Nomenclature of Pericytes * Intramural and Extramural by Norman Ashton

THE cellular structure of the retinal vessels as revealed by the digest technique recently introduced by Kuwabara and Cogan (1960) has aroused widespread interest and speculation, especially concerning the nature and function of the cells to be found embedded in the walls of the retinal capillaries. Cogan and his colleagues at first thought that these were the nuclei of perivascular glia, but later concluded that they were special cells confined to the retinal circulation (Kuwabara and Cogan, 1963), wherein they were probably concerned in the control of capillary blood flow, and, being selectively injured in diabetic retinopathy, were of significance in its pathogenesis (Cogan, Toussaint, and Kuwabara, 1961). Believing that these cells were an entity not previously described they gave them the special name of "mural cells", a term which is already becoming established in the literature as a specific component of the retinal capillaries. From our own observations and from a review of the literature, it has since become clear that this cell is not in fact unique to the retina, but corresponds to one type of cell found throughout the vascular system, and long classified under the confusing term of "pericyte". It has now become essential, not only in ophthalmological work, but also in general vascular studies, to clarify the nomenclature and select a term that accurately describes these cells wherever they may be found.