The hydrodynamic tail vein (HTV) injection of naked plasmid DNA is a simple yet effective in vivo gene delivery method into hepatocytes. It is increasingly being used as a research tool to elucidate mechanisms of gene expression and the role of genes and their cognate proteins in the pathogenesis of disease in animal models. A greater understanding of its mechanism will aid these efforts and has relevance to macromolecular and nucleic acid delivery in general.

In an attempt to explore how naked DNA enters hepatocytes the fate of a variety of molecules and particles was followed over a 24‐h time frame using fluorescence microscopy. The uptake of some of these compounds was correlated with marker gene expression from a co‐injected plasmid DNA. In addition, the uptake of the injected compounds was correlated with the histologic appearance of hepatocytes.

Out of the large number of nucleic acids, peptides, proteins, inert polymers and small molecules that we tested, most were efficiently delivered into hepatocytes independently of their size and charge. Even T7 phage and highly charged DNA/protein complexes of 60‐100 nm in size were able to enter the cytoplasm. In animals co‐injected with an enhanced yellow fluorescent protein (EYFP) expression vector and fluorescently labeled immunoglobulin (IgG), hepatocytes flooded with large amounts of IgG appeared permanently damaged and did not express EYFP‐Nuc. Hepatocytes expressing EYFP had only slight IgG uptake. In contrast, when an EYFP expression vector was co‐injected with a fluorescently labeled 200‐bp linear DNA fragment, both were mostly (in 91% of the observed cells) co‐localized to the same hepatocytes 24 h later.

The appearance of permanently damaged cells with increased uptake of some molecules such as endogenous IgG raised the possibility that a molecule could be present in a hepatocyte but its transport would not be indicative of the transport process that can lead to foreign gene expression. The HTV procedure enables the uptake of a variety of molecules (as previous studies also found), but the uptake process for some of these molecules may be associated with a more disruptive process to the hepatocytes that is not compatible with successful gene delivery. Copyright © 2006 John Wiley & Sons, Ltd.