Study Tip: When identifying epithelium under a microscope, look for the "free surface." If you see a clear boundary where cells end and a cavity begins, you are likely looking at the apical surface of an epithelium.
Classification of Epithelia
Histologists classify epithelial tissue using a two-part naming system. The first name describes the number of cell layers, and the second name describes the shape of the cells.
Cell Shapes
There are three primary shapes used in classification:
- Squamous: Cells that are flattened and scale-like.
- Cuboidal: Cells that are box-like or cube-shaped.
- Columnar: Cells that are tall and column-like.
Simple Epithelia
Simple epithelia consist of a single layer of cells. This structure is ideal for processes like diffusion and absorption where a thin barrier is required.
- Simple Squamous: These cells are extremely thin with sparse cytoplasm. They are found in areas like the lungs (alveoli) and the lining of blood vessels and the heart (where they are specifically called endothelium).
- Simple Cuboidal: A single layer of cube-shaped cells, typically forming the walls of small gland ducts and many kidney tubules.
- Simple Columnar: A single layer of tall, closely packed cells. These cells often feature microvilli to increase surface area or cilia to move substances. They may also contain goblet cells, which are specialized for secreting mucus.
- Pseudostratified Columnar: This is a unique type where cells vary in height, making the tissue appear multi-layered (stratified), but it is actually a single layer because every cell touches the basal lamina. These are often ciliated and found in the upper respiratory tract.
Stratified Epithelia
Stratified epithelia consist of two or more layers of cells. This structure provides increased protection against friction and environmental damage.
- Stratified Squamous: The most widespread type. It can be keratinized (containing a tough protein found in the skin) or nonkeratinized (found in moist linings).
- Stratified Cuboidal and Columnar: These are relatively rare and are typically found in specific locations like the pharynx or certain glandular ducts.
- Transitional Epithelium: A specialized type of stratified epithelium found in the urinary bladder, ureters, and urethra. Its unique ability to change shape when stretched allows for increased storage capacity in the bladder.
Glandular Epithelia
A gland is defined as one or more cells that make and secrete an aqueous fluid called a secretion. Glands are classified based on where they release their products.
Endocrine vs. Exocrine Glands
- Endocrine Glands: These are ductless glands. They secrete messenger chemicals known as hormones directly into the bloodstream, which then travel to target organs.
- Exocrine Glands: These glands use ducts to release their secretions onto epithelial surfaces. Examples include sweat, salivary, mucous, and oil glands. Exocrine glands can be unicellular (such as a single goblet cell) or multicellular.
Classification of Multicellular Exocrine Glands
Multicellular exocrine glands are categorized by their physical structure and how they release their contents.
By Structure:
- Tubular glands: The secretory cells form a tube.
- Alveolar (or Acinar) glands: The secretory cells form sac-like structures.
- Some glands may be branched, possessing both tubular and alveolar components.
By Mode of Secretion:
The method by which a cell releases its product is a critical distinction in histology:
- Merocrine: The most common mode, where products are released via exocytosis without any damage to the cell (e.g., sweat or pancreatic secretions).
- Holocrine: The cell accumulates its product until it eventually ruptures, releasing the entire contents along with the cell itself (e.g., sebaceous oil glands).
- Apocrine: A mode where the apex (top) of the cell ruptures to release the secretion. (Note: The existence of this type in humans is sometimes considered controversial, though it is often associated with mammary cells).
