Dynamic and morphological responses of draining lymph nodes to single and repeated applications of oxazolone to guinea-pig skin.

Auricular lymph nodes in guinea-pigs were studied for 3 weeks under continuous stimulation with oxazolone applied to the ear skin. Quantitation of 3H-Tdr labelled paracortical lymphocytes following pulse labeling, demonstrated a marked, but only transitory rise in the proportion of cells in DNA synthesis. In spite of this, the total number of cells in S-phase continued to rise during the remaining part of the observation time, as a result of a steady increase of the paracortical cell mass. The variation in the proportion of large, pyroninophilic blast cells, revealed a pattern similar to that of the labelling index. A high proportion of blast cells was found only in the initial phase of the reaction, while the chronic response was characterized by a low proportion, no different from the starting level. Paracortical enlargement appeared to be the most reliable morphological criterion on which a chronically stimulated paracortex could be distinguished from an "unstimulated". The initial paracortical response, similar to a primary reaction, was followed by an equally pronounced development of germinal centres and plasma cells. These were also persistent features under continued stimulation. The reported changes are most likely specific responses to oxazolone stimulation.