Two Types of Ribonucleoprotein in the Nucleolus of Mammalian Cells

Abstract

NUCLEOLAR differentiation has been recognized since the time of Gustav Valentin. Agassiz¹ referred to a body within the nucleolus of the turtle egg as the “Valentinian vesicle”. Montgomery² and Heidenhain³ called this structure the nucleolinus. Recently, it has been suggested that the appearance of vesicular or solid nucleolini is due to the presence of a reticulum or thread called the nucleolonema⁴. None of the procedures that reveal nucleolonemata^4–6 has any cytochemical significance. Serra⁷ suggested that the intranucleolar differentiation revealed by empirical procedures may be the result of physical rather than chemical inhomogeneity within the nucleolus. Chemical differences have, however, been found in the nucleoli of plant cells. Chayen et al.⁸ demonstrated one or more inner zones of protein material and an outer zone that probably contained ribonucleoprotein. Further evidence of cytochemical differentiation in the nucleolus has been provided by a new stain for nucleoproteins⁹.