The cilium is a thin protuberance on the surface of eukaryotic cells which projects from the much broader cell body.
Cilia are divided into two kinds, namely: motile and non motile cilia. Primary cilia are non-motile cilia that act as sensory organelles. An individual, non-motile primary cilium serves as a cellular antenna in certain mammalian types of cells. Exceptions comprise olfactory neurons, that have multiple non-motile cilia, and cells of the transient embryonic node, that have single motile cilia called nodal cilia, which are essential for the development of left-right body asymmetry.
Motile cilia and flagella (commonly referred to as undulipodia) were structurally identical in eukaryotes, however, distinctions are often made based on function or length. Main cilia (immotile cilia) transmit signals from the environment or through other cells.
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Primary Cilia: Non motile cilia can be present on virtually every form of cell in animals, with blood cells being the notable exception. In comparison to cells containing motile cilia, some cells have only one, with perhaps the exception of olfactory sensory neurons, that have approximately ten cilia each and contain odorant receptors.
While the primary cilium was identified in 1898, these were largely overlooked for the next century, and then was assumed to be a vestigial organelle with no significant function. Its physiological functions in signal transduction, chemosensation, and cell growth regulation have recently been discovered, revealing its significance in cell function. The discovery of its presence in a diverse group of diseases accompanied by the dysgenesis or deficiency of cilia, including congenital heart disease, polycystic kidney disease, and retinal degeneration, known as ciliopathies, further illustrated its relevance to human biology. Several human organs now have a primary cilium that plays a significant role in their operation.
Motile Cilia: Motile cilia cell are also found in larger eukaryotes, like mammals. Motile cilia are found in large numbers on the surface of cells and thus beat in synchronised waves.
Motile cilia are located on the respiratory epithelium lining the respiratory tract within humans, where they help with mucociliary clearance, which involves sweeping mucus and debris out of the lungs. The respiratory epithelium contains approximately 200 motile cilia per cell.
The ovum is moved from the ovary to the uterus by the beating of cilia in the oviduct in female mammals.
The epithelial cells including its choroid plexus epithelial cells possess motile cilia. These are found in greater numbers across each cell and travel slowly, putting them in the middle of motile and primary cilia. Within hair cells, there have been 9+2 cilia which are immobile in addition to the 9+0 mobile cilia.
Maintaining optimum levels of periciliary fluid bathing the cilia is important for motile cilia to function properly. ENaC epithelial sodium channels, which are distributed across the full length of cilia, appear to function as sensors that control the fluid level covering the cilia.
Nodal Cilia: The third form of the cilium is the nodal cilium, which is a motile 9+0 cilium. The nodal cilia tend to be present during the embryo's early development. It has no central apparatus, however, it does have dynein arms that allow it to travel or rotate in a circular direction, equivalent to the primitive cilium. A clockwise spin of a nodal cilium induces a movement of extraembryonic fluid all across the nodal surface, which is guided to the left. The directional flow is sensed by primary cilia across the nodal cilia, which stimulates nodal signalling and establishes left to right sidedness.
The axoneme seems to be a microtubule-based cytoskeleton found between cilia and flagella. The axoneme of such a primary cilium usually possesses a ring of nine outer microtubule doublets (referred to as a 9+0 axoneme), while the axoneme of a motile cilium includes two central microtubule singlets in contrast to the nine outer doublets (referred to as a 9+2 axoneme).
The axoneme serves as a scaffold for the axonemal anterior and posterior dynein arms, which transfer motile cilia cell, as well as tracks for molecular motor proteins like Kinesin II, which deliver proteins all along the length of the cilium via intraflagellar transport (IFT). IFT seems to be a bi-directional, retrograde device. To return to the cell body, IFT uses the cytoskeletal dynein motor 2. The cilium is enclosed by a membrane that is similar to but not identical to the plasma membrane in structure.
Ciliary Rootlet: A cytoskeleton-like, named ciliary rootlet structure that emerges from the proximal end of a cilium's basal body. Cross striae were spread at periodic intervals of around 55-70 nm on rootlets that are usually 80-100 nm in diameter. Rootletin is a major aspect of the rootlet.
Transition Zone: The proximal-most area of the cilium has a transition zone that is responsible for regulating which proteins must join and leave the cilium to attain a distinct composition. Y-shaped structures bind the ciliary membrane to the underlying axoneme throughout the transition region. A sieve-like feature of the transition zone could be used to monitor selective entry into cilia.
Cilia Versus Flagella: Motile cilia and flagella, despite their various names, have virtually similar structures and serve the same purpose: motion. The motion of the appendage could be compared to that of a wave. The wave is defined in terms of amplitude, frequency (ciliary beat frequency (CBF)), and wave duration, and it appears to arise from the cilium base. The cilia beat motion is caused by the slipping of external doublets by dynein arm structures, and it arises in the axoneme rather than the basal body. A main distinction between the two systems is that flagella drive eukaryotic organisms like humans, whereas cilia transfer substances around a surface.
The axoneme's dynein builds bridges amongst adjacent microtubule doublets. Once ATP stimulates dynein's motor domain, it tries to walk across the microtubule doublet next to it. If it weren't for the existence of Nexin between the microtubule doublets, it would cause the adjacent doublets to slip over each other. As a result, the force generated by dynein is transformed into bending motion.
Sensing the Extracellular Environment: In eukaryotes, several primary cilia along epithelial cells serve as cellular antennae, sensing the extracellular environment for thermosensation, chemosensation, and mechanosensation. Such cilia, therefore, mediate various signalling signals, such as soluble factors throughout the outer cell environment, secretory roles during which a soluble protein is produced to provide an impact downstream of fluid flow, and fluid flow mediation if the cilia become motile.
Most epithelial cells being ciliated, and these usually form a tube or tubule through cilia extending into the lumen as just a layer of polarised cells. Cilia play an important sensory and signalling role in preserving the local cellular environment, which may explain why ciliary defects produce such a wide variety of human diseases.
Q1. What is the Significance of Cilia?
Ans. Cilia are involved in locomotion and play a significant role. By assisting in the movement of fluids over the cell, Cilia may aid in the removal of pollutants from organs or tissue. Cilia line the inside of the nasopharynx and the trachea. Bacteria, Mucus, and other debris are removed from the lungs by such ciliated epithelial cells.
Q2. Mention the Major Difference Between Flagella and Cilia?
Ans. Cilia comprise hair-like appendages that protrude from a living cell's surface whereas Flagella tend to be the elongated, threadlike appendages on a living cell's surface.