A cilium is a slender protuberance projecting from a eukaryotic cell body. There are 2 types of cilia: motile and primary (non-motile).
Motile cilia are like the flagella on bacteria and some eukaryotic cells: a means of moving about. Sperm cells have motile cilium that lets them swim to their target and consummate a love connection.
A bacterium’s flagellum is a single lash-like tail that propels a bacterium through a fluid. A bacterium also uses its flagellum as a sensory organelle. Some bacteria, like Helicobacter pylori, which lives in the stomach, have multiple flagella.
Primary cilia are found in most animal cells. They were discovered in 1867 by Russian biologist Alexander Kovalevsky. Quickly dismissed as an evolutionary artifact of no significance, primary cilia were ignored for over a century.
The primary cilium is a cell’s transceiver: detecting a wealth of information about its surroundings and acting as a cell status transmitter. The primary cilium picks up protein-based chemical signals and information from mechanical forces, such as fluid flow and tensile force, in the immediate vicinity.
Cell status sent via primary cilium is essential to development and tissue maintenance, allowing coordination that otherwise would not happen. Primary cilia help orient stem cells in their direction of growth. Diseases result if primary cell cilia are not working properly.
For efficiency, there is only 1 primary cilium per cell. Multiple cilia would degrade signal reception quality as well as increase cellular complexity without advantage.