Glucocorticoids and evolution of haemopoietic tissue in the liver of the rat foetus.

The number of haematopoietic cells per unit of volume decreases after day 18 and falls rapidly until day 21. On day 21 it is reduced by three quarters as compared to the day 17 value. Maternal laparotomy causes, in the 24 h period following the operation, an anticipated and accelerated decrease. This effect is prevented by previous adrenalectomy. After maternal adrenalectomy, rat foetuses submitted to decapitation in utero, and therefore completely deprived of corticosteroids, do not present in their liver the involution of the haematopoietic tissue normally observed in their intact littermates. Cortisol administration restores the normal involution. These results show the importance of the role played by corticosteroids in the regulation of foetal liver hematopoietic function. The evolutions of proportions and frequencies of the different cellular types composing the haematopoietic tissue studied during normal pregnancy and after either overload or deprivation of corticosteroids, suggest that corticosteroids exert their action essentially at two levels of the maturation process. They decrease the ability of self-maintenance of the precursor cell pool and accelerate the maturation of the polychromatophilic cells, resulting in a progressive exhaustion of the haematopoietic tissue.