Classification of Epithelia
- Epithelial classification uses two main features: the number of cell layers and the shape of the superficial cells.
- Superficial cells are the cells at the free or luminal surface of an epithelium.
- Simple epithelium has one cell layer, and every cell contacts the basal lamina.
- Stratified epithelium has multiple cell layers, and only the deepest cells contact the basal lamina.
- Pseudostratified epithelium looks multilayered, but it is one cell layer because every cell reaches the basal lamina.
- Squamous cells are flat cells that support thin barriers for diffusion, filtration, or protection.
- Cuboidal cells are roughly cube-shaped cells often involved in secretion or absorption.
- Columnar cells are taller cells often specialized for absorption, secretion, or movement of material along a surface.
- Simple squamous epithelium has one layer of flat cells.
- Endothelium is simple squamous epithelium lining blood vessels.
- Mesothelium is simple squamous epithelium lining serous cavities such as the peritoneal and pleural cavities.
- Peritoneum is the serous membrane associated with the abdominal cavity and abdominal organs.
- Pleura is the serous membrane associated with the lungs and thoracic cavity.
- Simple cuboidal epithelium has one layer of cube-shaped cells.
- Distal kidney tubules, some gland ducts, and the ovary surface are examples of simple cuboidal epithelium locations.
- Simple columnar epithelium has one layer of tall cells.
- Intestine, stomach, and some excretory ducts are examples of simple columnar epithelial locations.
- Pseudostratified epithelium has cells of different heights, so nuclei appear at different levels even though the tissue is a single layer.
- The lumen is the open space inside a tube or hollow organ, and not all cells in pseudostratified epithelium reach it.
- Trachea, primary bronchi, nasal cavity, and parotid gland ducts are examples of pseudostratified epithelial locations.
- Stratified squamous nonkeratinized epithelium has many layers with living flattened surface cells that still contain nuclei.
- Nucleated superficial cells distinguish nonkeratinized stratified squamous epithelium from keratinized stratified squamous epithelium.
- Esophagus, vagina, mouth, and true vocal cords are examples of stratified squamous nonkeratinized epithelium locations.
- Stratified squamous keratinized epithelium has many layers with flattened surface cells that have lost their nuclei and are filled with keratin.
- Anucleate superficial cells are surface cells without nuclei, and they are characteristic of keratinized epidermis.
- The epidermis is the epithelial outer layer of skin and is made of stratified squamous keratinized epithelium.
- Stratified cuboidal epithelium has multiple layers with cube-shaped cells at the surface.
- Sweat gland ducts are examples of stratified cuboidal epithelium locations.
- Stratified columnar epithelium has multiple layers with tall cells at the surface.
- Large excretory ducts and the cavernous urethra are examples of stratified columnar epithelium locations.
- Transitional epithelium changes surface-cell shape as urinary passages stretch or relax.
- Dome-shaped superficial cells are seen in transitional epithelium when the tissue is relaxed.
- Flattened superficial cells are seen in transitional epithelium when the tissue is stretched.
- Urinary passages from the renal calyces to the urethra are lined by transitional epithelium.
- Renal calyces are urine-collecting spaces in the kidney that drain toward the renal pelvis.
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