Trypanosoma congolense: appearance and distribution of variable antigen types during metacyclic differentiation in vitro.

Differentiation of epimastigotes and production of infective metacyclic forms of Trypanosoma congolense were examined in a culture system which enabled manipulation of the population density of insect forms. Scanning electron microscopy of cultures revealed the attachment sites of epimastigotes in detail, showing them to be attached as 'clusters' or 'bundles' and having associated fibrillar structures. Dividing epimastigotes were observed either within individual bundles or in association with two bundles. Metacyclic forms were detected by an immunofluorescence antibody test (IFAT) using metacyclic variable-antigen type (M-VAT) specific monoclonal antibodies, by day 7 after seeding cultures. Trypanosomes expressing M-VATs appeared singly in bundles, observed by both IFAT and an immunogold labelling method. Statistical analysis using Poisson calculations suggested that, in general, the distribution of metacyclics expressing individual M-VATs was random throughout cultures.