Epithelial tissues line body cavities and spaces. They are avascular tissues that are characterized by a free surface and a basement membrane. The free or apical surface faces the space, lumen or body cavity. The basement membrane is attached to the underlying tissue.
The tissue is classified by the cell shape and by the number of layers of cells. Single cell layers are called simple. Simple epithelial tissues are found in internal environments where the tissue functions in diffusion or secretion and abrasion (protection function) is minimal. Stratified epithelial tissues have two or more layers of cells. Pseudostratified epithelial tissues are simple (single layer of cells) layers in which all of the cells contact the basal lamina, but not all cells reach the free surface.
Epithelial cells shapes include cuboidal, squamous and columnar. Cuboidal cells are as tall as they are wide. They look square and typically have a centrally located nucleus. Squamous cells are flat and platelike. In cross section they are very, very thin. In transverse sections they are polygonal. Columnar cells are rectangular.
Goblet cells are associated with epithelial tissues. Goblet cells are unicellular glands that secrete mucous.
To view a larger version of the thumbnails below, click the thumbnail. All of these are original images, photgraphed in our labs from the college's slide collection. Sensory structures from the skin are also shown, even though they are not epithelium.
|Simple Squamous Epithelium|
|Simple squamous epithelium: The arrows tips are pointing at the apical surface of the simple squamous epithelium of this blood vessel. In the cardiovascular system this layer is called the endothelium. Simple squamous epithelium is found in the lung (alveoli), lining the lumen of blood vessels and lymphatic vessels and in the kidney,|
|Simple squamous epithelium: The arrow tips are pointing at the simple squamous epithelium of renal corpuscle of the kidney. The renal corpuscle is composed of the glomerulus and Bowman's capsule. The glomerulus is a capillary network of simple squamous epithelium seen in the center of the image. Around the glomerulus is a space which is bounded by the simple squamous epithelium of Bowman's capsule|
|Simple squamous epithelium: This is the renal corpuscle again. The arrow points are pointing at the simple squamous epithelium of the glomerulus and Bowman's capsule.|
|Simple squamous epithelium: The arrows tips are pointing at the apical surface of the simple squamous epithelium of this blood vessel. In the cardiovascular system this layer is called the endothelium.|
|Simple squamous epithelium: The arrows tips are pointing at the apical surface of the simple squamous epithelium of this arteriole. In the cardiovascular system this layer is called the endothelium.|
|Simple Cuboidal Epithelium|
|Simple cuboidal epithelium: This is a section from the kidney again. The yellow arrows are pointing to simple cuboidal epithelium of the kidney tubules. Simple cuboidal epithelium is found in the kidney, surface of the ovary, and in the ducts and secretory portions of glands.|
|Simple cuboidal and simple squamous: This is a section from the kidney again. The black arrows are pointing to the simple squamous epithelium of the glomerulus and capsule. The yellow arrows are pointing to simple cuboidal epithelium of the kidney tubules.|
|Simple Columnar Epithelium|
|Simple Columnar Epithelium: This a section from the intestine. The epithelium is composed of a single layer of tall rectangular cells. The clear visible 'inclusions' are actually goblet cells. Goblet cells are unicellular glands that produce and secrete mucous. Simple columnar epithelium is found throughout the digestive tract.|
|Goblet cells: The black arrow in the thumbnail is pointing towards the goblet cell. The red arrow labeled 'A' is pointed a the mucous pouring out of the cell and the blue arrowed, labeled 'C' is pointing at the ciliated surface of the epithelium.|
|The black arrow is pointing at a goblet cell.|
|Pseudostratified Columnar Epithelium|
|Pseudostratified columnar epithelium is a simple (single) layer of cells in which the apical or free surface of some cells does not reach the lumen. This gives the tissue its stratified appearance. This cells in this tissue have cilia as a modification of their apical surface. Pseudostratified columnar epithelium is found in the trachea, upper respiratory system and in portions of the male reproductive tract.|
|This is a cross section through thick skin. The letter A is located in the reticular layer of the dermis. The letter B is in the papillary layer of the dermis. The letter C is located in the epidermis. The unlabelled black arrow is pointing at the stratum basale of the epidermis.The letter D is pointing to the location of the stratum lucidum. This layer found only in thick skin separates the stratum corneum (more superficial layer) from the stratum granulosum (slightly deeper, darker layer). Stratified squamous epithelium is found in the skin (keratinized) and at the stomach/esophageal boundary (cardiac junction) (non-keratinized).|
|The yellow arrow indicates the location of a Meissener's corpuscle. See below for details.|
|Meissener's corpuscles are located at the black arrow tips. These sensory organs are located in the papillary layer of the dermis and detect light pressure. The shorter yellow arrow is pointing toward the stratum basale. The longer yellow arrow is pointing toward the stratum lucidum.|
|Pacinian corpuscles are found deeper in the dermis than either Merkel's or Meissener's corpuscles. They detect heavy pressure.|
|Stratified Cuboidal Epithelium|
|This is not a stellar example of stratified cuboidal epithelium. Two layers of nuclei can be observed. Stratified cuboidal epithelium is observed in ducts of sweat glands, mammary and salivary glands.The epithelium of the seminiferous tubules is also stratified cuboidal.|
|Cross section of a semiferous tubule from the testis. The arrow points to the basement membrane/basal lamina of the tissue.|
|Stratified Columnar Epithelium|
|Stratified columnar is rare. It is found in the male urethra and some ducts. The basal cells typically appear more cuboidal and the apical cells more columnar. The black double tipped arrow is in the lumen of the duct.|
|Transitional epithelium: Transitional epithelium is a stratified tissue found in the bladder. In a relaxed bladder the cells appear round or cuboidal and the tissue folds into ridges or rugae. As the bladder fills the tissue stretches and the cells flatten and appear squamous. The blue arrow tips are pointing to the basement membrane of the epithelium. The black arrows are pointing to the free surface of the tissue.|
|Wider view of transitional epithelium. The yellow arrows are pointing to the basement membranes and the black arrows are in the lumen and pointing toward the apical surface of the tissue.|