Phagotrophy and two new structures in the malaria parasite Plasmodium berghei

Abstract

Blood collected from rats infected with Plasmodium berghei was centrifuged and the pellet was fixed for 1 hour in 1 per cent buffered OsO(4) with 4.9 per cent sucrose. The material was embedded in n-butyl methacrylate and the resulting blocks sectioned for electron microscopy. The parasites were found to contain, in almost all sections, oval bodies of the same density and structure as the host cytoplasm. Continuity between these bodies and the host cytoplasm was found in a number of electron micrographs, showing that the bodies are formed by invagination of the double plasma membrane of the parasite. In this way the host cell is incorporated by phagotrophy into food vacuoles within the parasite. Hematin, the residue of hemoglobin digestion, was never observed inside the food vacuole but in small vesicles lying around it and sometimes connected with it. The vesicles are pinched off from the food vacuole proper and are the site of hemoglobin digestion. The active double limiting membrane is responsible not only for the formation of food vacuoles but also for the presence of two new structures. One is composed of two to six concentric double wavy membranes originating from the plasma membrane. Since no typical mitochondria were found in P. berghei, it is assumed that the concentric structure performs mitochondrial functions. The other structure appears as a sausage-shaped vacuole surrounded by two membranes of the same thickness, density, and spacing as the limiting membrane of the body. The cytoplasm of the parasite is rich in vesicles of endoplasmic reticulum and Palade's small particles. Its nucleus is of low density and encased in a double membrane. The host cells (reticulocytes) have mitochondria with numerous cristae mitochondriales. In many infected and intact reticulocytes ferritin was found in vacuoles, mitochondria, canaliculi, or scattered in the cytoplasm.