Evaluation of the morphological and functional characteristics of murine thymic non-lymphoid cells grown in vitro.

During the past few years, several investigators have attempted to grow the epithelial component of the murine thymus as a monolayer in vitro in order to study its proposed physiological role in T-lymphocyte differentiation. However, using for the most part identical culture techniques, we have provided morphological, histochemical and functional evidence that the majority of murine thymic nonlymphoid cells grown in culture are epithelioid macrophages and that these macrophages and/or their soluble factors may be responsible for many functions previously attributed to thymic epithelium or "purified thymic hormones". This study emphasizes the absolute necessity and importance of extensive morphological and histochemical evaluation of cultured cells prior to their use in physiological studies. Furthermore, any conclusion regarding the function of thymic non-lymphoid cells in vitro must first take into account numerous species differences in the morphological and histochemical characteristics of these cells as noted both in our laboratory as well as by other investigators.