Cilia

There are two types of cilia; motile and primary, and they tend to be around 10um in length and around 0.2 µm in diameter^[1]. Motile cilia grow in clusters e.g. the cells in the bronchial epithelium, primary cilia grow as a single cilium e.g. those on the embryonic node. Motile cilia are found on a limited range of different cell types whereas primary cilia are found on nearly every cell in the human body. Primary cilia's function is to receive signals from other cells or fluids.

Contents 1 Motile Cilia 2 Primary Cilia 3 Example describing how the aberrant structure of cilia can lead to human disease 4 References

Motile Cilia

Cilia (singular: Cilium) are cylindrical organelles^[2][3], built from microtubules, in a “9 + 2” arrangement^[4], with 9 doublet microtubules surrounding the circumference and a central pair in the middle. The microtubules are regularly cross-linked with accessory proteins to make up the axoneme, a central core that provides stability. Certain accessory proteins such as dynein (see Figure 1) play a key role in the organelles movement. The 9 doublet microtubules around the circumference consist of one complete and one incomplete tubule, which are shared by common tubule wall. The organelle is also anchored to the cell membrane by the basal body, which is composed of triplet microtubules that transition into the doublet ones seen in the axoneme^[5]. The majority of cilia in different eukaryotes have this characteristic arrangement^[6]. They are motile structures whereby the rhythmic beating of cilia creates movement. Cilia are involved in mucociliary clearance in which bacteria, mucus and dust are swept up the respiratory tract into the mouth and eliminated by swallowing. Moreover, ciliated cells move oocytes along the fallopian tubes, and sperm from testes into the male reproductive tract^[7]. Furthermore, by the beating of cilia, single cells are able to propel through fluid^[8]. The movement of cilia is caused by axoneme, which can reach a length of 10-200 micrometres^[9]. Dynein is a cytoskeleton motor protein within cilia which enables them to have movement. This occurs by the conversion of chemical energy in ATP to mechanical energy^[10].

Primary Cilia

Cilia can also be immotile (primary cilia) and have a “9+0” arrangement where they have 9 peripheral doublets microtubules and 0 single microtubules in the centre^[11][12]. One of the roles of primary cilia is to act as a sensory antennae on receptor cells, they extend out from the apical surface of the cell into the surrounding medium and are able to detect the presence of ligands better than receptors on the cell surface due to the fluid further away from the cell membrane being more mobile^[13].

Example describing how the aberrant structure of cilia can lead to human disease

Primary Ciliary Dyskinesia (PCD) is a genetic disorder caused by abnormal cilia and flagella, which can lead to respiratory tract infections. Some of the symptoms of this disorder include a chronic cough, nasal congestion and chronic chest infections. In people with PCD, the cilia found in the respiratory tract are immobile and stiff due to defects in the axonemal dynein. This means they cannot move the mucus away, so the bacteria remain in the respiratory tract and cause infection. Although there is no cure, PCD can be treated with antibiotics, anti-inflammatory drugs or a bronchodilator and more importantly regular exercise is required to help clear the airway^[14][15].

References

1. ↑ The Histology Guide - The University of Leeds. Epithelia: Specialisations; Available from: http://www.histology.leeds.ac.uk/tissue_types/epithelia/epi_specialisations.php
2. ↑ 5) “Fundamental role of microvilli in the main functions of differentiated cells: Outline of an universal regulating and signalling system at the cell periphery” K. Lange, J Cell Physiol. 2011, 226, 896-927 Abstract. https://www.ncbi.nlm.nih.gov/pubmed/20607764
3. ↑ Lynn H. (2008) The Ciliated Protozoa Characterization, Classification, and Guide to the Literature, New York: Springer
4. ↑ Lynn H. (2008) The Ciliated Protozoa Characterization, Classification, and Guide to the Literature, New York: Springer
5. ↑ Jeff Hardin, Gregory Bertoni, Lewis J. Kliensmith, Becker's World of the Cell, Page 453, Eighth Edition, Benjamin Cummings and Pearson, San Francisco, 2010
6. ↑ Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P. Molecular Biology of the Cell. 6th Ed, New York; Garland Science. 2015 (page 941-942)
7. ↑ Martini FH, Nath JL, Bartholomew EF. Fundamentals of Anatomy and Physiology. 9th Ed, San Francisco; Pearson Education Inc: 2012 (page 70-71)
8. ↑ Alberts, Bruce et al. (2008). Molecular Biology of the Cell, 5th edition, New York: Garland Science. (Page 1031)
9. ↑ Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P. Molecular Biology of the Cell. 6th Ed, New York; Garland Science 2015 (page 941-942)
10. ↑ https://www.ncbi.nlm.nih.gov/pubmed/20607764
11. ↑ Satir P, Pedersen LB, Christensen ST. The primary cilium at a glance. Journal of Cell Science 2010; 123(499-503): 1. http://jcs.biologists.org/content/123/4/499 (accessed 14 November 2016).
12. ↑ 5) “Fundamental role of microvilli in the main functions of differentiated cells: Outline of an universal regulating and signalling system at the cell periphery” K. Lange, J Cell Physiol. 2011, 226, 896-927 Abstract. https://www.ncbi.nlm.nih.gov/pubmed/20607764
13. ↑ Marshall WF, Nonaka S. Cilia: Tuning in to the Cell’s Antenna. Current Biology, Volume 16, Issue 15, p604-614. 2006
14. ↑ 13. primary ciliary dyskinesia [Internet]. Genetics Home Reference. 2017 [cited 02 December 2017]. Available from: https://ghr.nlm.nih.gov/condition/primary-ciliary-dyskinesia#diagnosis
15. ↑ 14. Daniels M, Ghosh S, Noone P. Primary ciliary dyskinesia. Current Pulmonology Reports. 2016;5(4):191-198.