Fetal thymic medulla functional alterations in Down's syndrome

Although the thymus is usually considered a specifically T cell organ, occasional mature B lymphocytes, B cell germinal centres and rarely plasma cells can be found in this organ, particularly in children. Ultrastructural studies have shown that these lie outside the epithelial enclosure and within the perivascular spaces of the medulla, the corticomedullary junction and septa, and this has led to a concept of 2 functional compartments of the thymus, namely an intraepithelial compartment and an extraepithelial (perivascular) compartment. These cells do not originate from the thymus but enter the organ via the bloodstream, at the 16th week of gestation. Our study points towards possible alterations in the functional histology of this lymphoid organ associated with immunization.

A quantitative comparison of B lymphocytes in the thymic medulla in embryos after involuntary abortion during the 1(st) and 2(nd) trimester of gestation and embryos with Down's syndrome, respectively, was performed.

Our results showed: 1) A statistically significant difference in the number of B cells together with infrequent primary follicles in the cases of embryos with Down's syndrome over those after voluntary abortion, during the 20(th) week of gestation (p=0.013). 2) No statistically significant difference in the number of B cells in both categories during the 1(st) trimester of gestation (p=0.34) and the 24(th) week of gestation (p=0.14).

The occurrence of increased number of B cells along with primary follicles in the cases with Down's syndrome, in the 2(nd) trimester of gestation imply that the thymic medulla - at least part of it - works and behaves as a peripheral lymphoid organ receiving mature lymphocytes, and turning them from inactive to immunoefficient cells, in the affected fetuses.