Nuage material and the origin of dense-core vesicles in oocytes of Nassarius reticulatus (Mollusca,Gastropoda)

Summary

In late previtellogenic oocytes nuage material accumulates in the vicinity of the nucleus and is often seen to be intimately associated with cisternae of endoplasmic reticulum (ER). A similar association observed in Ilyanassa has given rise to the proposition that nuage granules become completely enclosed in an envelope of ER and that this structure is transformed into so-called double-membrane vesicles, organelles which have only been found in eggs of Ilyanassa and Nassarius. This study provides evidence that, in Nassarius, the association of nuage material with ER has a temporary character. At later stages the nuage granules dissociate from the ER, move away from the nucleus, and become surrounded by mitochondria. Eventually they disintegrate. Evidence is presented that double-membrane vesicles originate from cisternae of ER by the accumulation and transformation of material within the lumen of the cisternae. Since only a single membrane is present in these vesicles, and a dense core is consistently found if the appropriate fixation is employed, I suggest that these vesicles be called dense-core vesicles instead of double-membrane vesicles.