Epithelial Tissue

Humans are made of four different types of tissues: muscular, neural, epithelial and connective.

The epithelial tissue is a thin layer of tissue covering all body parts, including the inner and outer surfaces. It forms a layer of specialised cells on the outer covering of the exposed surface. Also, it lines the internal body cavity, the digestive, reproductive, respiratory, excretory tracts, eyes and nose.

Three types of primary germ cells lead to the development of epithelial cells – 

1. Ectoderm
2. Mesoderm
3. Endoderm. 

These specialised cells are attached with ‘tight junctions’ and are supported by connective tissue as the basement layer. They are devoid of nerves and blood supply and may be multilayered or single-layered. 

Biology of epithelial cells of the epithelial tissue

A cell is the smallest unit that makes up all living organisms.

Human bodies are made up of about 30 trillion cells depending upon the function and type of each cell. Like any other cell, the epithelial cell is made up of cytoplasm, cell membrane and nucleus. 

1.  Cell membrane- surrounds each cell and controls the movement of substances into and out of it. 
2. Nucleus- present within the cell, composed of genetic material / DNA.
3. Cytoplasm- fluid inside the cell housing the other components. 

What are the nine types of epithelial tissues? 

The nine different types of epithelial tissue are based on the epithelial cells’ shape, layers, and functioning. They are as follows-

Based on the arrangement of cells/ layer of cells 

• Simple/unicellular:  

• Comprised of only one layer of epithelial cells.
• Directly attached to the basement layer.
• Further divided into – 

1.  Simple squamous
2.  Simple cuboidal 
3.  Simple columnar

• Stratified / multicellular 

• It Comprises more than one layer of epithelial cells.
• Further divided into 

1.  Stratified squamous
2. Stratified cuboidal 
3. Stratified columnar

• Pseudostratified 

• Comprised of different sizes of cells closely packed with each other, giving it a stratified appearance but is just a single layer of cells
• It may be ciliated or nonciliated
• Lines the respiratory tract

Based on the shape of the cells 

Squamous epithelium-

• Cells having a width more than the height
• Have a sheet-like appearance
• Found lining the mouth, oesophagus, alveoli and blood vessels

Cuboidal epithelium- 

• Cells having equal height, depth, and width 
• Have a cube-like appearance
• Lining the ducts of kidneys, pancreas and salivary glands

Columnar epithelium

• Cells have a height more than the width, also called goblet cells.
• Have a column-like appearance
• Further divided into two types of cells 

1. ciliated columnar epithelium
2. glandular columnar epithelium
3. Lining the stomach, colon and rectum.

Based on function

Traditional epithelium

• It Consists of several layers and stretches when tightened.
• Also called urothelium
• Lines the urinary tract.

Glandular epithelium

• Layers of cells that produce and secrete substances such as hormones, water and chemicals.
• It lines the specialised glands

Olfactory epithelium

• The cells have specialised cilia and are olfactory receptor cells. 
• Odour is trapped when it enters the olfactory tube, and these cells pass them across the epithelial layer.
• It lines the nasal cavity. 

Characteristics of the different structures of epithelial tissue

• Tight junctions

The epithelial tissue comprises specialised closely attached cells, which prevents tissue leakage.

• Adherence junction

Helps in tight cementation with the neighbouring tissue layer.

• Gap junction

 This junction allows free entry and exit of ions and molecules across the different tissue layers. 

• Cilia

Hair like structure that helps in the mobility of the cell. These delicate structures also help in the movement of substances across.

• Microvilli.

Finger-like non-motile projections increase the surface area of the cell for better absorption. 

Functions of epithelial tissue notes

Based on the structure and location, epithelial tissues in different body parts have different roles: 

Secretion 

• Epithelial cells lining the mucous glands release hormones and enzymes that help digestion.
• The endocrine glands secrete hormones like insulin and thyroxine, which maintain the sugar and thyroid levels in the body. 
• This is the glandular epithelium.

Absorption

• Presence of microvilli on the epithelial cells lining the intestine increases the surface area and allows the absorption of nutrients from the food.
• Cells lining the lungs and liver also help absorb certain substances.

Excretion 

Cells lining the urinary tract and anal tract help remove waste from the body.

Protection 

•  The layers of cells help protect the deeper layers of the body.
• The presence of cilia also protects the inner surfaces from bacteria and harmful chemicals.

Other functions include 

Filtration, diffusion and sensory reception.

Conclusion 

The epithelial tissue notes have shown us how important it is for the proper functioning and maintenance of health. This tissue protects bodies externally and is a significant part of the different functions within. Adequate nutrition and care will lead to a longer life by strengthening this tissue and the other tissues that make up the human body.