abnormal cell morphology abnormal endoplasmic reticulum morphology The endoplasmic reticulum membranes form a complex meshwork of tubular channels, which are often expanded into slitlike cavities called cisternae. The ER takes two forms, rough (or granular), with ribosomes adhering to the outer surface, and smooth (with no ribosomes attached). any structural anomaly of irregular network of unit membranes, visible only by electron microscopy, that occurs in the cytoplasm of many eukaryotic cells abnormal ER morphology MPheno.ontology MP:0009378