Undulipodium
{{short description|Eukaryotic motile appendage that includes cilium and flagellum}}
An undulipodium or undulopodium (Greek: "swinging foot"; plural undulipodia) is a motile filamentous extension of eukaryotic cells, composed of a membrane protrusion held by a cytoskeletal structure called the axoneme.{{Cite journal |last1=Lindemann |first1=Charles B. |last2=Lesich |first2=Kathleen A. |date=2021 |title=The many modes of flagellar and ciliary beating: Insights from a physical analysis |journal=Cytoskeleton |language=en |volume=78 |issue=2 |pages=36–51 |doi=10.1002/cm.21656 |issn=1949-3592 |pmc=8048621 |pmid=33675288}} It is divided into cilia and flagella – which are differing terms for structurally identical organelles used on different types of cells,
{{cite journal |vauthors=Haimo LT, Rosenbaum JL |date=December 1981 |title=Cilia, flagella, and microtubules |journal=J. Cell Biol. |volume=91 |issue=3 Pt 2 |pages=125s–130s |doi=10.1083/jcb.91.3.125s |pmc=2112827 |pmid=6459327}} but are distinguished according to function and/or length,{{Cite book |last=Alberts |first=Bruce |title=Molecular biology of the cell |date=2015 |publisher=Garland Science, Taylor and Francis Group |isbn=978-0-8153-4432-2 |edition=Sixth |location=New York, NY |pages=941–942}} and usually corresponds to different waveforms of the organelles beating motion. The Gene Ontology database does not make a distinction between the two, referring to most undulipodia as "motile cilium", and to that in the sperm as sperm flagellum.{{cite web |title=Term Details for "motile cilium" (GO:0005929) |url=http://amigo.geneontology.org/amigo/term/GO:0031514 |website=AmiGO 2 |language=en |quote=Synonyms: motile cilia, microtubule-based flagellum, motile primary cilia, motile primary cilium, motile secondary cilium, nodal cilium}}
The name was coined to differentiate from the analogous structures present in prokaryotic cells,{{cite journal |author=Hülsmann N |title=Undulipodium: End of a useless discussion |journal= European Journal of Protistology |volume=28 |issue=3 |pages=253–257 |year=1992 |pmid=23195228 |doi=10.1016/S0932-4739(11)80231-2}}{{cite web |vauthors =Margulis L, Lovelock JE |url=https://history.nasa.gov/CP-2156/ch2.1.htm |title=CP-2156 Life In The Universe: Atmospheres and Evolution |work= history.nasa.gov |access-date=8 July 2013}} although "flagellum" would be a misnomer for the prokaryotic structure as they function more like propellers or corkscrews and, thus, do not "whip".
The usage of the term was early supported by Lynn Margulis, especially in support of endosymbiotic theory.{{cite journal |author= Sagan L |year=1967 |title=On the origin of mitosing cells |journal=J Theor Biol |volume=14 |issue=3 |pages=255–274 |pmid=11541392 |doi=10.1016/0022-5193(67)90079-3|bibcode=1967JThBi..14..225S }}
Usage
In the 1980s, biologists such as Margulis advocated the use of the name "undulipodium", because of the apparent structural and functional differences between the cilia and flagella of prokaryotic and eukaryotic cells. They argued that the name "flagellum" should be restricted to prokaryotic organelles, such as bacterial flagella and spirochaete axial filaments.{{cite journal |author=Margulis L |title=Undulipodia, flagella and cilia |journal=Biosystems |volume=12 |issue=1–2 |pages=105–108|year=1980 |pmid=7378551 |doi=10.1016/0303-2647(80)90041-6|bibcode=1980BiSys..12..105M }} A distinction was drawn between "primary" cilia which function as sensory antennae, and ordinary cilia: it was argued that these were not undulipodia as they lacked a movement mechanism.{{cite journal |vauthors=Satir P, Christensen ST |title=Structure and function of mammalian cilia |journal=Histochem. Cell Biol. |volume=129 |issue=6 |pages=687–93 |date=June 2008 |pmid=18365235 |pmc=2386530 |doi=10.1007/s00418-008-0416-9 }}
However, the term "undulipodium" is not generally endorsed by biologists, who argue that the original purpose of the name does not sufficiently differentiate the cilia and flagella of eukaryotic from those of prokaryotic cells. For example, the early concept was the trivial homology of the flagella of flagellates and the pseudopodia of amoebae. The consensus terminology is to use "cilium" and "flagellum" for all purposes.{{cite journal|author=Corliss JO |title=Objection to "undulipodium" as an inappropriate and unnecessary term |journal=Biosystems |volume=12 |issue=1–2 |pages= 109–110 |year=1980 |pmid=7378552 |doi=10.1016/0303-2647(80)90042-8|bibcode=1980BiSys..12..109C }}
Gallery
File:Axoneme.JPG|The axoneme, common structural motif of cilia and flagella.
File:Eukaryotic cilium diagram en.svg|Schematic of the cilium, with a cross section showing the axoneme.
File:Schematic representation of mammalian spermatozoa and flagellum structure - Fcell-07-00230-g001.jpg|Schematic of mammalian spermatozoon and its flagellum.