Lymphoid cells from bursa of Fabricius, bone marrow, thymus and spleen of 3 ½- to 16-week-old donors were used to study the reconstitution of the bursadependent lymphoid system in cyclophosphamide-treated chickens. Parameters used to assess the development of humoral immunity and other effects of transplanted cells were: production of natural and immune antibodies, plasma immunoglobulin levels, microscopic morphology of spleen and bursa, gain of body weight and survival pattern. Bursa cells from young donors, 3 ½ to 10 weeks of age, were capable of a functional restoration. Bone marrow cells had the same restorative capacity only when obtained from donors of older age, starting at 10 weeks. At the age of 14 weeks, the spleen, and also the thymus cells to a more limited extent, had the same capacity. Functional restoration of humoral immunity was paralleled by a normalization of the microscopic structure of the spleen but restoration of the bursal morphology was not achieved with any of the cell types used.

These findings suggest that the postembryonic stem cell responsible for humoral immunity emigrates from bursa to bone marrow at the time of bursal involution. Subsequently, a cell with the same reconstituting capacities appears in spleen, and to some extent also in thymus.

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