Epithelium
- One of the four major tissue types of the human body.
Key principles of epithelial tissues:
- Epithelial tissues comprise sheets of cells that are connected via intercellular junctions
- Epithelial tissues are polar:
- Some general functions of epithelial tissues include:
- Protection of underlying tissues from damage due to abrasion and/or dehydration
- Secretion
- Absorption
4 subcategories of epithelial tissue types
Named according to their shape at the apical surface:
- Squamous epithelium comprises flattened cells; "squamous" means scaly.
- Cuboidal epithelium comprises cube-shaped cells.
- Columnar epithelium comprises cells that are taller than they are wide; in other words, column-shaped.
- Transitional comprises irregularly shaped cells that can alter, or "transition," between flat and round.
- Simple = single layer
- Stratified = multiple layers
Locations and Functions:
Simple Squamous:
- Thin shape facilitates rapid diffusion and filtration across surfaces
- Located in the alveolar sacs of the lungs, where gas exchange occurs between the blood and the air, and in the linings of the cardiovascular system (where it is called endothelium).
- Keratinized = surface layers of dead, anucleic cells that contain the protein keratin.
- This thick, toughened layer provides extra protection in areas, such as the palms of the hands, where abrasion is most common.
- Non-keratinized = No toughened layer of keratinized cells.
- Stratified squamous epithelium protects the underlying tissues from abrasion, dehydration, and penetration of foreign materials
- Located in epidermis of the skin and in the linings of the gastrointestinal tract, anus, and vagina.
- Comprises a single layer of block-like cells.
- Nuclei are located near the center of each cell, and line up in a row across the cells.
- Can have microvilli on the apical surface; microvilli are microscopic cytoplasmic extensions that increase the surface area for reabsorption and secretion of substances.
- Particularly important in the kidney tubules, where nutrients, ions, water, and other substances are absorbed from and secreted as part of urine formation, and, in the ducts and secretory regions of many glands.
- Looks like stacked blocks; nuclei are centrally located.
- Stratified cuboid epithelium provide protection of underlying tissues and secretion of substances.
- It is located in the large ducts of some exocrine glands, such as the salivary glands.
- Comprises mucus-secreting goblet cells and epithelial cells that are taller than they are wide.
- Nuclei of columnar epithelia are centrally located, ovoid in shape.
- Some simple columnar cells have microvilli, which increase the surface area for absorption and secretion in much of the digestive tract.
- Some columnar epithelia have cilia, which are hair-like extensions that "sweep" materials along passageways.
- Ciliated simple columnar epithelia is present in the large bronchioles, where it continuously removes foreign matter from the lower respiratory tract, and in the uterine tubes, where it moves ova away from the ovaries and towards the uterus
- Comprises more than one layer of cells; the apical layer has the truly column-like cells.
- Provides protection and secretion in the large ducts of salivary glands, in the membranous portion of the male urethra, and, in the lining of the uterus.
- Appears (at first) as if it comprises multiple layers, but, upon closer inspection, does not.
- Cells fit together like puzzle pieces, and their nuclei are at various distances from the basal membrane.
- Provides absorptive and secretive surfaces.
- Is often ciliated and has mucus-secreting goblet cells; the mucus traps foreign materials, and the cilia sweep it along passageways, such as in the nasal cavities, larynx, and bronchi.
- Non-ciliated pseudostratified epithelium is rarer; it provides protection in parts of the male urogenital tract.
- Comprises multiple layers of irregularly shaped cells, some of which are multi-nucleated.
- Is located within the ureters and urinary bladder, where its cells stretch and retract in response to changes in urine volume.