Binding of 3H-actinomycin to vitelline platelets of amphibian oocytes

The 3H-actinomycin binding sites in mature ovarian oocytes of Xenopus laevis labelled after fixation were studied by electron microscopic autoradiography. Two different methods were compared: (a) labelling of floating ultrathin sections, (b) labelling of fixed oocytes before embedding. In each case, the oocytes were fixed either with 6% glutaraldehyde in Millonig buffer or with acetified methanol. Two different emulsions, Ilford L4 and Gevaert Scientia Nuc 307, were applied to the sections. Some autoradiograms were quantitatively analysed. The first method (a) failed to give satisfactory results as it proved impossible to eliminate the high nonspecific background. Method (b) on the contrary produced highly reproducible autoradiograms with identical distribution of radioactivity: strong labelling of the nuclei of follicular cell, significant labelling of the yolk platelets and very weak cytoplasmic background (table 1). Moreover it has been shown that the superficial layer of the yolk platelets contains at least twice as many grains as their crystalline core (table 2).