Pinocytosis of Ferritin from the Gut Lumen in Larvae of a Sea Star (Patiria miniata) and a Sea Urchin (Lytechinus pictus): (pinocytosis/ferritin/larval gut/sea star/sea urchin).

Pinocytosis of macromolecules from the gut lumen is demonstrated for the first time in larval stages of invertebrates. Developing sea stars (Patiria miniata) and sea urchins (Lytechinus pictus) were incubated in seawater containing ferritin, which was detected in cell organelles by transmission electron microscopy. Pinocytotic uptake of ferritin by gut cells of Patiria could be detected as soon as the larval mouth opened before the esophagus, stomach and intestine could be distinguished from one another; in contrast, no pinocytosis was detected at the comparable developmental stage (the prism larva) of Lytechinus. Pinocytosis was first detected in developing Lytechinus in pluteus larvae, especially in the stomach and intestine. In gut cells of both kinds of echinoderm larvae, the ferritin progressed rapidly from coated pits at the luminal cell membrane to secondary lysosomes (e.g. this progression took only about 10 min in stomach cells of Patiria larvae). Phagocytosis from the gut lumen was never observed after latex beads or starch granules were fed to larvae of Patiria and Lytechinus. Moreover, there was no evidence of pinocytosis of ferritin or phagocytosis of large particles by epidermal cells of larvae of either species.