Induction of human lampbrush chromosomes.

We previously demonstrated that sperm heads from amphibians (Xenopus and Rana) and zebrafish (Danio) could form giant lampbrush chromosomes when injected into the nucleus of amphibian oocytes. However, similar experiments with mammalian sperm heads were unsuccessful. Here, we describe a slightly modified procedure and demonstrate that human sperm heads can form giant lampbrush chromosomes when injected into the oocyte nucleus of the frog Xenopus laevis or the newt Notophthalmus viridescens. Human and other mammalian chromosomes do not form recognizable lampbrush chromosomes in their own oocytes or in any somatic cells. These experiments thus demonstrate that the lampbrush condition is an inducible state and that the amphibian oocyte nucleus contains all factors required to remodel the inactive chromatin of a mammalian sperm into a transcriptionally active state. They also demonstrate that absence of lampbrush chromosomes from human oocytes must relate to specific features of mammalian oogenesis, not to permanent genetic or epigenetic changes in the chromatin.