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 in-jected 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 reticu-loendothelial cells of the liver.
Fig: Life cycle of malarial parasite
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 crypto-zoite 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.