Annulate lamellae are interconnected by three distinct cisternal structures

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 cyt oplasm, then are recruited back to the nuclear envelope and AL. To investig ate structural contribution of ER to AL and interlamellar relationship of t he AL stack, we observed serial ultrathin and thick sections of rat intesti nal epithelial cells with conventional and high voltage electron microscope s. 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 bifurcat ed into successive AL cisternae. Type 3; several lamellae of a single AL st ack were connected by smooth tubular structures. We also found multiple bif urcations from ER to AL on freeze-fracture replicas. Thus, annulate lamella e 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 p roteins to the peripheral subdomains of ER and for formation of AL.