<sub>Definition of Epithelium:

Epithelium</sub> is a type of _tissue that covers the surfaces of organs, lines body cavities, and forms barriers between different environments in the body.

<sub>Characteristics of Epithelium:

1. Cellularity:</sub> Epithelium is composed of closely-packed cells with minimal extracellular matrix.

_{2. Polarity:} Epithelial cells have an apical surface facing the lumen or external environment and a basal surface facing the underlying connective tissue.

_{3. Attachment:} Epithelial cells are attached to each other and to the basement membrane through specialized junctions.

_{4. Avascularity:} Epithelium lacks blood vessels and receives nutrients through diffusion from underlying connective tissue.

_{5. Regeneration:} Epithelial cells have a high regenerative capacity, allowing for repair and renewal of the tissue.

<sub>Classification of Epithelium:

Epithelium</sub> can be classified based on the shape of its cells and the number of cell layers present.

<sub>Shapes of Epithelial Cells:

1. Squamous:</sub> Flat and scale-like cells.

_{2. Cuboidal:} Cube-shaped cells.

_{3. Columnar:} Tall, rectangular cells.

<sub>Layers of Epithelium:

1. Simple:</sub> Consists of a single layer of cells.

_{2. Stratified:} Composed of multiple layers of cells.

_{3. Pseudostratified:} Appears stratified but actually consists of a single layer of cells of varying heights.

<sub>Functions of Epithelium:

1. Protection:</sub> Epithelial layers act as barriers to protect underlying tissues from mechanical and chemical damage.

_{2. Absorption:} Certain epithelia have specialized cells for absorbing nutrients and ions from the external environment.

_{3. Secretion:} Epithelial cells can produce and release substances such as hormones, enzymes, and mucus.

_{4. Sensation:} Epithelium contains sensory receptors that allow for the detection of stimuli such as touch, temperature, and taste.

<sub>Examples of Epithelial Tissues:

– Simple squamous epithelium: Found in lung alveoli, blood vessels, and serous membranes.

– Stratified squamous epithelium: Forms the outer layer of the skin and lines the oral cavity, esophagus, and vagina.

– Simple cuboidal epithelium: Found in kidney tubules, glands, and the thyroid follicles.

– Simple columnar epithelium: Lines the digestive tract, gallbladder, and parts of the respiratory system.

– Pseudostratified columnar epithelium: Lines the respiratory tract, including the nasal cavity and trachea.</sub>