Assembly, Continuity, and Exchanges in Certain Cytoplasmic Membrane Systems

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.

We have examined the blood of man and of the rabbit, dog, guinea pig, sheep, and goat. There exists a great difference in the size of the red blood cells of these animals, but the total surfaces of the chromocytes See PDF for Structure from 0.1 cc. blood do not show a similarly great divergence, because animals having very small cells (goat and sheep) have much greater quantities of these cells in their blood than animals with blood cells of larger dimensions (dog and rabbit). We give all the results of our experiments, omitting only those in which we were unable to avoid losses in the procedure of evaporation of the acetone. It is clear that all our results fit in well with the supposition that the chromocytes are covered by a layer of fatty substances that is two molecules thick.