Pictures in cell biology: Lampbrush chromosomes from sperm chromatin

Abstract

The largest known chromosomes occur in the nuclei of amphibian oocytes. These so-called lampbrush chromosomes (LBCs) consist of a central axis of inactive chromatin from which hundreds or thousands of transcriptionally active loops extend laterally (Fig. 1). Although LBCs were discovered over 100 years ago, little is known about the factors that cause interphase chromatin to transform into these extraordinary structures during oogenesis. Recently, Gall and Murphy[1]found that typical LBCs can be produced `artificially' by injecting Xenopus sperm heads into the nucleus of a Xenopus oocyte. The injected sperm head first swells, then transforms into a loose aggregate of chromosome-like threads and eventually gives rise to 18 giant chromosomes that closely resemble the endogenous LBCs. Heterologous injections also work: sperm heads from the leopard frog and the zebrafish form LBCs in Xenopus oocytes, as do Xenopus sperm heads injected into newt oocytes (Fig. 2). Somewhat surprisingly, the injected sperm heads form LBCs with some characteristics corresponding to the species in which they are injected, suggesting that both donor DNA and recipient nuclear factors govern the formation of LBCs. These results suggest that it will be possible to produce LBCs from mammals and other organisms that do not go through a typical lampbrush phase or whose nuclei are too small to be handled by current techniques. Human LBCs would be particularly valuable for high-resolution analysis of transcriptionally active genes by in situ hybridization and immunofluorescence.