ONTOGENY OF CIRCULATING LEUKOCYTES IN THE FETAL GUINEA-PIG

The ontogeny of leucocytes in the fetal guinea pig was investigated. T he total leucocyte concentration increased between days 40 and 56, but thereafter was unchanged at term (day 68). T and B lymphocytes were d etected on day 56 and at term, and while T cell concentrations remaine d constant between day 56 and term, the concentration of B cells decre ased. The ratio of CD4+ to CD8+ T cells increased from 0.9 on day 56 t o 3.0 at term. All fetal cell concentrations were significantly lower as compared with adult animals. The fetal peripheral blood mononuclear cell proliferative responses to mitogenic stimulation increased from day 40 to 56, but tended to decrease between day 56 and term. The feta l proliferative responses were significantly lower as compared with ad ults. Comparison of these data with previous analyses of the pattern o f human fetal lymphocyte development indicated that, although there we re similarities, such as the timing of the first appearance of circula ting lymphocytes, there were also differences, for example the changes which occur between birth and adulthood, between the two species. Thi s suggests that the guinea pig may have a limited use as a model of so me aspects of the ontogeny of the human immune system.