Epithelium is what covers all the internal and external surfaces of our bodies. This article dives into the diverse types of epithelium, from simple squamous to transitional, understanding their structures and functions. Learn about their locations and roles in various bodily systems. To learn how to correctly identify and name epithelia, see the article on histology basics.

Simple squamous (pavement) epithelium

This is one of the simplest yet most common types of epithelium. The epithelium consists of a single layer of thin, flat scale like cells with little cytoplasm. Additionally, The nucleus bulges into the lumen. Pinocytotic vesicles are present in relation to cell membrane as well as in the cytoplasm.

Sites:

• Surfaces involved in passive diffusion -alveoli of lung.
• Blood capillaries endothelium
• Lining body cavities mesothelium
• Subarachnoid and subdural spaces
• Eye chambers
• nephron

Simple cuboidal epithelium

This epithelium lines the small ducts of glands. And, when you cut sections at an angle to the surface, the cells appear as cubes. Nuclei are rounded. It functions in secretion, excretion, and absorption.

Sites:

• Ducts of salivary glands
• Proximal convoluted tubules of the kidney
• Germinal epithelium of the ovary
• thyroid
• secretory portions of small glands.

Simple columnar epithelium

The height of the cells of this epithelium may vary. They are taller than they are wider, and nuclei are elongated and located at the base at right angles to the base of the cell, found in highly absorptive and secretory sites.

In sites where there is absorption, the luminal surface of the epithelial cells contains numerous microvilli forming a brush border to increase the absorptive surface area.

Sites:

• Sites where the chief function is protection.
  • Ducts of glands
  • Bronchioles
• Sites where cells are specialized to secrete mucous. (In such sites, cells appear frothy in hematoxylin and eosin stains)
  • Surface lining of stomach
  • Cervical canal
  • Small and large intestine where goblet cells secrete mucin
• Sites where cells are specialized for absorption
  • Lining of the small intestines.

Simple columnar ciliated epithelium

You can find cilia on the luminal surface of most of the lining cells. Beating of cilia facilitate movement of mucus along the surface and transportation of the ova.

Sites:

• Uterine tube

Pseudostratified columnar epithelium

This epithelium is a simple epithelium with single layer of columnar cells. All the cells rest on the basement membrane, but all cells do not reach the lumen.

Therefore, the nuclei are found at different levels. In certain sites the epithelium is ciliated whereas in some other site stereocilia are found on the luminal surface in ciliated epithelium in the respiratory system-respiratory epithelium the cilia trap and move “pollutants” towards the mouth and then swallowed.

In parts of the male genital system, stereocilia are present on the luminal surface. Stereocilia are large, branched microvilli which provide an absorptive function.

Sites:

• Respiratory tract
• Male reproductive tract epithelium with streocilia
• Epididymis

Stratified squamous epithelium

The epithelium consist of several cell layers and such epithelium serves mainly a function of protection. Basal cells are columnar type and the next ceil layers are cuboidal and when reaches the surface cells becoine squamous type, Surface cells are degenerated and are shed. Replaced by the cells of the deeper layers as there are stem cells.

Two types of epithelium:

• Keratinized stratified squamous epithelium-dry type
• Non keratinized stratified squamous epithelium-wet type

Non keratinized stratified squamous (wet) epithelium

Found in wet surfaces which are exposed to certain amount of wear and tear and sites where some protection is needed. Surface cells are squamous in type, and they are living cells with elongated nuclei. However, the basal cells at the bottom layer take up more of a cuboidal shape.

Sites:

• Found in the upper part of the digestive tract: oral cavity and oesophagus.
• Anal canal
• Larynx
• vagina

Keratinized type stratified squamous (dry) epithelium.

Found in parts of the body exposed to the atmosphere. A waterproof covering made of keratin covers this epithelium. A thick keratin layer protects the body from abrasion and desiccation.

Sites:

• Skin

Stratified cuboidal and stratified columnar epithelia.

In the stratified cuboidal epithelium, there are 2-3 layers of cuboidal calls and in stratified columnar epithelium there are several cell layers having columnar cells in the most superficial layer. For instance, you can see this type of epithelia in places where more protection is needed than that given by a simple epithelium. No significant absorptive or secretory function.

Sites

• Large ducts of exocrine glands
• Ducts of salivary glands
• Ducts of sweat glands

Transitional epithelium (Urothelium)

These types of epithelium are specialized to stretch and to withstand the toxicity of urine. Moreover, they are made up of 5 to 6 layers of cells. In a relaxed state superficial cells are large rounded and pale stained. Luminal surface shows a scalloped outline. Superficial cells of the transitional epithelium have special features. The membrane of luminal surface forms thick plates separated by narrow bands of thin membrane and responsible for osmotic barrier between urine and tissue fluids.

The cell membrane of the luminal surface appears thinner with the flattening of the superficial layers. Tight junctions between the surface epithelial cells prevent intercellular diffusion. In the deeper layers, desmosomal junctions are fewer. Numerous interdigitations and infoldings are present between two adjacent cells.

Sites

• Ureter
• Urinary bladder
• Prostatic urethra (proximal part)

Summary