An examination with light and electron microscopes has been made of the cell type which is considered by some authors to be a sporozoan parasite, Rhabdospora thelohani, and by others to be a tissue cell of fishes and amphibians.
The cell type was found in the minnow Phoxinus phoxinus in many tissue sites, including olfactory and pharygneal epithelia, kidney, eyes and brain. In some specimens it could not be found in any tissue.
The cell has a thick fibrous wall, a basal nucleus, an apical mitochondrial zone, a Golgi body, and from 8 to 50 characteristic refractile rodlets.
The rodlets give a negative reaction to the Feulgen test for nuclear material, but are positive to the periodic acid–Schiff stain, a test for polysaccharide. Glycogen does not appear to be present in the rodlets.
Under certain circumstances the rods rupture the cell wall and may protrude in the apical region of the cell.
The structures described are discussed with the evidence for and against a parasitic interpretation of the cell. It is concluded that Rhabdospora is a protozoan parasite, whose exact functional and taxonomic status is as yet unknown.
I am grateful to Professor G. E. H. Foxon and to Professor P. C. C. Garnham for their encouragement, advice, and helpful criticism of the manuscript. I am further indebted to the National Spastics Society for the use of the electron microscope housed in Guy&s Hospital Medical School, and to Mr I. D. Bradbrook for his assistance with the histological techniques.