RESUMO
After an exponential phase of growth, HK9 strain amebas, kept in the axenic medium PEHPS, spontaneously acquire a form morphologically similar to various natural cysts, as well as a resistance to hypotonic shock, due to the effect of a wall, partially composed of polysaccharides. The number of differentiated amebas increases gradually, although their viability diminishes, in function of the incubation time. On the ninth day, 96% of the population is made up of these cells, although only 6% are viable. The ultramicroscopic structure of the great majority of differentiated amebas corresponds to that of immature cysts. These, and the PEHPS medium, constitute a good model for a characterization of the start of the differentiation of E. histolytica, and open the opportunity of obtaining in axenic conditions, massive cultures of mature cysts.