Epithelial Tissue Structure and Classification

Epithelial Tissue Characteristics

Epithelial tissue is composed of cells with little intercellular substance. The cells are close together and form one or more layers. In addition, there are cells that divide rapidly to replace those that die, as the tissue is subjected to constant friction and injury.

These cells have points of union between them, which can be:

• Tight junctions: These exist between two adjacent membranes. They are very numerous in the epithelium of the small intestine, preventing molecules from passing between cells and forming an impermeable barrier.
• Communicating junctions: These form bridges between cells, connecting the membranes and the cytoplasm. They are found in cardiac muscle cells to facilitate the transmission of contraction from one heart cell to another.
• Desmosomes: These are mechanical joints between two cells. Shaped like a button, they provide stability to the cell. They are mainly found in skin cells subject to continuous friction.

Types of Epithelial Tissues

Epithelial Coating

This covers the outer surfaces of the body and lines the internal cavities of the body.

1. Simple Epithelium

Consists of a single layer of cells and can have different forms:

• Cuboidal: Part of the renal tubules and glands.
• Columnar: Coating the stomach, intestine, and part of the respiratory system.
• Squamous: Found in the alveoli and pulmonary capillaries. It has protective, secretory, and absorptive functions.

2. Stratified (Laminated) Epithelium

Formed by several layers of cells, it plays a role in secretion and protection. It is found in the mouth, esophagus, vagina, sweat glands, urinary bladder, anal mucosa, etc.

3. Pseudostratified Epithelium

Formed by a single layer of cells with nuclei at different levels, making it seem like it is formed by more than one layer. They often have cilia and are usually part of the lining of the airways, such as the trachea and bronchi, where they secrete mucus.

Glandular Epithelium

This is part of the glands of the body and has a secretory function. The glands are of two types:

1. Exocrine glands: These secrete their products into ducts and then to the outside or other surfaces. Examples include sweat, salivary, and sebaceous glands.
2. Endocrine glands: These secrete their products directly into the blood. These products are hormones. Examples include the hypophysis (pituitary), thyroid, pancreas, ovaries, and testes.