Thymic proliferative response during different physiological states: a comparative study

ABSTRACT

Objective - To study the thymic proliferative response during different physiological states to distinguish those changes due to alterations in steroid hormone secretion from those resulting from the presence of spermatozoa and/or early conceptual products in the female reproductive tract. Method - Using mature female rats of an inbred AO [RT1u] strain, observations on the thymus were made at 24 hour intervals during the oestrous cycle, early pseudopregnancy and early syngeneic pregnancy. Each daily group contained a minimum of 6 animals. Results- During the oestrous cycle, a significant mid-cycle increase of thymocyte proliferation occurred during dioestrus which peaked on day 2, and as a repetitive response may be a preparation for a coital challenge. This response may be oestrogen-dependent since oestrogen levels begin to increase during early dioestrus. The induction of pseudopregnancy generates a comparable but delayed increase in thymic proliferative activity. Since thymocyte proliferation and oestrogen secretion both peak on day 3 of pseudopregnancy, such a response may indeed also be oestrogen-dependent. After syngeneic mating, there was a significant depression in thymic proliferative activity on day 3 followed by a significant increase on day 5 compared with the same days of pseudopregnancy. Conclusion - This initial depression of proliferative activity may be induced by the immunosuppressive action of seminal plasma, to safeguard the preimplantation conceptus while the day 5 increase in cellular proliferation suggests a response to implantation