A cilium, or cilia when it is plural, are small hair-like protuberances on the outside of eukaryotic cells. They are responsible for the locomotion of the cell itself, or the fluids on the cell surface. They are involved in mechanoreception too. There is also a class of microorganisms that are named for these small structures. Ciliates are the protozoans that possess cilia that they use for both locomotion's and also for feeding.
Ciliated columnar epithelial cells are rectangular. They have between 200 to 300 hair-like protrusions known as cilia. The mitochondria are found towards the apical region of the cell. Whereas the cell nuclei are found towards the mostly elongated base.
Cells are interconnected through the desmosomes and the tight junctions and create a semipermeable membrane that is more selective than the membrane found in other types of cells.
Ciliated columnar epithelial cells are found generally in the tracheal as well as in the bronchial regions of the pulmonary system. They are also found in the fallopian tubes of a female reproductive system.
A simple ciliated epithelium cell present in the pulmonary system is always interspersed with the goblet cells. It secretes mucus to form a mucosal layer apical to the epithelial layer. The rowing-like action of epithelial cilia always works in tandem with goblet cells to propel mucus that too away from the lungs. It helps in preventing particulate matter from causing infection.
A study by Lawson in 2002 concluded that ciliated cells play a crucial role in repairing the distal airway injury. These pulmonary epithelial cells are always believed to be terminally differentiated cells that never divide. But data from Park 2006 suggested that the ciliated cells undergo morphological transitions from squamous to cuboidal to columnar forms because the bronchiolar epithelium is restored after the tissue damage, indicating the differentiation potential.
Pseudostratified ciliated columnar epithelia are the tissues that are formed by only one layer of cells and give the appearance of being made from multiple layers, especially when seen in a cross-section. The nuclei of the epithelial cells are at very different levels that will lead to the illusion of being stratified.
However, the ciliated epithelial tissue is made of a single layer of cells, whereas the cells do not share a common apical surface. Every cell is in contact with the basement membrane.
It is rarely found that pseudostratified epithelia are made from cuboidal or squamous cells. Therefore, the common subtype is the pseudostratified columnar epithelium.
This particular tissue is found in the regions where there is a need to secrete mucus for trapping the foreign bodies and also sweep those particles away through the coordinated action of cilia. They are also seen in the places where the epithelia have a combined secretory and also absorptive function. Such as in the vas deferens and the epididymis. These types of tissues usually contain stereocilia, which are cytoplasmic projections made of actin microfibrils.
Mostly, these tissues are made of goblet-shaped cells that secrete mucus and longer columnar cells. It traverses the entire epithelium. They also contain short basal cells whose apical surfaces never reach the lumen. Basal cells can be precursors of goblet cells or columnar cells.
Cilia play an important role in locomotion that includes movement of the cell itself, or other substances and objects past the cell.
Q1. Write a Short Note on the Structure of Cilium.
Ans: Cilium is made up of microtubules coated in the plasma membrane. The microtubules are like small hollow rods that are made up of the protein tubulin. Each cilium contains nine pairs of microtubules that form the outside of a ring and also the two central microtubules.
This structure is called an axoneme, and the arrangement is called ‘9+2’, an arrangement ubiquitous in motile cilia. The microtubules are held together through the cross-linking proteins. There are nine outer pairs between motor proteins called dynein.
Cilia attach to the cell at a basal body which is made up of microtubules arranged as nine triplets. The triplets are formed just like the doublets from the cilia. They are always joined by an additional microtubule from the cell. The two central microtubules will end before taking an entry to the basal body.
The motor proteins that are dynein are large flexible molecules that allow the cilia to be motile. The protein always hydrolyzes ATP for energy. When proteins are activated, they undergo conformational changes which in turn allow for complex movements. The dynein molecules essentially crawl along the microtubules, and pull the neighbouring doublet up, and also reattach further down. Because the doublets are attached by the cross-linking proteins, they can only slide a short distance from each other. This movement causes bending in the cilium.
Cilia are very tiny structures that measure approximately 0.25 μm in diameter and up to 20 μm in length. They are found in huge numbers on the surface of the cell. The cilia always act like the oars, beating back and forth to create movement.
Q2. What are the types of Pseudostratified Columnar Epithelia?
Ans: These tissues are classified as ciliated or nonciliated. They are classified based on the cellular organelle involved with motility and sensory activity.
Approximately every eukaryotic cell has a single, primary cilium that plays an important role in the developmental signalling pathways. It is also involved in maintaining tissue homeostasis. Despite the presence of primary cilium, most cells are often known as the ‘non-ciliated to distinguish them from cells containing numerous motile cilia. These specialized structures are made of microtubules and can beat in a coordinated manner to move particles in a particular direction. These particles could be dust and also pathogens in the respiratory tract or even could be an ovum that is propelled with the fallopian tube towards the uterus.
Non-ciliated tissues of these epithelia contain tuft-like cytoplasmic projections known as stereocilia. These structures are stiffer and also made from actin microfilaments. They generally have an absorptive or mechanosensory function. They are found in the male reproductive tract.