A light and electron microscopic quantitative analysis of the innervation of axillary lymph nodes in juvenile and old rats.

The innervation of axillary lymph nodes from 20 juvenile rats (aged < 6 wk) was compared with that of 20 old rats (aged > 2 y). One half of each group was investigated by light microscopy on silver-impregnated paraffin sections, the other half by electron microscopy. The lymph nodes of the old animals were larger than those of juvenile animals, as derived from the volumes analysed by light microscopy. By both light and electron microscopy, the nerves were found to be confined almost exclusively to the medulla and were frequently associated with groups of plasma cells. Nerves identified as cortical in location were consistently found only in cortical areas adjacent to the medulla. In the old animals the nerves were thicker and more prominent in the light microscopic preparations. No such differences were noted at the ultrastructural level. Myelinated nerves were found in all lymph nodes, except in a single old animal. Quantitative analysis of the nerves revealed a significant increase in the density of innervation of the medulla in the old animals both by light and electron microscopy. By light microscopy there also appeared to be a significant increase in the innervation density of the cortex, but these nerves were considered to be mislocalized because of difficulty in precise localisation at this level of magnification. On light microscopy, a significant increase in the incidence of nerve branching was found in the nodes of the old animals.