The Pseudostratified Columnar Epithelium
The pseudostratified columnar epithelium is tissues made up of a single layer of cells that appear to be made up of multiple layers, especially when viewed in cross-section. The nuclei of these epithelial cells are found at different levels, giving the illusion of being stratified. However, this tissue is made up of a single layer of cells, and although the cells do not share an apical surface, each cell is in contact with the basement membrane.
Pseudostratified epithelium consisting of cuboid or squamous epithelial cells is rarely found, thus the most common subtype is the pseudostratified columnar epithelium.
What are the Types of Pseudostratified Columnar Epithelia?
These tissues can be classified as ciliated or non-ciliated based on a cellular organelle involved in motility and sensory activity.
Almost all eukaryotic cells have a single primary cilium that plays a crucial role in developmental signaling pathways and is involved in the maintenance of tissue homeostasis. However, despite the presence of primary cilia, most cells are often called “non-ciliary” to distinguish them from cells with numerous motile cilia. These specialized structures are made up of microtubules and can beat in a coordinated fashion to move particles in a specific direction. These particles can be dust and pathogens in the airways or an egg that is released along the fallopian tube into the uterus.
The upper part of this image shows human pseudostratified columnar epithelium with motile cilia.
Even the non-ciliated tissues of these epithelia contain tuft-like cytoplasmic protrusions called stereocilia. Unlike motile cilia, these structures are more rigid and are made up of actin microfilaments. They usually have an absorbent or mechanosensory function and occur in the male reproductive system.
What are the Functions of Pseudostratified Columnar Epithelia?
This tissue is found in regions where mucus must be secreted to trap foreign objects and sweep away these particles through the coordinated action of the cilia. In addition, they are also seen in places where the epithelia have a combined absorptive and secretory function, such as in the vas deferens and epididymis. These tissues usually contain stereocilia, which are cytoplasmic processes made up of actin microfibrils.
Examples of Pseudostratified Columnar Epithelia
Many of these issues are made up of cup-shaped cells that secrete mucus and longer columnar cells that run throughout the epithelium. In addition, they also contain short basal cells, the apical surfaces of which do not reach the lumen. Basal cells can be goblet cell precursors or column cells.
Upper Respiratory Tract Epithelium
The main function of the airways is to bring the air in close contact with the blood flowing in the capillaries to allow gas exchange in the alveoli. Capillaries and alveoli are made up of thin squamous cells to maximize gas diffusion. However, these sensitive surfaces must be protected from the abrasive effects of large dust particles, pollutants, pollen, and a large number of corrosive substances and pathogens. The presence of pseudostratified columnar epithelium in the upper respiratory tract (consisting of the nose, trachea, and bronchi) protects the lungs from these irritants. The goblet cells of the epithelium secrete mucus to trap particles and prevent them from traveling further down the airways. The coordinated action of the cilia on the longer columnar cells facing the lumen pulls mucus along with particles away from the lungs. While the smaller particles are eliminated as part of normal nasal drainage, larger or heavier loads of particles are sneezed or coughed.
Vas Deferens and Epididymis
The pseudostratified columnar epithelium is seen in the vas deferens and the epididymis of the male reproductive system. In these organs, cells contain stereocilia, which behave like microvilli in the digestive tract. When sperm leave the testicles and enter the epididymis, they are immobile and therefore need to be propelled by large amounts of fluid. Stereocilia increase the surface area of the epithelial cells that line this tubular structure and allow sperm to concentrate by absorbing excess fluid before ejaculation.
The Pseudostratified Columnar Epithelium