To understand the lineage relationship and to define morphological characteristics of each thymocyte subset, we have performed ultrastructural analysis of highly purified thymocyte subpopulations. By flow cytometry, five subpopulations were sorted based on the expression of CD4 and CD8 and on cell size (forward scatter): large and small CD4+8+, CD4-8-, CD4+8-, and CD4-8+ thymocytes. Small CD4+8+ thymocytes were the smallest among lymphoid cells, and had a round and smooth cell outline with condensed nuclei, the cytoplasm was scanty and the cell organelles were not developed, suggesting the majority of this subset might be inactive by morphological criteria. CD4+8- thymocytes appeared to be similar to peripheral CD4+ T cells. The CD4-8- thymocyte subset contained morphologically immature cells in terms of cell size, presence of cell surface villi, and euchromatic appearance of the nucleus. CD4-8+ thymocytes, heterogeneous in cell size, nuclear chromatin contents and amount of cytoplasm, could be divided into two distinct types. Type 1 CD4-8+ thymocytes were intermediate in size, and therefore similar to peripheral mature CD8+ T cells. Type 2 CD4-8+ thymocytes were large and irregular in shape (large CD4-8+) with irregular-shaped and euchromatic nuclei. Large CD4-8+ cells were, thus, considered to be at the transitional stage from CD4-8- to CD4+8+. At least two groups of large CD4+8+ cells were ultrastructurally classified by the nuclear chromatin content. Large CD4+8+ cells with heterochromatic nuclei were round with a smooth cell membrane, whereas large CD4+8+ cells with euchromatic nuclei were spherical with projections. Cytological features of heterochromatic large CD4+8+ cells are similar to those of small CD4+8+ thymocytes except for cell size. Euchromatic large CD4+8+ cells could be regarded as active blasts potentially leading to mature cells. Taken together, this is the first report that describes the ultrastructural characteristics of each thymocyte subset highly purified by flow cytometry.
ASJC Scopus subject areas
- Immunology and Allergy