In vitro behavior of thymic nurse cell-like complexes from mechanically and enzymatically dissociated frog tadpole thymuses

Cellular complexes, analogous by virtue of their external appearance, size, and number of seemingly internalized thymocytes to thymic nurse cells (TNCs) of endothermic vertebrates, were seen in short-term cultures (6-8 days) of mechanically and enzymatically dissociated thymuses of leopard frog tadpoles. Most TNC-like complexes from mechanically disrupted thymuses were covered with many thymocytes that morphologically resembled the "internalized" thymocytes. With time in culture, most complexes remained spherical and lost their externally adherent and "internalized" thymocytes. Some complexes, however, adhered to the glass substratum by means of macrophage-like cells. After one typically appearing TNC from a mechanically dissociated thymus had released its "internalized" thymocytes and spread completely over the glass substratum, it could be seen to consist actually of 9-10 stromal cells with the appearance of epithelial cells, macrophages, and dendritic cells. TNC-like structures from enzymatically dissociated thymuses had few, if any, attached thymocytes. Although these structures closely resembled murine TNCs initially, they displayed abnormal transformations within a few days of culture. Our observations led us to question the assumption that all TNCs from mechanically as well as enzymatically isolated TNCs from vertebrate thymuses are single cells. Rather, some if not all of the so-called TNC may actually be entities composed of several stromal cell types that enclose thymocytes. We suggest that this configuration seen in vitro may reflect the architecture of the compartmentalized reticular stromal cell meshwork that characterizes the intact thymus.