Pseudostratified columnar epithelium definition
The pseudostratified columnar epithelium is a type of epithelium consisting of a single layer of cells that gives the appearance of being multiple layers because the nuclei of the cells are present at different levels. This epithelium is histologically a simple epithelium even though in a crosssection, it might appear as a stratified epithelium. The pseudostratified epithelium made up of cuboidal or squamous cells is reasonably rare, whereas the pseudostratified columnar epithelium occurs in different regions throughout the body.
Structure of pseudostratified columnar epithelium
- The pseudostratified columnar epithelium consists of column-like cells of varying heights.
- The cells are not of equal heights and as a result, not all cells the apical surface of the tissue.
- This creates an illusion of multiple layers as the nuclei of these varying cells end up in different levels of the tissue.
- The cells, like is all other epithelial tissues, are connected through gap junctions, adhesions, or desmosomes, ensuring that they are tightly packed together without any gaps or leaks.
- The cells are arranged tightly adjacent to one other, and goblet cells are present in between cells in the layer of ciliated pseudostratified columnar epithelium. These mucus-producing goblet cells extend up to the apical surface.
- Similarly, the cells of the columnar epithelium are also provided with a large number of organelles. They thus are capable of a higher level of secretion and absorption than are other cells.
- The complexes formed by the junctions and desmosomes occur at the apical surface of some cells which gives them a bulging appearance at the apical surface.
- As an epithelial tissue, this epithelium also doesn’t have its own blood supply. The nutrients, water, and exchange of gases occur through diffusion with the vasculated underlying tissues.
- The epithelium is, however, innervated, having its own nerve supply.
- Some cells in the pseudostratified columnar epithelium have cilia on the apical surface that is involved with motility and sensory activity.
Based on the presence and absence of cilia, the pseudostratified columnar epithelium has two types:
1. Ciliated pseudostratified columnar epithelium
- The epithelium that consists of irregularly sized columnar cells containing cilia on the apical surface form the ciliated pseudostratified columnar epithelium.
- The cilia are typically 5-10 μm long and 0.2 μm in diameter. Each cilium has a core structure consisting of nine peripheral microtubule doublets arrayed around by two central microtubules.
- Cilia exhibit rapid beating patterns that move a current of fluid and suspended matter in one direction along the epithelium.
- These cilia direct the movement of molecules in a particular direction aids the function of excretion and secretion.
- In the female reproductive system, the cilia assist the movement of the ovum through the fallopian tube towards the uterus.
- In addition, few mucus-secreting goblet cells are also interspersed between these cells that extend up to the apical surface to release the secretion.
- Before release, the mucus accumulates in the upper portion of the cell, causing it to bulge and making the whole-cell resemble a goblet or wine glass.
2. Non-ciliated pseudostratified columnar epithelium
- The non-ciliated pseudostratified columnar epithelium consists of irregularly sized columnar cells without any cilia.
- The cells lack cilia but have stereocilia, which are typically much longer and less motile than microvilli, and may show branching distally.
- Stereocilia resemble microvilli in containing arrays of microfilaments and actin-binding proteins, with similar diameters
- Like microvilli, stereocilia increase the cells’ surface area, facilitating absorption.
- These stereocilia are present on the non-ciliated pseudostratified columnar epithelium of the male reproductive system.
Functions of the pseudostratified columnar epithelium
The primary function of the simple columnar epithelium includes secretion and absorption. The non-ciliated pseudostratified columnar epithelium is mainly involved in absorption and secretion. In contrast, the ciliated columnar epithelium aids the transport or movement of molecules and cells from one place to another and also provides protection against certain infections.
- The epithelium in different regions throughout the body acts as a protective barrier and provides protection against the non-specific movement of luminal substances.
- The complex formed by the junction on the apical surface of the cells acts as gatekeepers filtering the unwanted molecules and allowing the entry of nutrients and water.
- Similarly, the mucus secreted by the goblet cells in the ciliated pseudostratified columnar epithelium traps foreign particles, and cilia sweep away mucus for elimination from the body.
- The presence of pseudostratified columnar epithelium in the upper respiratory tract protects the underlying tissue against large dust particles, pollutants, pollen, and a variety of corrosive agents and pathogens.
- The stereocilia present on the apical surface of the cells increases the absorptive surface of the epithelium in the male reproductive system that allows the sperm to be concentrated by absorbing fluid before ejaculation.
- The cells are also provided with accessory membrane-bound proteins that help in the active absorption of these nutrients.
- The goblet cells in between the columnar cells in the tissue secrete mucus that lubricates linings of respiratory and reproductive tracts, and most of the urinary tract.
- The mucus in the respiratory tract provides protection against foreign particles and prevents their entry through the nasal passage into the inner respiratory tract.
- The non-ciliated pseudostratified columnar epithelium lines the large excretory ducts of some glands which helps in the movement of hormone and enzymes to their site of action.
- In the same way, the cilia present in the ciliated columnar epithelium of the respiratory tract beat in unison moving mucus and foreign particles toward the throat, where they can be coughed up and swallowed or spit out.
- The pseudostratified columnar epithelium lining the epididymis of the male reproductive system has cilia which helps move immotile sperm expelled from testes into the epididymis.
Location and examples
- The non-ciliated pseudostratified columnar epithelium lines the epididymis and vas deferens of the male reproductive system.
- In the urinary system, the epithelium lines some parts of the urethra in the male.
- In the respiratory system, the pseudostratified columnar epithelium covers most of the upper respiratory tract.
References and Sources
- Mescher AL (2016). Basic Histology. Fourteenth Edition. McGraw-Hill Education.
- Tortora GJ and Derrickson B (2017). Principles of Physiology and Anatomy. Fifteenth Edition. John Wiley & Sons, Inc.
- Waugh A and Grant A. (2004) Anatomy and Physiology. Ninth Edition. Churchill Livingstone.
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