Complex Heterogeneity of the Thymic Stroma

Abstract

Despite the simple structural features of the thymus which standard histological examination reveals, the very intricate pathways of T lymphocyte differentiation occuring in this organ (1) would predict that the stromal elements which regulate this process must be equally complex. Tissue culture of dispersed thymic stromal cells (2), ultrastructural studies (3) and the distribution of MHC antigens (4) provided the first indications that this was so. More recently, use has been made of M.Abs. raised against enriched preparations of thymic stromal cells to identify a limited number of specific regions of the thymus which may reflect different roles in T cell maturation (5-7). Using the chicken thymus as a model, we have been able to markedly extend these observations by finding numerous antigenically distinct regions within which were multiple populations of stromal cells and their products (8; Boyd et al. submitted). This extensive panel of M.Abs to chicken stromal antigens probably reflects the phylogenetic distance between chickens and the mice used for immunization. We were interested to see, however, whether modified immunogen preparations and immunization protocols would reveal if similar complexities exist in the mouse and human thymus. This paper summarizes the major findings of these studies which clearly indicate that this is so.