Prenatal ontogeny of lymphocyte subpopulations in pigs

Although porcine lymphocytes have been classified into numerous subpopulati ons in postnatal animals, little is known about the ontogeny of these compl ex cell subsets. Using double- and triple-colour flow cytometry (FCM), we i nvestigated the surface phenotype of fetal lymphoid cells in the thymus, co rd blood, spleen and mesenteric lymph nodes at different stages of gestatio n. It was found that the major lymphocyte subpopulations started to appear at the beginning of the second third of the gestation period, with B cells being the earliest lymphocyte subpopulation to appear in the periphery. The T-cell receptor (TCR) gamma delta(+) cells were the earliest detectable T- cell subset, developing first in the thymus and subsequently arriving in th e periphery. Later in ontogeny, however, the number of TCR alpha beta(+) ly mphocytes rapidly increased, becoming the predominant T cells both in the t hymus and in the periphery. Cells with the phenotype of adult natural kille r cells were also identified in pig fetuses, though their nature and functi onal roles remain to be investigated. In addition, CD2 was expressed on mos t B cells whilst very few CD4(+) TCR alpha beta(+) cells or CD2(+) TCR gamm a delta(+) cells expressed CD8, suggesting that the expression of CD2 and C D8 may reflect the functional status of the cells in postnatal animals. Tak en together, this study has provided a systematic analysis of fetal porcine lymphocyte subpopulations and may provide the base for studies to establis h the physiological roles of these lymphocyte subsets.