Studies on the mechanisms supporting the continuous growth of Trypanosoma (Trypanozoon) brucei as bloodstream-like form in vitro.

The conditions which enable growth of Trypanosoma (T.) brucei stock S 427 as bloodstream-like form over bovine fibroblast-like cells (BHF) were analyzed. The experiments with different BHF conditioned media indicated that there is no stable factor produced by the BHF which can trigger growth as bloodstreamlike form. Only living, actively metabolizing BHF could support continuous growth. The intercellular localization of trypanosomes in the BHF feeder-layer was a main feature of an intact culture. The number of trypanosomes per mm(2) BHF-layer was fairly constant. Light- and scanning electron microscopy revealed that there is no regular arrangement of the intercellular trypanosomes. No contact structures such as desmosomes or hemidesmosomes could be observed between trypanosomes and the BHF by transmission electron microscopy. Whenever trypanosomes were separated from the BHF by micropore membranes in different chamber systems the continuous growth as bloodstream-like form was no longer possible. An obligatory short-term trypanosome/BHF association is therefore assumed as growth trigger. Whether this inductive shortrange signal is transmitted by an exchange of unstsable, low concentrated factors or by cell contact interactions is not yet clear. The fibroblast surface antigen, fibronectin, did not seem to be involved in mediating the trypanosome/BHF association.