Cilia Function

A subunit of eukaryotic cells is called the cilium, and multiple units are called cilia. Cilium is a slender, lumped shape unit of cells that appears to have a bigger cell body. Mainly, there are two varieties of cilia: non-motile and motile cilia. Non-motile or primary cilia function as a sensory subunit. Generally, mammal cells have an individual non-motile or primary cilium, and it plays the role of the cellular antenna. There are a few exceptions, like neurons of the olfactory have many non-motile kinds of cilia and embryonic node (transients) or primitive node cells have a single motile cilium, also called nodal cilia. Flagella and Motile cilia, together, are called undulipodia. They are present in eukaryotes and have a similar structure.  However, distinctions can be made based on function and length. Primary cilia or immotile cilia commute with signals from different cells or environments. 

Types of cilia

Cilia can be divided into three categories: Primary Cilia, Motile Cilia and Non-classical cilia.  

• Primary cilia

Non-motile or primary cilia can be found in all animals and in almost every cell type, but prominently in blood cells. The exception is olfactory neurons, in which the receptors of odorant are situated and have at least ten cilia. All other cells have only individual non-motile cilia, which is contradictory to those with motile cilia. Cilia are organised during phase G1 and are disorganised before the occurrence of mitosis. In modern research, the physiological role of chemosensation is discovered, along with the transduction of signals and control of cell growth. 

The importance of cilia in cell function has been discovered in modern research. The discovery of cilia’s roles in a varied group happened due to the cause of cilia dysfunction, which led to issues complications like polycystic kidney illness, congenital disease of the heart, mitral prolapse and degeneration of the retinal, also called ciliopathies. In many human organs, primary cilium is known to have essential functions.  

• Motile cilia

Mammals, which are the bigger eukaryotes, also have motile cilia. It is present on the cell’s surface in greater numbers and beats in certain coordinated waves. Motile cilium is located in the lining of the human respiratory epithelium of the respective respiratory tract. Its function is to clear the mucociliary, or in simpler words, wipe dirt and mucus from the lungs. Two hundred motile cilia are present in the respiratory epithelium. Female mammals have a different location for the cilia beating. It is present inside a fallopian tube and shifts the ovum towards the uterus from any ovary. 

The motile cilia functions are based on the optimal level to maintain the fluids like periciliary bathing any cilia. Motile cilia are present in ciliates. A microscopic organism uses motile cilia for movement and to move the liquid over the respective surface.  

• Non-classical cilia

The classification of any cilia usually has 2+9 motile cilia and 9+0 leading cilia with no central apparatus. The 9+0 type of cilia does not happen in multiple cells. The 9+0 type cilia, also known as the nodal cilium, is visible in embryonic development. It has a very similar construction to the present primitive cilium and has no primary apparatus. But the non-classical cilia does have arms (dyein) that help them move and spin in a proper circular direction. The spinning causes the fluid to flow to the left of the nodal surface. 

Cilia structure

A microtubule-like cytoskeleton, called an axoneme, is present inside the flagella and cilia. An axoneme of the primary cilium has nine outer rings called microtubule doublets or axoneme 9+0. In contrast, the motile cilium axoneme has a 9+2 axoneme, with nine outer doublets in multiple to two central microtubules. 

Functions of cilia

The function of cilia in the human cell is to sense the extracellular environment. In eukaryotes, some primary cilia behave like cellular antennae and provide a sense of processes like chemosensation, mechanosensation, and thermosensation. It also has a part to play in arbitrating particular signals, cues which also include soluble factors. 

The defect of the ciliary can cause several human diseases. Chronic disorders including PCD, senior Loken syndrome, or nephronophthisis can happen because of the genetic mutations affecting the cilia functioning. Additionally, it can also cause a defect to the primary cilium in the renal tubule cells and lead to PKD. Cilia defects are related to obesity and often turn into type 2 diabetes. In females, lack of cilia or its dysfunction in the fallopian tubes can cause ectopic pregnancy.  

Conclusion

Cilium, the singular form of cilia, is a slender, lumped shape subunit of cells with a bigger cell body. It is a subunit of eukaryotic cells. Broadly, there are two kinds of non-motile and motile cilia. The primary cilia or non-motile cilia functions as a sensory subunit. These are found in all animals and in almost every cell type, most prominently in blood cells, except olfactory neurons. Motile cilia are present on the surface of the cell in greater numbers and beat in coordinated waves. Motile cilia are located in the lining of the human respiratory epithelium of the respiratory tract. The most important function of the cilia is to sense the extracellular environment. A bridge is formed between the neighbouring doublets of the microtubule.