangement of cells, it gives a false impression of stratified epithelium. Pseudostratified epithelium is generally referred to as respiratory epithelium. These types of epithelium are found in the nasal cavity, trachea, and bronchi.
2. Stratified epithelium
These are the epitheliums in which the cells are arranged in multiple layers. These epitheliums are quite thick which makes it poor for absorption, and secretion but makes it suitable for protection. Stratified epitheliums are further classified into the following types
a.) stratified squamous epithelium
Stratified squamous epithelium consists of multiple layers of squamous cells. They are further classified into keratinized stratified squamous epithelium and non keratinized stratified squamous epithelium. They are not classified on the basis of absence or presence of keratin but they are classified on the basis of the amount of keratin present inside the epithelium. They are present in the hard palate, dorsum of the tongue, and skin.
b.) Stratified cuboidal epithelium
Stratified cuboidal epithelium consists of two or three layers of cuboidal epithelial cells. These types of epithelium are quite thin. These types of epithelium are found in the salivary and sweat glands but are not involved in the absorption and secretion.
c.) stratified columnar epithelium
Stratified columnar epithelium consists of multiple layers of columnar cells and is quite rare. These types of epithelium are involved in the mucous secretion and are involved in the protection. They are found in the conjunctiva.
d.) transitional epithelium
Transitional epithelium is also called urothelium. These types of epithelium is called transitional epithelium because it contains features of two epithelium. Transitional epithelium is special because they have the capability of distention. Transitional epithelium is impermeable to water and salt and is also resistant to the toxicity of urine.
Location of some epithelium
Blood vessels simple squamous epithelium
Dorsum of tongue stratified squamous keratinized epithelium
Hard palate stratified squamous keratinized epithelium
Esophagus stratified squamous non keratinized epithelium
Stomach simple columnar epithelium
Small intestine simple columnar epithelium
Large intestine simple columnar epithelium
Gall bladder simple columnar epithelium
Thyroid follicle simple cuboidal epithelium
Ependyma simple cuboidal epithelium
Lymph vessels simple squamous epithelium
Ovaries simple cuboidal epithelium
Fallopian tubes simple columnar ciliated epithelium
Ejaculatory duct columnar epithelium
Development of epithelial tissue
Epithelial tissue developed from all three germ layers for example epidermis arises from the ectoderm, lining of the gastrointestinal tract arise from the endoderm and lining of body cavities derived from the mesoderm.
Endothelium and mesothelium that are derived from the mesoderm are not true epithelial tissue because pathologists label cancer in mesothelium and endothelium as sarcomas where epithelial cancer is called carcinoma.