Epithelial tissue is one of the four basic types of animal tissue, that covers the surfaces of the body, lines the internal cavities, and forms glands. It is composed of closely packed cells that are arranged in sheets or layers. This tissue has several functions, such as protection, absorption, excretion, secretion, and sensation. Here is a flow chart of epithelial tissue:

Types of Epithelial Tissue:
- Simple Squamous Epithelium: thin, flat cells that allow for diffusion and filtration.
- Simple Cuboidal Epithelium: cube-shaped cells that secrete and absorb substances.
- Simple Columnar Epithelium: tall, narrow cells that secrete mucus and absorb nutrients.
- Pseudostratified Columnar Epithelium: appears layered, but all cells touch the basement membrane; secretes and moves mucus.
- Stratified Squamous Epithelium: layers of flat cells that protect against abrasion and dehydration.
- Stratified Cuboidal Epithelium: layers of cube-shaped cells that protect and secrete.
- Stratified Columnar Epithelium: layers of tall, narrow cells that protect and secrete.

Functions of Epithelial Tissue:
- Protection: provides a barrier against physical, chemical, and biological damage.
- Absorption: absorbs nutrients, water, and ions from the environment.
- Excretion: eliminates waste products from the body.
- Secretion: produces and releases substances such as hormones, enzymes, and mucus.
- Sensation: contains sensory receptors that detect touch, pressure, temperature, and pain.

Characteristics of Epithelial Tissue:
- Avascular: lacks blood vessels and receives nutrients by diffusion from underlying tissues.
- Basement Membrane: a thin layer of extracellular matrix that anchors the epithelium to the underlying connective tissue.
- Apical Surface: faces the external environment or internal cavity and may have specialized structures such as cilia or microvilli.
- Lateral Surface: faces neighboring cells and may have intercellular junctions.
- Regeneration: has a high rate of cell division and can regenerate quickly after injury.

In conclusion, epithelial tissue is a vital component of the body that performs a variety of functions. Understanding its types, functions, and characteristics can help in the diagnosis and treatment of diseases that affect this tissue.