Ultrastructure of embryonated eggs of the cestode Gyrocotyle urna (Gyrocotylidea) using cryo-methods

Abstract

Ultrastructure of eggs of the cestode Gyrocotyle urna Grube and Wagener 1852 with fully formed shells and early embryonic stages is described for the first time. Multiple techniques used included cryo-methods of high-pressure freezing with freeze substitution combined with field emission scanning electron microscopy and transmission electron microscopy (TEM). After 20 days of in vitro maturation, eggs are composed, from the exterior to the interior, of a thick egg shell of vitelline origin, an early embryonic envelope, a ring of numerous large vitellocytes, and a group of differentiating blastomeres situated in the center, forming the early presumptive larva. The thick resistant egg shell is similar to that reported from other polylecithal neodermatan flatworms. The embryonic envelope is poorly formed at this stage and not yet divided into inner and outer envelopes as occurs in all neodermatans described to date. Vitellocytes are separate, not forming a syncytium, and are filled with diverse vitelline materials as well as lipid droplets and associated endomembrane components having ultrastructure consistent with macroautophagy systems seen among many animals in apoptotic or autophagic cells. These results largely corroborate the basic observations and interpretations of previous TEM studies on other neodermatans and provide insight into a powerful suite of cryo-techniques that can enhance our work with these challenging subjects.