Ramification of microglia, monocytes and macrophages in vitro: Influences of various epithelial and mesenchymal cells and their conditioned media

Microglial cells are able to switch between an ''active'' amoeboid and a ramified ''resting'' morphology during development and after experiencing lesions. We have previously shown that in vitro microglial morphology is controlled by their cellular environment, i.e. cells become ramified in astrocyte coculture but amoeboid on monolayers of fibroblasts. In the present study we have extended the analysis of the control of macrophage morphology by maintaining macrophages of different origins in coculture with different epithelial or mesenchymal cells and their conditioned media. Microglia, monocytes and spleen macrophages seeded onto monolayers of astrocytes, kidney epithelia or hepatoma cells developed the ramified morphology but remained amoeboid in fibroblast coculture. Ramification was also induced by media conditioned by these cells as well as by phorbolic esters, i.e. activators of protein kinase C. In double coculture assays, even small numbers of fibroblasts were able to override the ''epithelial'' influence. Likewise, microglia remained amoeboid, when incubated on several constituents of the extracellular matrix. These results indicate that macrophage ramification is an active process initiated by diffusible factors secreted by various epithelial cells, possibly acting upon a protein-kinase-C-related receptor. We interprete the modification of macrophage morphology as a functional adaptation to the surrounding type of tissue that is enforced by its constituent cells. Thus, the specific morphologies of microglia, hepatic von Kupffer's cells or peritubular kidney macrophages could be explained by similar epithelium-macrophage interaction.