The ultrastructural features of B-, T-, and surface Ig(sIg)-bearing cells have been studied on cell suspensions from lymphoid organs of mice at different stages of immunization. The cells were identified by exposure to rabbit antibodies against mouse-specific lymphocyte antigens (MSLA) or brain-associated θ antigen (BAθ) for T cells, mouse-specific bone marrow-derived lymphocyte antigens (MBLA) for B cells, and mouse Ig for sIg-bearing cells. The rabbit antibodies fixed on the cell surfaces were detected by peroxidase-labeled sheep anti-rabbit Ig antibodies or by a "bridge" technique using southern bean mosaic virus or bacteriophage T4 as the final markers. In some experiments, short-lived lymphoid cells were labeled in vivo with repeated tritiated thymidine and the ultrastructural detection of their surface antigens was combined with radioautography.

MBLA+ lymphoid cells showed a whole range of ultrastructural patterns. Most were small and medium-sized lymphocytes with a clear cytoplasm containing mono- and polyribosomes, but they comprised also blasts and large cells with various amounts of endoplasmic reticulum, as well as plasma cells at different stages of maturation. sIg-bearing cells appeared to be identical with MBLA+ cells, except that sIg was less easily detectable on large blasts, and only very rarely observed on plasma cells. MSLA+ and BAθ+ cells fell into three categories. One of them (T1 cells) consisted of small to medium-sized lymphocytes with a clear cytoplasm and few organelles, mostly monoribosomes. A second consisted of large cells (T2 cells) characterized by numerous polyribosomes often in a "rosette"-like pattern, occasional dark, membrane-bound granules, and a developing "filamentous network." The third, very characteristic type, (T3 cells) was represented by dark small to medium-sized lymphocytes, usually containing large amounts of closely packed ribosomes and showing a striking accumulation of filamentous network, often condensed in large areas devoid of cell organelles. This filamentous network appeared to correspond to the cytochalasin B-sensitive system of microfilaments found in other cells and considered to be one of the contractile elements of the cell. The T3 lymphocytes showed frequently vesicles suggestive of a strong pinocytic activity, and assumed a variety of shapes, including uropods. Evidence is presented that T1 lymphocytes represent "virgin" T cells, T2 "activated," and T3 "differentiated" lymphocytes.

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