Phagocytic response of planarian reticular cells to heat-killed bacteria

Abstract

The defence mechanism of the planarian Dugesia dorotocephala (Woodworth) against invasion of foreign material was studied by inserting heat-killed bacteria into an incision and then examining the tissues around the incision by light and electron microscopy. The incision was made behind the right eye of each planarian, and a small aggregate of heat-killed Mycobacterium tuberculosis H37Ra was inserted into it using a fine needle. Samples of tissues were collected at 2, 4, 6, 8, 10, 12, 24, and 48 h after insertion of bacteria. As early as 10 h after insertion, bacteria were found in phagosomes of reticular cells, which are mesenchymal cells similar to fixed parenchymal cells and known previously to phagocytize degenerate tissues. By 12 h, aggregates of bacteria were found encapsulated in extensions of reticular cells, and by 24 h encapsulated bacteria were found between epithelial cells of the intestinal wall. These findings indicate that foreign material can be phagocytized or encapsulated by reticular cells and expelled into the intestinal cavity; it is thus conceivable that reticular cells could act as an immune surveillance system against foreign invaders.