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Metacyclic Trypanosoma cruzi transforms in muscle tissue to a regressive trypanosome which broadens and thickens while changing from a ribbonlike to a crescentic body form, developing many volutin granules and vacuoles. The elongated, oval, laterally compressed nucleus becomes oval and then round while the oval kinetoplast becomes triangular. Disappearance of the trypanosome shape results in a rounded, regressive, transition parasite which, by shortening or disappearance of the flagellum accompanied by development of the rod shaped kinetoplast, becomes the regressive leishmaniform parasite.
Large size of all organelles characterizes the dividing leishmaniform stage. The reduction of volutin, vacuoles and cell volume, with retention of rod-shaped kinetoplast and round nucleus characterizes the progressive leishmaniform parasite. The progressive transition shows an oval kinetoplast and enveloping flagellum with undulating membrane forming. If elongation occurs, the parasite, by flow of the cytoplasm and nucleus along the line of growth of undulating membrane and flagellum, transforms into the short, stout trypanoform organism. If flagellar growth continues over the circular, progressive, transition cytosome, a V-shaped indentation appears between the base of the free flagellum and kinetoplast area and the parasite untwists becoming a stout, progressive trypanoform parasite.
These progressive trypanoform parasites, by subsequent slimming of the body, elongation and compression of the nucleus, and condensation of the kinetoplast, become the progressive trypanosome which on arrival in the peripheral blood becomes the broad, deeply basophilic regressive trypanosome with anteriorly displaced nucleus. Since leishmaniform stages differ in size, small forms develop into small, progressive, transition and eventually short, slender, progressive trypanosomes. Large, leishmaniform parasites elongate under tissue pressure, resulting in long, slender, progressive trypanosomes. Small, intermediate and large trypanosomes, under the changing chemical environment of the hosts' blood, transform into regressive trypanosomes which may reinvade the tissues and repeat the cycle.
Visiting Scholar, 1950–51, Division of Zoology, University of California at Davis.