Developmental changes in the cellular composition of the chicken thymus

The cellular composition of the chicken thymus has been analyzed at different ages by using size distribution analysis in combination with preparative cell electrophoresis. The combination of these two physical methods was able to clearly resolve two major cellular subpopulations in the young chicken thymus and suggested the exsistence of a third one. Microscopically, all three cell types appeared to be small lymphocytes. Medium and large lymphocytes are not detected as distinct peaks by the settings used.The analysis revealed dramatic developmental changes in the cellular composition of the thymus. The adult chicken thymus, which is known to have practically no cortex, contained mainly one relatively large cell type. This cell type may, therefore, represent the medullary lymphocyte and may be active in graft-versus-host (G.v.H.) reactions. In the early postnatal thymus that is known to contain little graft-versus-host reactivity this larger cell type was not detectable. Instead, smaller cell types were found to be dominant. The developmental shift from smaller to larger cells was discontinuous. Before thymus involution at 16 weeks of age, smaller and larger cells were both found to be present and to have the same typical size and electrophoretic mobility that is characteristic for the postnatal or the adult chicken thymus, respectively. Size and electrophoretic mobility were therefore taken as markers indicating distinct cellular subpopulation in the thymus.