Differences in the fine structure and chemical constitution of nucleolar bodies and the true nucleolus in Xenopus laevis embryos.

Abstract

Numerous bodies resembling nucleoli, named “prenucleolar bodies”, were seen in the interphase nucleus of Xenopus laevis embryos between stages 7 and 11 of Nieuwkoop and Faber (1956) but not at stage 12. These bodies are composed of thick strands, 200 A in diameter, and apparently differ from the fibrillar component of the true nucleolus which consists of thin fibrils, 50 A in diameter. The granular component of the true nucleolus consists of fibers and granules which are both also 150–200 A in diameter, but which differ in chemical nature from the prenucleolar bodies. The granular component and fibrillar component are readily digested by RNase with or without pretreatment with trypsin, while the prenucleolar body is only digested with RNase after pretreatment with trypsin. This suggests that the prenucleolar body consists of strands of RNA coated with protein. At stage 9, another type of nucleolus-like body is formed, which is larger (2–2.6 μ in diameter) than the prenucleolar body (0.2–1 μ) and consists of thin fibrils of 50 A. This body resembles the fibrillar component of the true nucleolus in the size of the elemental fibrils as well as in its susceptibility to actinomycin D, RNase and trypsin. It seems to be a precursor of the true nucleolus and for this reason was named the “primary nucleolus.” From stage 9 to stage 10, each nucleus in the presumptive ectodermal and mesodermal areas contains 2 primary nucleoli together with multiple prenucleolar bodies. At stage 12, the prenucleolar body is not seen at all, but a new type of nucleolus-like body appears. There are usually 2 of these bodies in each nucleus, and they consist of 2 components: a network of 50 A fibrils, and a group of strands, 150–200 A in diameter, containing some granule-like elements. The former has the same susceptibility to actinomycin D, RNase and/or trypsin as the fibrillar component of the definitive nucleolus and the primary nucleolus, while the latter has the same susceptibility as the granular component of the definitive nucleolus. Thus, this body may