What We Can See by Observing Primary Cilia through Light Microscopy

Abstract

Cilia are slender projections with a length of 1 to 10 μm and a diameter of about 200 nm, and are evolutionarily conserved very well from protozoans such as Paramecium to vertebrates including humans. In multicellular organisms such as vertebrates, a primary cilium protrudes on a cell, and in mammals almost all cells in the body possess it. The primary cilium is a fine structure as thin as the optical limit of light microscopy. It is easily overlooked as only a tiny structure is present on the surface of cells with a size of several tens of μm, while those who know the existence can find it out through careful observation. In addition, primary cilia are an unstable structure compared to many motile cilia of Paramecium and tracheal epithelium. The fragility of primary cilia makes findings false negative, i.e. loss of cilia, or false positive, i.e. abnormal structures, as artifacts unless the samples are handled carefully. I herein introduce difficulties and some tips of observing such “fragile” primary cilia, and show what are observed with an optical microscope—the morphology and behavior of primary cilia.