Observations on the ultrastructure of the test cell ofCiona robusta, with special reference to the localization of vanadium and iron

Summary

The ultrastructure of the inclusions in the test cell of the unfertilized egg and the oocyte in later vitellogenesis of the ascidian,Ciona robusta, was studied. The granules first appear in the cytoplasm of the test cell when it is embedded in the cortex of the oocyte, and the granules gradually condense into a mass. The electron-density of the condensed masses of test granules gradually increases and the number of masses increases until the cytoplasm of the test cell is filled with them. When the test cell leaves the oocyte, the mass gradually loses the electron-density from its central part to its periphery and finally becomes a myeline-like structure enveloped in a large vacuole. In later vitellogenesis small vacuoles with moderately electron-dense granules newly appear in the test cell. The granules are scattered in these vacuoles or sometimes gathered to form a net.

X-ray microanalysis revealed that considerable vanadium and iron are detectable in the condensed masses of test granules but there is little in the myeline-like structure of the unfertilized egg or the oocyte in later vitellogenesis. The relationship between the morphological transition of the inclusions in the test cell and the accumulation of vanadium was discussed with respect to the observed results.