Heterogeneity and age dependency of human thymus reticulo-epithelium in production of thymosin components.

The specific compartmentalization of intrathymic lymphoid cells in different steps of T-cell maturation and the involvement of thymosin components in these steps prompted us to evaluate the localization of epithelial cells synthesizing various thymosin components. Thymosin fractions 5 and 6 were found in epithelial cells throughout the thymus cortex and medulla. Thymosin alpha 1 and beta 4 were observed in subcapsular and perivascular epithelial cells, and thymosin alpha 1 was observed in the medullary epithelium. Thymosin alpha 7 was exclusively found in Hassall's corpuscles and adjacent epithelial cells. We conclude that the organ localization of epithelium synthesizing different thymosin components is associated with the organ localization of intrathymic T-cell maturation steps. The involution of the human thymus, starting during the third decade of life, has been related to the age-dependency of thymus humoral factors in the circulation. However, quantitative data on humoral factor synthesizing epithelial cells are lacking. We found a strong intensity of thymosin fraction 6 expression by epithelium in thymus tissue from donors in the early postnatal period (2-4 months of age) and from all four donors aged 20-30 years. This phenomenon was paralleled by a high number of thymosin fraction 6 positive cells per unit area.