ONTOGENY OF LEUKOCYTE POPULATIONS IN THE SPLEEN OF FETAL LAMBS WITH EMPHASIS ON THE EARLY PROMINENCE OF B-CELLS

The presence and distribution of B cells and other early leucocyte pop ulations are described in the spleen of fetal lambs from 40 to 134 day s of gestation (length of gestation 150 days). Computer-assisted morph ometric analysis and flow cytometry were used to quantify the early pr edominance of B cells in mid-gestation. B cells appeared at about 48 d ays and increased in number to occupy over 20% of the spleen area at 7 7 days. All spleens were collected on their respective livers and at n o stage did the livers contain more than a few IgM-positive (+) cells, which were usually close to blood vessels. Two-colour flow cytometry demonstrated that only 1-2% of IgM(+) cells expressed CD5 at 81 days. Beyond 77 days, with the expanding presence of T cells, the percentage of area occupied by IgM(+) cells declined to stabilize at about 7% du ring late gestation. The conventional organization of the splenic whit e pulp was observed from 90 days along with 5' nucleotidase-positive p rimary follicles. Double staining technique using immunohistochemical methods demonstrated that IgM(+) cells were proliferating in the splee n from as early as 51 days and that clusters of proliferating IgM(+) c ells were prominent between 60 and 77 days. The results of the present study suggest that during the ontogeny of fetal lambs the spleen is a site of B-cell development or expansion before colonization of the il eal Peyer's patch and the subsequent generation of the preimmune antib ody repertoire.