The foundation of Human life can be categorized into numerous groups, each larger than the last. First, we identify atoms that make molecules. Things such as Oxygen, Carbon, and Nitrogen are critical atoms that create the foundation of molecules. Molecules are combinations of atoms that result in the creation and foundation of cells, the smallest life form. Groups of Cells can combine to create one of four primary tissues (Epithelial, Connective, Muscle, and Nervous). Together, a group of tissue will function to create an organ such as the stomach, brain, and kidney. Finally, organs work together to create organ systems that provide the functions of an organism.
Epithelial tissue has characteristics that make it completely different from other tissues. These Characteristics include Cellularity, Polarity, Attachment, and Avascularity.
Cellularity is the characteristic that cells are bound closely together and there is no intercellular space.
Polarity means that the cells have structural and functional differences based on the area of the cell. The apical surface of an epithelial cell is exposed and has things such as cilia and microvilli for absorption.
The basal surface is attached to deeper tissue to stabilize the cell. This is also called Attachment and the epithelial cell’s basal layer is attached to the basal lamina.
Finally, avascularity is the characteristic that epithelial tissue does not contain blood vessels.
The functions of Epithelial tissue vary based on the shape, polarity, and many other factors. The main functions include:
- Physical protection from abrasion, dehydration, and destruction
- Controlling permeability
- Provides sensations (Neuroepithelia)
- Produces secretions with special gland cells
Within Epithelial cells, there are specializations when it comes to the apical surface. The major specializations are Microvilli, Stereocilia, and Ciliated Epithelium. Microvilli help increase surface area to increase absorption of nutrients and materials. They are often found in the urinary and digestive tracts. Stereocilia are long microvilli found in the inner ear and male reproductive tract. In the ear, they help with balance and hearing. Finally, Ciliated Epithelium helps transport material along the apical surface to another location. These are found in the lining of the respiratory tract and are used to push mucus out of your airways.
Maintaining Integrity of Epithelium
Epithelial cells are often small, fragile, and numerous. So, how do they keep their shapes and overall integrity to perform their functions? Three major factors are involved in this:
- Intercellular Connections
- Attachment to the basal lamina
- Self-perpetuated epithelial maintenance and renewal
Intercellular Connections are extensive connections present between epithelial cells that hold them together and prevent the passage of chemicals and pathogens. Some examples of Intercellular Connections include Cell junctions and CAMs (Cell adhesion molecules)
Attachment to the Basal Lamina is created between the cell membrane and the basement membrane. Within this connection, there are typically two layers called the Basal Lamina and Reticular Lamina. The Basal Lamina is the layer closest to the epithelial cell and connects to the Plasma membrane. The Reticular Lamina is the next layer and attaches to the underlying tissue. These two layers help create a boundary between the Connective Tissue and Epithelial cells.
Maintaining Epithelium is extremely important to assure all functions are operating properly. They are replaced frequently because of exposure to enzymes, toxic chemicals, abrasion, and pathogens. So, how are these cells replaced? Epithelial cells are replaced through stem cell division located near the basal lamina.