Thymocyte subpopulations during early fetal development in sheep.
Phenotypic analysis of thymocytes during fetal development may identify subpopulations which are either absent or difficult to detect in postnatal thymus. A panel of monoclonal antibodies specific for sheep lymphocyte antigens (SBU-T1, -T4, -T8, -T6) was used to identify thymocyte subpopulations in postnatal and fetal sheep. Thymuses were analyzed by two-color immunofluorescence and flow cytometry or by immunohistology. Two-color immunofluorescent staining of postnatal sheep thymus with anti-SBU-T4 and anti-SBU-T8 revealed four relatively distinct subpopulations with particular localizations: a) SBU-T4-T8-, predominantly outer cortex (12%); b) SBU-T4+T8+, inner cortex (74%); c) SBU-T4+T8-, medulla (10%), and d) SBU-T4-T8+, medulla (4%). One- and two-color immunofluorescent analysis of cells from early fetal thymuses demonstrated the appearance of SBU-T8+ cells well before SBU-T4+ cells. Immunohistologic staining of fetal sheep thymus at various stages of gestation (term = 150 days) revealed that lymphoid cells and MHC class II-positive dendritic cells first appeared at 35 days, at which stage the thymic epithelium was weakly positive for class I MHC antigens but negative for class II MHC antigens. The earliest lymphocyte antigens detectable on fetal sheep thymocytes were SBU-LCA and SBU-T1. By 40 days, the antigens SBU-T6, SBU-T4, and SBU-T8 were detectable on a small number of thymocytes; SBU-T8 preceded SBU-T4, and the number of SBU-T8+ thymocytes always exceeded the number of SBU-T4+ thymocytes throughout early gestation. At 50 days, a thymic medulla appeared and thereafter grew rapidly in size. Immunoperoxidase staining of serial sections of the fetal neck revealed cortical-type thymocytes outside the thymus from 40 days onward, before the appearance of a thymic medulla. However, by 60 days, only medullary-type thymocytes were observed either extrathymically or within the interlobular septa of the thymus, indicating that only thymocytes with a medullary phenotype leave the thymus from this stage of gestation.
C R
CR
Mackay
J F
JF
Maddox
M R
MR
Brandon
D000818:Animals; D000831:Animals, Newborn / Q000276:immunology; D000911:Antibodies, Monoclonal; D005314:Embryonic and Fetal Development; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D005865:Gestational Age; D006649:Histocompatibility Antigens / Q000276:immunology; D007958:Leukocyte Count; D012756:Sheep / Q000276:immunology*; D013194:Staining and Labeling; D013601:T-Lymphocytes / Q000145:classification* / Q000276:immunology / Q000502:physiology; D013950:Thymus Gland / Q000166:cytology / Q000196:embryology / Q000276:immunology*
D000911:Antibodies, Monoclonal; D006649:Histocompatibility Antigens