Occurrence of membrane particle aggregates on the luminal surface of goldfish intestine revealed by quick-freezing

During the quick-freezing study of goldfish posterior intestinal epithelial cells, we found a hitherto unknown form of membrane particle specialization on the apical surface membrane. Numerous small plaques of intramembrane particles densely packed in polygonal or random patterns were observed on the PF face of intermicrovillous luminal surface membrane. The aggregated particles appeared smaller and shorter than ordinary intramembrane particles randomly distributed around the plaques. These particle aggregations were often localized in close proximity to microvilli and openings of well-developed surface invaginations, but not found on microvilli, interiors of invaginations, basolateral cell membranes of absorptive epithelia, or surface membranes of goblet cells. The functional significance of these particle aggregations remains to be clarified.