Coated Vesicle-Mediated Transport and Deposition of Vitellogenic Ferritin in the Rapid Growth Phase of Snail Oocytes

Abstract

The main yolk component in oocytes of the pulmonate freshwater snails Planorbarius corneus L. and Lymnaea stagnalis L. consists of the iron storage protein ferritin and iron-free apoferritin. Both compounds are deposited in the yolk in the form of large paracrystalloids, tubular structures and randomly dispersed particles. In addition, the plasm contains lysosome-like inclusions with depositions of haemosiderin. Haemosiderin is interpreted as the product of proteolytic degradation of ferritin. During the rapid growth phase of the oocytes vitellogenic ferritin is transported across the basement lamina and taken up by adsorptive endocytosis via coated pits and vesicles. Formation of yolk bodies occurs by fusion of ferritin-containing vacuoles and empty vesicles that are probably derived from the Golgi apparatus. Uptake of ferritin is restricted to the basal region of the oocyte. No involvement of the follicle cells in synthesis and deposition of ferritin could be detected. Secretory cells of the midgut gland are the most likely site of synthesis of vitellogenic ferritin. Under conditions of iron overload large masses of ferritin are encountered in the basement lamina of the oocytes. However, no significant increase in the uptake of ferritin could be observed. With the use of a tannic acid-glutaraldehyde fixation procedure a hitherto unobserved filamentous or rod-like material was detected inside the lamina and in coated pits. This material is probably also taken up by the oocytes and integrated into yolk platelets. Though ferritin is a rather unusual vitellogenic protein, the mode of its uptake and deposition in the oocyte plasm is highly reminiscent of that of typical hormone-induced vitellogenins in other animal groups.