ABSTRACT
Through the single lens of a homemade microscope, Antonie van Leeuwenhoek visualized the incredibly tiny feet of “animalcules.” The function of these beating motile cilia for locomotion of protists in pond water was obvious to van Leeuwenhoek.1 He also observed motile cilia and flagella in human cells, noting that their undulations may move fluid or propel the sperm. A few hundred years later, embryologist Alexander Kowalevsky identified single cilia projecting from a variety of vertebrate cells.2,3 These “primary cilia” are immotile and are far more widespread than the motile cilia types within our organ systems. Besides motile and primary cilia, “sensory cilia” make up the third and most structurally diverse cilia type. Sensory cilia, as their name implies, are found in sensory tissues such as olfactory neurons and photoreceptor cells. In general, sensory and primary cilia are thought to function as cellular antennae—receiving and integrating stimuli (chemical and other) through the activation of molecular effectors such as G protein-coupled receptors (GPCRs) and ion channels which are concentrated in these projections.4,5 However, beyond fluid motion and motility, some evidence suggests that motile cilia may also carry out a sensory function for cells.6 Thus, there might be considerable functional overlap between these classifications of cilia. Below, we will discuss the structural, functional, and component differences among these three cilia types.
