YOLK PROTEIN UPTAKE IN THE OOCYTE OF THE MOSQUITO <i>AEDES AEGYPTI</i>. L

TLDR

Yolk proteins are believed to enter eggs via a micropinocytosis‑like process, though the detailed mechanism has not been illustrated. The study examined protein yolk formation in Aedes aegypti by fixing ovaries before and after a blood meal for electron microscopy. Pits form by invagination of the oocyte membrane, acquire a 20 µm bristle coat on the cytoplasmic side, adsorb protein on the extracellular side, pinch off as bristle‑coated vesicles that deliver protein into the oocyte, lose the coat, and fuse to form crystalline yolk droplets that coalesce into mature yolk bodies. Yolk deposition correlated with a 15‑fold increase in 140 µm pit‑like depressions, which accumulate protein by selective adsorption, and preliminary data suggest the midgut is the main site of yolk protein synthesis.

Abstract

Yolk proteins are thought to enter certain eggs by a process akin to micropinocytosis but the detailed mechanism has not been previously depicted. In this study the formation of protein yolk was investigated in the mosquito Aedes aegypti L. Ovaries were fixed in phosphate-buffered osmium tetroxide, for electron microscopy, before and at intervals after a meal of blood. The deposition of protein yolk in the oocyte was correlated with a 15-fold increase in 140 mµ pit-like depressions on the oocyte surface. These pits form by invagination of the oocyte cell membrane. They have a 20 mµ bristle coat on their convex cytoplasmic side. They also show a layer of protein on their concave extracellular side which we propose accumulates by selective adsorption from the extraoocyte space. The pits, by pinching off from the cell membrane become bristle-coated vesicles which carry the adsorbed protein into the oocyte. These vesicles lose the coat and then fuse to form small crystalline yolk droplets, which subsequently coalesce to form the large proteid yolk bodies of the mature oocyte. Preliminary radioautographs, and certain morphological features of the fat body, ovary, and midgut, suggest that the midgut is the principal site of yolk protein synthesis in the mosquito.