Image analysis of cell proliferation in rat thymus throughout development.

A System for Analytical Microscopy in Biological Applications (SAMBA 200) was used to digitize, process and statistically analyze the cell proliferation of rat lymphoid cells throughout thymus development. This cytometric study shows that lymphoid cell proliferation starts in 14-day-old embryos, remains high throughout embryogenesis with a maximum around day 18, decreases after birth then rises again in 21-day-old rats, after which proliferation decreases gradually with age, this decrease appears even before the onset of the sexual maturation, and is very marked in 2-year-old animals. Image cytometry analysis makes it possible to distinguish 5 lymphoid cell subpopulations (lymphoid stem-cells, lymphoblasts, large lymphocytes, medium lymphocytes, and small lymphocytes), and to discriminate in each cycling and non-cycling cells. The S-fraction of the lymphoid stem-cells decreases in 16-day-old rats, while the S-fraction of the lymphoblasts increases rapidly during the embryonic period. The large lymphocytes show the highest S-fraction at the stage E18, while the medium lymphocytes show the more stable S-fraction. The S-fraction variation for small lymphocytes can be divided into three periods: 1. up to E16 (S-fraction increases rapidly); 2. between newborn and 21 days (S-fraction increases slowly and irregularly); 3. after 21 days (S-fraction decreases progressively).