Fig. 5. Interactions between mitochondria and ER in the ascidian egg and in the Ca²⁺ and ATP microdomain. (A-C) Imaging of the ER network (red) and of mitochondria (green) in the vegetal contraction pole (v) of a fertilized Phallusia egg (A) shows the cortical ER-rich domain closely apposed to the subcortical mitochondria-rich domain. At higher resolution (B), rod-shaped mitochondria are observed densely packed in the vegetal subcortex (0.5 µm under the surface of the egg). Mitochondria are in close proximity to ER-rich domains and tubes of ER (C; reveals ER tubes between ER-rich domains shown in B). a, animal side. (D) Probable Ca²⁺ fluxes in the Ca²⁺ microdomain forming at the interface between a mitochondria (green) and an ER tubule (red) with clustered IP₃Rs. In the same space an ATP microdomain is formed between mitochondria (producing and exporting ATP) and Mg-ATP-consuming pumps (SERCAs) and ATP^4--using channels (IP₃Rs). Calcium released in the cytosol can be sequestered by the mitochondria where it stimulates oxidative phosphorylation. ATP^4- is exported from the mitochondria into the cytosol where it stimulates Ca²⁺ release by sensitizing the IP₃Rs to Ca²⁺. Finally, Mg-ATP generated by mitochondria can energize Ca²⁺ pumping back into the ER lumen and replenish Ca²⁺ stores.

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