Isolation of Giant Lampbrush Chromosomes from Living Oocytes of Frogs and Salamanders

We describe methods for studying the giant transcriptionally active lampbrush chromosomes (LBCs) found in the oocyte, or unlaid egg, of frogs and salamanders. Individual LBCs can be up to 1 mm in length and they reside in a gigantic nucleus, itself up to 0.5 mm in diameter. The large size of the chromosomes permits unparalleled observations of active genes by light optical microscopy, but at the same time special techniques are required for isolating the nucleus, removing the nuclear envelope, and spreading the chromosomes on a microscope slide. The oocyte nucleus, also called the germinal vesicle (GV), is isolated in a medium that allows partial gelling of the nuclear actin and preserves the delicate structure of the LBCs. This step is carried out manually under a dissecting microscope using jeweler's forceps. Next, the nuclear envelope is removed, again manually with jeweler's forceps. The nuclear contents are quickly transferred to a medium that disperses the actin gel and allows the undamaged LBCs to settle onto a microscope slide. At this point the LBCs and other nuclear organelles can be viewed by phase contrast or differential interference contrast microscopy, although finer details are obscured by Brownian motion. For high resolution microscopical observation or molecular analysis, the whole preparation is centrifuged to attach the delicate LBCs firmly to the slide. A brief fixation in paraformaldehyde is then followed by immunofluorescent staining or in situ hybridization. LBCs are in a transcriptionally active state and their enormous size permits molecular analysis at the individual gene level using confocal or super-resolution microscopy.