Question

‘Signet ring’ stage of Plasmodium is found in
a. RBC of man
b. RBC of Anopheles
c. Liver of man
d. Salivary gland of Anopheles

Answer 1

Hint: Plasmodium is a protozoan parasite. It has four species- P. vivax, P. falciparum, P. ovale, P. malarae. All the four species cause malaria. It was discovered by Laberon in the Red blood cells of humans.

Complete answer:
Plasmodium is a protozoan parasite. It has four species- P. vivax, P. falciparum, P. ovale, P. malarae. All the four species cause malaria. Plasmodium is digenetic, that means it completes its life cycle in two hosts- primary host (female Anopheles mosquito) and secondary host (human). It completes the reproductive cycle in the primary host.

In man Plasmodium reproduces asexually by a process called Schizogony. It occurs in liver and Red blood cells (RBCs). In the liver it is called hepatic schizogony and in Red blood cells (RBCs) it is called erythrocytic schizogony.

Hepatic schizogony was discovered by Shortt and Garnham. Whenever a plasmodium infected mosquito bites man, nearly 2000 sporozoites are released in the bloodstream of the man. Within half an hour, they reach liver cells, also known as hepatocytes, where they undergo pre-erythrocytic and post erythrocytic cycles.

Pre-erythrocytic cycle: whenever the sporozoite reaches the liver cell, it gets transformed into Trophozoite. They feed on the contents of hepatic cells, the parasite assumes spherical shape and attains maximum size called schizont. In schizont, the nucleus divides multiple times mitotically forming 12000 cryptozoites. This entire process is completed in 8 days. Now these cryptozoites or first generation merozoites can either enter liver and can continue exo-erythrocytic cycle or can enter RBC and continue erythrocytic cycle.

The erythrocytic cycle is initiated by cryptozoites of the pre-erythrocytic cycle. In fresh RBC they assume spherical shape and transform into trophozoites. It develops a small vacuole which gradually enlarges in size, pushing the cytoplasm and nucleus to the periphery. Now the plasmodium looks like a finger ring. Hence this stage is called the signet ring stage.

Hence, The correct answer is option (A).

Note: Soon after the signet stage it loses vacuole and is then called ameboid stage. It feeds on RBCs and thus RBCs grow double in size. It digests globin part of hemoglobin and converts soluble haem to insoluble hemozoin. This cycle is completed in approximately 48 hours. Hemozoin is responsible for causing high fever in malaria which is followed by profuse sweating. A very peculiar symptom of malaria.