Electron microscopic study of thymus and bone marrow derived mouse lymphocytes: morphological differences and...

T and B lymphocytes from spleen, lymph nodes, thymus and bone marrow of unstimulated CBA mice have distinct ultrastructural features: respectively the Th type (dense dark appearance, smooth margin, high nucleocytoplasmic ratio, rare cytoplasmic organelles) and the Bm type (low electronic density, villous margin, low nucleocytoplasmic ratio, numerous cytoplasmic organelles). Correlations between Th or Bm morphology and presence of specific T or B surface markers (theta antigen or surface Ig) have been established. The Th/T and Bm/B equivalence do not however hold in all circumstances: first, there are morphologically intermediate types termed In (less than 13%) that may be theta- or Ig-positive; second, some Bm lymphocytes are theta-positive in CBA thoracic duct and some Th lymphocytes are Ig-positive in Nude mice spleen. Purified T-or B lymphocyte populations stimulated by selective mitogens (ConA or LPS respectively) undergo ultrastructural modifications before their surface markers (theta or Ig respectively) disappear. A time, some theta-positive T lymphocytes show a Bm-like morphology. The results suggest that the basis for the usual T-B ultrastructural differences in unstimulated mice resides in the normally different functional state of metabolic activity of these two types of cells: the cell metabolism would be higher in Bm (the usual form of unstimulated B lymphocytes) than in Th (the usual form of unstimulated T lymphocytes). This view may explain the paradoxical results observed in thoracic duct and Nude mice spleen as well as conflicting data reported by several authors.