Significant immunological restoration of 45-day old, neonatally thymectomized C3Hf mice was obtained by the cooperation of syngeneic newborn or embryonic hemopoietic liver cells with thymic function. Thymic function or cells alone are almost ineffective or restore approximately 10% of the animals. Newborn liver cells are effective in association with thymus grafts or humoral thymic function (thymoma grafts and thymus or thymomas in diffusion chambers). Embryonic liver cells are ineffective, even in large numbers, when associated with humoral thymic function. On the other hand, embryonic liver cells are effective in the cooperative effect only in association with viable thymus grafts, preferably syngeneic, whether the grafts were placed subcutaneously, intraperitoneally, or under the kidney capsule. Dispersed viable thymic cells are ineffective in association with embryonic liver cells. Cells capable of cooperating with humoral thymic function start to appear to embryonic liver by day 19–21 of gestation and are detectable until day 5–6 postbirth. Embryonic hemopoietic liver cells from 12 to 18 days of gestation contain cells capable of cooperation only with viable free thymus grafts and not with humoral thymic function.

A prethymic cell population of partially differentiated cells of hemopoietic origin, insensitive to humoral activity of the thymus but requiring thymic stroma and traffic through the thymus is postulated to explain our results. This population of prethymic cells can become postthymic through this process and eventually develop into competent cells. Postthymic cells are characterized by their sensitivity to humoral activity of the thymus and by their wide distribution in the lymphohemopoietic tissues of newborn and young adult mice.

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