The effect of pre-natal thymectomy on lymphocyte sub-populations in the sheep

Ten sheep foetuses were thymectomized between 55 and 77 days gestation. The subsequent growth of the lambs was not affected for periods up to 500 days after birth. Prior to 240 days of age the thymectomized lambs were markedly lymphopenic and the response of their blood lymphocytes to phytohaemagglutinin (PHA) was significantly reduced. Peanut agglutinin-binding cells were found to be depleted in the blood of thymectomized lambs, while an unmarked 'null' cell population was virtually absent. The absolute numbers of E-rosette forming cells and sIg+ cells were similar for both groups. These findings indicated that 'null' cells in sheep may be immature thymus-dependent lymphocytes. The effect of thymectomy on blood counts, PHA responsiveness and the numbers of 'null' cells were less evident in thymectomized sheep that survived beyond 240 days. Possible differentiation pathways for sheep T-cells are discussed, together with the role played by the thymus in the maturation of T-cells.