Uptake of ferritin by the mosaic egg surface of Brachydanio

The internalization of membrane from the mosaic egg surface of the zebra fish, Brachydanio, was investigated using anionic ferritin and transmission electron microscopy. The cortical cytoplasm of the 5-min activated egg showed numerous membrane-bound vesicles not found in the unactivated egg cortex. Two types of vesicles were identified: uncoated (smooth) and coated. Coated vesicles measured about 0.7 to 0.9 micrometer in diameter. Coated pits, considered to be precursors to the formation of coated vesicles, were frequently observed at the base of membrane-lined cortical granule crypts. Anionic ferritin was localized over coated pits and in both smooth and coated vesicles. The absence of any morphological evidence of a surface origin for smooth vesicles suggested these ferritin-labeled organelles might be formed by coated vesicle fusion. Our results indicate that the plasma membrane redundancy created by the exocytosis of cortical granules in Brachydanio appears to be resolved in part by the internalization of membrane through endocytosis.