Life Cycle of Plasmodium in Man – Schizogony
There are two phases in the life cycle of malarial parasite in man.They are
(1) Pre erythrocytic cycle or Exoerythrocytic cycle (in liver cells) and
(2) Erythrocytic cycle or Endo-erythrocytic cycle (inside the red blood corpuscles)
The pre-erythrocytic cycle comprises the asexual reproduction of the parasite in the liver. When an infected female Anopheles mosquito bites a person, thousands of slender, sickle shaped nucleated sporozoites are injected in the blood. The sporozoites first enter the capillary vessels of the skin and then enter the general circulation. These parasites circulate in the blood for about 30 minutes and enter into the pre-erythrocytic cycle in the reticuloendothelial cells of the liver.
The sporozoites penetrate the liver cells and develop into forms known as cryptozoites. A cryptozoite has a compact nucleus and no pigment or vacuoles. Cryptozoites rapidly grow feeding on the liver cells. When a cryptozoite has reached its full growth it fills the entire cell. In this stage it is known as the crypto-schizont. It undergoes schizogony and the resulting cells known as crypto-merozoites are set free in the blood by the rupture of the liver cells. The released crypto-merozoites invade fresh liver cells or red blood corpuscles. This cycle is considered as a period of incubation before the para- sites could start the erythrocytic cycle. During this period of 7 – 17 days, the parasites are not seen in the blood stream.
Erythrocytic or Endo-erythrocytic cycle
Each cryptomerozoite makes its way into a red blood corpuscle and feeds on its contents. After some time, the parasite gets an amoeboid shape. This growing stage is known as the trophozoite stage. Soon it develops a vacuole which gradually increases in size. Thus the nucleus is pushed to one side. This stage is called the signet ring stage. With further growth the vacuole disappears and the amoebula occupies the entire interior of the corpuscle. This stage is known as the schizont stage.
In the schizont, the nucleus breaks up into bits (6-24) and each becomes surrounded by a small amount of cytoplasm. These cells are known as merozoites. By the rupture of the wall of the red blood corpuscles the merozoites along with wastes(haemozoin) are released into the blood. This causes the malarial fever. The liberated merozoites attack another set of corpuscles and start the life cycle anew. This method of infection is known as autoinfection. The life cycle in the blood of man is called the cycle of Golgi or schizogony or endoerythrocytic cycle.
Schizogony keeps up the multiplication of the parasites and their maintenance in the blood. After schizogony has taken place for several generations some of the meroziotes which invade the red corpuscles, instead of developing into trophozoites and schizogonts, develop into gametocytes. The gametocytes are of two types – marco-gametocytes and micro-gametocytes. The macrogametocyte has a small nucleus and a dense food laden cytoplasm. The micro-gametocyte has a relatively large nucleus and clear cytoplasm. Their further development depends on their entry into the stomach of a female anopheles. If it does not take place they disintegrate.