"The arrangement is so universal that one can call it the ,conventional architecture' of the cell nucleus", says Boris Joffe at the Biocenter of the Ludwig Maximilians University (LMU) Munich. "Hence we were very surprised to find marked differences in this architecture - and that they depend on an animal's lifestyle". An interdisciplinary team of researchers at the LMU, the Max Planck Institute for Brain Research in Frankfurt and the Cavendish Laboratory in Cambridge could show that in nocturnal mammals the chromatin arrangement in the rod nuclei is inverted: Here the heterochromatin is lumped in the nuclear interior whereas the less compacted euchromatin with the active DNA regions forms a peripheral shell.
This unusual nuclear architecture is explained by the requirements of vision. In humans and all other vertebrates, the light has to traverse the entire retina to reach the light-sensitive outer segments of the photoreceptors. "And here nocturnal animals face a dilemma. They need lots of rods to detect the little available light - but that makes their retinas thicker, so that more light is lost by scatter and diffusion before it reaches the photoreceptor outer segments", explains Leo Peichl at the Max Planck Institute for Brain Research.
Evolution solved the problem by using a physical property of heterochromatin: Because of its denser packing it has a higher optical refraction index than euchromatin. This has no effect when the heterochromatin lies in the periphery of the nucleus. However when it is lumped in the nuclear core, the heterochromatin acts as a minute collecting lens. As the rod nuclei are arranged in columns, sever
|Contact: Dr. Leo Peichl|