Ontogeny of leucocyte 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 populations are described in the spleen of fetal lambs from 40 to 134 days of gestation (length of gestation 150 days). Computer-assisted morphometric analysis and flow cytometry were used to quantify the early predominance of B cells in mid-gestation. B cells appeared at about 48 days and increased in number to occupy over 20% of the spleen area at 77 days. All spleens were collected on their respective livers and at no 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% during late gestation. The conventional organization of the splenic white pulp was observed from 90 days along with 5' nucleotidase-positive primary follicles. Double staining technique using immunohistochemical methods demonstrated that IgM+ cells were proliferating in the spleen from as early as 51 days and that clusters of proliferating IgM+ cells 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 ileal Peyer's patch and the subsequent generation of the preimmune antibody repertoire.
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Department of Morphology, Genetics and Aquatic Biology, Norwegian College of Veterinary Medicine, Oslo.
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Landsverk
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D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D015703:Antigens, CD / Q000032:analysis; D000954:Antigens, Surface / Q000032:analysis; D001402:B-Lymphocytes / Q000166:cytology*; D018956:CD5 Antigens; D002454:Cell Differentiation / Q000276:immunology; D005865:Gestational Age; D007091:Image Processing, Computer-Assisted; D007124:Immunoenzyme Techniques; D007075:Immunoglobulin M / Q000032:analysis; D012756:Sheep / Q000196:embryology* / Q000276:immunology; D013154:Spleen / Q000196:embryology* / Q000276:immunology; D013601:T-Lymphocytes / Q000276:immunology
D000911:Antibodies, Monoclonal; D015703:Antigens, CD; D000954:Antigens, Surface; D018956:CD5 Antigens; D007075:Immunoglobulin M