In vitro migration of avian hemopoietic cells to the thymus: preliminary characterization of a chemotactic mechanism.

Hemopoietic cells differentiating in the thymus have an extrinsic origin; it has been suggested that, in avian embryos, the thymic epithelium can recruit precursor cells at several precisely determined periods by a chemotactic mechanism. We have used a chemotactic chamber (Zigmond, S.H.: J. Cell Biol. 75, 606-616 (1977] to analyse quantitatively the behaviour of hemopoietic precursor cells confronted with thymuses. The embryonic bone marrow was found to be a rich source of precursor cells carrying thymic lymphocyte potentialities. In the Zigmond chamber, precursor cells exhibited a strong chemotactic response to a concentration gradient of substances secreted by attractive thymuses from 7 1/2-day-old chick embryos; the mesonephros from embryo of the same age had no effect. In addition 7 1/2-day-old attractive and 10-day-old refractory chick thymuses induced a net increase of adhesiveness and speed of locomotion. The chemotactic factor secreted by an attractive thymic epithelium was not retained by a 12 kdalton cut-off dialysis membrane, whereas the chemokinetic activity was associated with material of molecular weight higher than 50 kdaltons. Refractory thymuses freed of most of their homopoietic cells became attractive and produced a chemotactic factor of a molecular weight close to 1 kdalton. When introduced into the two separate compartments of the chemotactic chamber, the chemokinetic and the chemotactic activities could complement each other to produce a full response identical to that obtained with attractive thymuses. The two factors, synthesized exclusively by the thymus, were effective on basophilic precursor cells but not on granulocytes. A theoretical model for random cell migration described in the Appendix showed that the standard deviation of the chemotactic index was greatly influenced by the degree of persistence in the direction of movement, the duration and the number of cells recorded. The high values obtained for the standard deviations are fully explained by the persistence in the direction of movement of the basophilic bone marrow cells. In all cases, a clear distinction could be made between a chemokinetic and a chemotactic response. For the first time, a chemotactic mechanism has been demonstrated to control the directed migration of embryonic cells. Our data fully support the hypothesis proposed by Le Douarin (1978, in: Cold Spring Harbor Conferences on Cell Proliferation, pp. 5-31) that the homing of hemopoietic precursor cells into the thymus is controlled by a chemotactic mechanism.