Using an improved fixation for electron microscopy, we have observed an ordered cytoplasmic organization during pattern formation in the alga Micrasterias. At the earliest stages of morphogenesis (the bulge to 3-lobe stage), as observed previously, electron-dense vesicles and endoplasmic reticulum were found towards the cell periphery, while mitochondria and Golgi material were located internally. In contrast to previous studies, which found no organization in the cytoplasm of growing lobes, we observed a single cluster of electron-dense vesicles at the tip of each lobe at or beyond the 5-lobe stage. In lobes about to branch, two separate clusters of electron-dense vesicles were present in each lobe, apparently foreshadowing the sites of new lobe outgrowth. Individual lobes also possessed an ordered distribution of organelles. Immediately basal to the lobe tip, the concentration of electron-dense vesicles decreased and rough endoplasmic reticulum was more prevalent, followed more deeply in the cytoplasm by increased concentrations of mitochondria and then Golgi material. This cytoplasmic organization in individual lobes is characteristic of tip growth, thus suggesting that individual lobes elongate by this process. We have also investigated the effect of the ionophore A23187 on this cytoplasmic organization. The polarization of rough endoplasmic reticulum, mitochondria and Golgi material was destroyed by ionophore treatment in the presence of 2 mM-MgCl2 or 2 mM-CaCl2. The tip-associated clusters of electron-dense vesicles were dispersed by A23187 in 2 mM-CaCl2 but not significantly affected by A23187 in 2 mM-MgCl2. The latter observation indicates that the vesicle pattern can be disrupted by high calcium, but not high magnesium concentrations.

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