Epithelial tissues line and cover the body. While diverse, epithelial tissues all share the same basic traits including:
- cellularity (closely packed cells);
- specialized contacts (cell to cell junctions);
- polarity (contain two distinct regions);
- relationship to connective tissue (supported by connective tissue);
- avascularity with innervation (have no blood vessels, but do have nerves); and
- regeneration (quickly replaced by new cells).
The functions of epithelial tissues include protection, diffusion, absorption, secretion, ion transport and filtration. Epithelial cells form the slippery surfaces which line the body, and also form the major covering of the body -- skin.
Simple epithelial tissues are composed of one layer of cells and include simple squamous, simple cuboidal, simple columnar and pseudostratified epithelium. Stratified epithelium is made of two or more layers and functions primarily in protection.
Simple Squamous Epithelium
Simple squamous epithelium is one layer of flat shaped cells which primarily play a role in diffusion and filtration. Simple squamous cells also function in secretion. These cells are located in:
- kidneys;
- lungs;
- linings of the heart, blood and lymphatic vessels; and
- lining of front body cavity sacs (which hold organs) called serosa.
Simple Cuboidal Epithelium
Simple cuboidal epithelium are large cube shaped cells which function in secretion and absorption. They are found within:
- kidney tubules;
- secretory portions of small glands; and
- ovary surface.
Simple Columnar Epithelium
Simple columnar epithelium are rectangular shaped cells which absorb and secrete substances. Some have cilia, which function to move secreted mucous and reproductive cells. These cells are found in linings of the:
- digestive tract;
- gall bladder;
- some gland ducts;
- small bronchi (ciliated);
- uterine tube (ciliated); and
- uterus (ciliated).
Pseudostratified Columnar Epithelium
Pseudostratified columnar epithelium appear falsely stratified -- they appear to have two layers. However, all pseudostratified cells begin at the base layer (called the basement membrane), while cell heights vary. Pseudostratified columnar epithelium function to secrete and propel mucous and can be found in the:
- ducts of male reproductive glands;
- ducts of large glands; and
- upper respiratory tract.
Stratified Squamous Epithelium
Stratified squamous epithelium is found in the skin and is either keratinized or non-keratinized. Keratinized stratified squamous epithelial cells make up the protective, dead outer skin cells. Non-keratinized cells form the moist linings of body openings including the esophagous, mouth, anus, vagina and urethra.
Stratified Cuboidal Epithelium
Stratified cuboidal epithelium is composed of two layers of cuboidal cells and provides protection. Stratified cuboidal cells are found in the ducts of:
- mammary glands;
- salivary glands; and
- large sweat glands.
Stratified Columnar Epithelium
Stratified columnar epithelium is composed of several layers and functions to protect and secrete. This rare epithelium is found in the male urethra and some gland ducts.
Transitional Epithelium
Transitional epithelium is involved with the functioning of the urinary system and has characteristics of stratified cuboidal and stratified squamous cells. The outer cell layer of transitional epithelium stretches when the bladder expands and flattens when the bladder empties.
Summary
Overall epithelial tissues function to line, cover and protect all of the inner and outer surfaces of the body. The shape and arrangement of each type of epithelium plays a role in its function and location within the body. Thin simple squamous cells allow substances to be filtered and diffused, while layered stratified epithelium provide protection.
Cuboidal and columnar versions of epithelium play a role in secretion of mucous and other substances. Columnar and pseudostratified columnar epithelium may be ciliated and play a role in moving, or propelling, mucous or reproductive cells. Scroll down and click on the image thumbnail to compare squamous and columnar epithelial cells.
Source:
- Marieb, Elaine, Patricia Brady and Jon Mallatt. Human Anatomy, sixth ed. Pearson Education. 2011.
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