- provides physical protection;
- controls permeability;
- provides sensations;
- produces specialized secretions.
Epithelia are classified according to the number of cell layers and the shape of the exposed cells.
There are two basic types of layering – simple and stratified.
There are three cell shapes – squamous, cuboidal, and columnar.
Types of epithelial tissue – (type – structure – location – function)
- Simple squamous – one layer of fat cells – lines ventral body cavities, heart and blood vessels, and portions of the kidney tubules (thin sections of loop of Henle), inner lining of the cornea, and exchange surfaces of the lungs (alveoli) – permits difusion of gases, reduces friction, controls vessel permeability, and performs absorption and secretion
- Simple cuboidal – one layer of cube-shaped cells – found in glands, ducts, portions of kidney tubules – provides limited protection, secretion of hormones, and / or absorption
- Simple columnar – one layer of column-shaped cells – line the stomach, intestine, gallbladder, uterine tubes, and collecting ducts of kidneys – protection, secretion of gastric juices and enzymes, and absorption of digestive end products (if microvilli are present)
- Pseudostratified columnar – line the nasal cavity, bronchi, and trachea – provides protection and secretion
- Ciliated – one layer of columnar cells with cilia on their free surfaces – lines trachea and fallopian tubes – sweeps mucus and dust to the pharynx and sweeps ovum toward uterus.
- Transitional – many layers of cells, but surface cells change from rounded to flat – line the urinary bladder, renal pelvis, and ureters – permits expansion and recoils after stretching
- Stratified squamous – many layers of cells (surface – flat, lower cells – rounded, lower layer undergoes mitosis) – located on the skin surface and line the mouth, throat, esophagus, rectum, anus, and vagina – provides physical protection against abrasion, pathogens, and chemical attack. Surface cells of epidermis are dead providing a barrier to pathogens while interior cells are living and also provide a barrier for pathogens.