Presence of eosinophilic precursors in the human thymus: Evidence for intra‐thymic differentiation of cells in eosinophilic lineage

Abstract

The distribution of myeloid cells in the human thymus was investigated by light and electron microscopy, immunohistochemistry, and/or flow cytometry. A series of 74 thymic samples, from newborn to 37 year old patients, were studied. By light microscopy, aggregates of mononuclear cells were frequently present in intralobular septa and outer medulla. Among those cells, eosinophilic precursors (promyelocyte, myelocytes and metamyelocytes) were readily identified. These immature granular cells were present in all pre-involutional thymi, and were particularly frequent in the thymi of patients who were younger than 5 years of age. The cells 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 pre-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 that cells expressing CD33 or CD34 constituted 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 the eosinophilic lineage in particular areas such as the intralobular septa and fibroreticular network of the outer medulla in preinvolutional human thymi.