English: Microscopy of kidney tissue showing tubules. One tubule is highlighted to show epithelial cells (blue), cell nuclei (green) and the tubule lumen (dark center). Source: my personal image. The copyright to this image is retained by John Schmidt (JWSchmidt). Permission is granted to copy, distribute and/or modify this image under the terms of the Wikipedia GFDL, as indicated in the fine print at the bottom of this page. If you do not want to use this image under the terms of the GFDL, you can alternatively use it under the terms of the cc-by-nc-sa license.