Electron microscopic and cytochemical studies on the cytoplasmic RNA in sea-urchin eggs

Abstract

Immature, mature unfertilized and fertilized eggs of Japanese seaurchin,Helocidaris crassispina were observed both with the light- and electron microscope, in order to examine various states of ribonucleic acid containing structures. 1. A great number of minute granules and microvesicles which were comparable to the microsomes reported by Claude ('43) were found to be actually scattered freely in the cytoplasm of sea-urchin eggs. 2. So-called yolk-nuclei of sea-urchin eggs are composed of canaliculi or double membrane structures, whose morphology is reminiscent of the endoplasmic reticulum. Features to be considered as transitional forms between this component and microsomes were noted. 3. The aster and spindle of the mitosis were seen in tubular shape in their full-formed states and as granular chains at the beginning of their development and at their reducing stage. 4. The nuclear envelope, which is directly continuous with the cytoplasmic double membranes or canaliculi at the resting stage of the cell, may be formed by the reorganization of microsomes at late telophase. 5. After the light microscopic examination with various basic dyes, free scattered microsomes, asters and spindle as well as the cytoplas- mic double membranes or canaliculi were recognized to contain appreciable amount of RNA. 6. It is conceivable from the above facts that the microsomes can serve as the common origin of cytoplasmic double membranes or canaliculi, nuclear envelope, and the achromatic figure in mitosis. Thus these structures were tentatively combined to a single cytoplasmic system, here called“microsome system”. The components of this system may be transformed from one into another associating with the physiological condition of the cell.