Trypan blue inhibition of yolk deposition--a clue to follicle cell function in the cecropia moth.

The yolk of insect eggs consists largely of proteins derived pinocytotically from the maternal blood (Telfer, 1965). Vitellogenic oocytes can also sequester the acidic colloid trypan blue: when injected into the blood, the dye, like the blood proteins, is deposited in yolk spheres in the cortex of the oocyte. This behavior of trypan blue was first described in the scorpian fly by Ramamurty (1964), and has since been confirmed in a cricket (Sander & Vollmar, 1967) and in the cecropia moth (Telfer & Anderson, 1968). Ovarian follicles of the cecropia moth have also been exposed to trypan blue in vitro; under these conditions the dye is not only incorporated into cortical yolk spheres but is in addition bound at the oocyte surface and in the intercellular spaces which serve as passageways for blood proteins across the follicular epithelium. The occurrence of extracellular trypan blue binding, particularly along the route of blood protein entry, suggested that the dye may be interacting with a normal component of the mechanism for vitellogenic blood protein accumulation. This concept is supported by the finding that trypan blue can be an effective inhibitor as well as a participant in yolk formation (Ramamurty, 1964; Korfsmeier, 1966; Telfer & Anderson, 1968). The follicular structures and passageways which have proved relevant to blood protein accumulation and trypan blue effects in cecropia were examined with the electron microscope by Stay (1965) and by King & Aggarwal (1965). To enter a follicle blood proteins must first cross the porous wall of the ovariole, and then an acellular basement lamina. They next pass between the singlelayered, columnar follicle cells (Telfer, 1961), and finally penetrate the vitelline membrane, which is too thin to be readily visualized by light microscopy, to reach the folded oocyte surface, the brush border, where pinocytosis occurs. Pinocytotic vesicles in the oocyte cortex then fuse to yield layer upon layer of 15—20 /A wide, membrane-bound yolk spheres. The structural relationships of the basement lamina, the follicle cells, and the oocyte with its contained yolk spheres may be observed in Figs. 1-4 and 7-10.