The utilization of yolk platelets by tissues of Xenopus embryos studied by a safranin staining method Available to Purchase

The amphibian yolk platelet is a particular kind of food-reserve granule which may be easily recognized by microscopy and which is abundant in the cytoplasm of amphibian eggs and embryos. Wallace & Karasaki (1963) developed a method by which intact yolk platelets were isolated from eggs of Rana pipiens and were shown by electron microscopy to be practically free from other materials. Chemical analysis of such yolk platelets by Wallace (1963,a, b) showed that the crystalline main body is made up of two components, a phosphoprotein of similar amino-acid composition to avian phosvitin and a lipoprotein similar to avian a-lipovitellin, the molecular proportions being 2 to 1 respectively. Surrounding this crystalline main body of the yolk platelet there is a granular peripheral zone which has been reported to contain both protein resembling histone (Horn, 1962) and polysaccharide (Ohno, Karasaki & Takata, 1964). Histochemical work by Ohno, Karasaki & Takata (1964) showed there was no nucleic acid detectable in yolk platelets, and although nucleic acids have often been reported present in isolated yolk fractions subjected to biochemical analysis, it seems probable in view of the experience of Wallace (1963a) that such reports were due to contamination either by follicle cells or cytoplasm. The ultrastructure of the amphibian yolk platelet has been described by Karasaki (1962, 1963a), Ward (1962) and Lanzavecchia (1965). Wallace (1963Ô) proposed a molecular structure for the main body component of a yolk platelet which appears to fit all known data from previous biochemical, biophysical and electron-optical analyses.