Ultrastructural Differences Between Thymic and Lymph Node Small Lymphocytes of Mice: Nucleolar Size and Cytoplasmic Volume

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Abstract
Ultrastructural differences between small lymphocytes of the thymic cortex and those located in the cortical zone of lymph nodes of mice were examined by electron microscopy and cytometric methods. The particle size analyzer TGZ 3 (Zeiss) was used to measure the size-frequency distribution of nucleolar cross-section diameters, and the resulting data were computed mathematically and statistically to determine the corresponding mean volumina. It could be shown that thymic, as compared to lymph node cortical small lymphocytes, (1) have a smaller mean nucleolar volume (0.11µ3 versus 0.46µ3); (2) represent a more uniform population with regard to the frequency distribution of the nucleolar cross section diameters; and (3) are of smaller size due to less cytoplasm, while the mean nuclear volume does not differ significantly from that of small lymphocytes located in the cortex of lymph nodes. These findings are discussed in relation to qualitative ultrastructural peculiarities and probable differences in the magnitude of RNA turnover as estimated by the use of tritiated cytidine. The possible significance of cytometrically defined populations of small lymphocytes with regard to sensitized and nonsensitized cells is considered.