Stratified columnar epithelium definition
The stratified columnar epithelium has multiple layers of cells in which the apical layer is made up of columnar cells while the deeper layer can be either cuboidal or columnar. As in the case of other stratified epithelium, the cells in the deeper layers might be different than the layer on the top. The modification of the cells on the apical surface is based on the location and function of the epithelial tissue. This epithelium is relatively rare and is only found in some areas throughout the body.
Structure of the stratified columnar epithelium
- The stratified columnar epithelium consists of many layers of cells, where the cells in the deeper layers are irregular and of different shapes. In contrast, the cells in the apical layer are column-like in shape.
- The cells in the stratified columnar epithelium, as in the case of simple columnar epithelium are taller than they are wide.
- The cells in the epithelium are present close to each other, ensuring no gap between the cells aiding to their protective function.
- Similarly, the cells are connected to each other through the gap junctions, adhesions, and desmosomes to maintain the structural integrity of the epithelium.
- Characteristic to the stratified epithelium, only a single lowermost basal layer is attached to the basement membrane while the rest of the layers are connected to each other.
- The stratified epithelium is formed when the cells in the basal layer divide to form more cells which then form layers on top of the basal layer.
- Eventually, the cells on the uppermost layer or the layer towards the lumen die or are sloughed off, and new cells are formed, leading to new layers.
- The cells in the apical layer contain complexes of gap junctions and desmosomes which act as filtering agents keeping the unnecessary particles out of the cells.
- The epithelium is doesn’t have its own blood supply, but instead gets the nutrients, water, and oxygen from the underlying vasculated tissues via diffusion.
- Nonetheless, the epithelium has a nerve supply of its own.
Functions of the stratified columnar epithelium
The stratified epithelium has multiple layers of cells that aid in the protection of the underlying tissues. Additionally, based on the location of the tissues, they perform a function of limited secretion.
- The primary function of stratified epithelium is protection.
- As the epithelium has multiple layers, it protects the underlying tissues and internal organs against several physical and microbial damages.
- The columnar epithelium in the eyes protects the conjunctiva of the eyes and other delicate structures in the eyes.
- The gap junctions and desmosomes present on the cells create an impermeable layer preventing the entry of foreign particles.
- At the same time, this epithelium also acts as gatekeepers, filtering out the unwanted particles while allowing the entry of nutrients and water into the cells.
- Because the cells are continuously replaced and repaired, they act as the first line of defense for the protection of the body.
- The stratified columnar epithelium in the excretory system and the glands perform the function of secretion of waste materials into the ducts and out of the body.
- Similarly, the columnar epithelium in the conjunctiva of the eyes secretes mucus to keep the eyes moist and clean.
Location and examples of stratified columnar epithelium
- In the excretory system, the stratified columnar epithelium forms the lining of a small area of the mucosal membrane of the anus and some parts of the male urethra.
- In the endocrine system, it is found in the large excretory ducts of glands like the oesophageal glands.
- Besides, the stratified columnar epithelium lines some parts of the conjunctiva of the eyes.
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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