Lymphocyte subpopulations in mediastinal milky spots of mice: light- and electron-microscopic immunohistochemical observations.

The structure and distribution of cellular components in the mediastinal milky spots of C57BL/6 mice were studied by electron microscopy and immunohistochemistry. The mediastinal milky spots were generally covered with plump mesothelial cells with hemidesmosome-like structures in small projections of the cytoplasm, and consisted mainly of clusters of lymphocytes, macrophages and fibroblasts. Macrophages and lymphocytes were often seen passing between mesothelial cells and migrating into the pleural cavity. Neither high endothelial venules (HEV) nor lymphatics were present in the mediastinal milky spots. Thy-1(CD5)-, L3/T4(CD4)- and Lyt-2(CD8)-positive cells were located predominantly at the basal area adjacent to fat cells under the lymphoid tissue. IgG- and Ia-positive cells dominated throughout the lymphoid tissue, whereas IgM-positive cells were observed in the lymphoid tissue other than the basal area. Mediastinal milky spots showed a specific distribution of lymphocyte subsets: the T cell area comprised the basal area, and the B cell area was mainly other areas. The mean percentages of CD4-, CD8-, IgM-, IgG- and IgA-positive cells in the mediastinal milky spots were 16.5%, 20.1%, 28.8%, 37.3% and 2.1%, respectively; the percentage of B cells was higher than that of T cells. IgG- and IgM-positive cells were the main lymphocyte subsets in the mediastinal milky spots. The most characteristic finding was that IgA-positive cells were a very minor cellular component.