Annulate Lamellae Are Interconnected by Three Distinct Cisternal Structures.

Abstract

Annulate lamellae (AL) are observed as a stack of specialized parts of the endoplasmic reticulum (ER) in intestinal epithelial cells. At mitosis, the components of the nuclear pore (NP) are known to disassemble into ER or cytoplasm, then are recruited back to the nuclear envelope and AL. To investigate structural contribution of ER to AL and interlamellar relationship of the AL stack, we observed serial ultrathin and thick sections of rat intestinal epithelial cells with conventional and high voltage electron microscopes. We found three distinct types of interlamellar connections of AL at the transitions between AL and their extended rough endoplasmic reticulum (RER). Type 1; several lamellae forming a single AL stack were connected by the reflection or rolling of single ER cisterna. Type 2; the RER often bifurcated into successive AL cisternae. Type 3; several lamellae of a single AL stack were connected by smooth tubular structures. We also found multiple bifurcations from ER to AL on freeze-fracture replicas. Thus, annulate lamellae appeared to be interconnected by three distinct cisternal structures. The presence of these various interlamellar connections suggests that ER would be an essential basis for concentration of integral nuclear pore complex proteins to the peripheral subdomains of ER and for formation of AL.