Ontogeny of circulating leucocytes in the fetal guinea pig

The ontogeny of leucocytes in the fetal guinea pig was investigated. The total leucocyte concentration increased between days 40 and 56, but thereafter was unchanged at term (day 68). T and B lymphocytes were detected on day 56 and at term, and while T cell concentrations remained constant between day 56 and term, the concentration of B cells decreased. The ratio of CD4+ to CD8+ T cells increased from 0.9 on day 56 to 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 fetal proliferative responses were significantly lower as compared with adults. Comparison of these data with previous analyses of the pattern of human fetal lymphocyte development indicated that, although there were similarities, such as the timing of the first appearance of circulating lymphocytes, there were also differences, for example the changes which occur between birth and adulthood, between the two species. This suggests that the guinea pig may have a limited use as a model of some aspects of the ontogeny of the human immune system.