| Summary: | Hormonal bursectomy significantly reduces thymocyte numbers in 18 day embryonic thymuses when it is carried out on or before the fifth day of incubation. Early embryonic surgical bursectomy (at 72 hours in ovo) similarly reduces thymocyte numbers. When hormonal bursectomy and early embryonic surgical bursectomy are combined, the effects appear to be additive producing a nearly two-fold depression of thymocyte numbers. Such results suggest that hormonal bursectomy, on or before the fifth day of incubation, and early surgical bursectomy affect thymocyte numbers by two different mechanisms. It is postulated that the effects of hormonal bursectomy prior to the fifth day of incubation represent a testosterone-thymus interaction. This interaction appears to result in an interference with the initial population of the thymus by migrating stem cells, resulting in a reduction of the numbers of thymocytes. Early surgical bursectomy would presumably circumvent such an interaction. It is suggested that early surgical bursectomy does indeed reduce thymocyte numbers by removing a potential bursal contribution to thymocyte numbers. Based on the evidence of this study and other investigations, it is suggested that the bursa contributes significantly to a secondary inflow of thymic stem cells occurring later in the embryonic period. === Land and Food Systems, Faculty of === Graduate
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