Eosinophilic Precursors in the Fibroreticular Network of Human Thymus

Summary

The distribution of myeloid cells in the human thymus was investigated using a series of seventy four thymic samples from newborn to 37 year-old patients. By light microscopy, eosinophilic precursors (promyelocyte, myelocytes and metamyelocytes) were readily identified. These immature granular cells were present in all pre-involutional thymi; they made up 30-50% of the total eosinophilic population and were frequently observed as a group of cells at various stages of differentiation, suggesting that they differentiate from existing precursors in the thymus. These eosinophilic precursors were mostly located in the intralobular septa and fibroreticular network at the corticomedullary junction, while mature eosinophils were scattered throughout the thymus. Flow cytometric analyses using stem cell-enriched preparations showed cells expressing CD33 or CD34 to constitute on average 2.55% and 3.33% (0.09% and 0.12% of the total cells) respectively. CD33⁺/CD34⁺ coexpressors were also identified, and they constituted 0.36% of the analyzed cells (0.01% of the total cells). No statistical difference in the proportions of CD33⁺ and/or 34⁺ cells was noted between any age groups. It is concluded that eosinophilic precursors present in the thymus differentiate into cells in eosinophilic lineage in particular areas such as intralobular septa and the fibroreticular network of the outer medulla in pre-involutional human thymi.