An Electron Microscope Study of Lampbrush Chromosomes

Lampbrush chromosomes were isolated from germinal vesicles of oocytes from Necturus maculatus, Triturus viridescens, Pseudotriton montanus and Rana pipiens. After treatment of isolated nuclei with 10 per cent sucrose, chromosomes free of nuclear sap are obtained for examination in either the light microscope or in the electron microscope. For electron microscopy the chromosomes were prepared either by Anderson's critical-point procedure or were embedded in methacrylate and sectioned. The evidence presented in favor of the view that the loops, axis, and the chromomeres of lampbrush chromosomes are formed by two chromonemata is based on the following observations: 1. Treatment of isolated chromosomes with 0.002 M KCN loosens the structure of the loops, and a more or less coiled organization is then observed in most of them with the light microscope. At the electron microscope level, each loop consists of a bundle of microfibrils. The latter are 500 A in diameter, and their complex arrangement within the loops is best studied in stereoscopic preparations. 2. Treatment of chromosomes with 0.002 M KCN also unravels the "chromomeric" regions of the axis. A fibrillar organization then becomes visible in the light microscope. In the electron microscope, wide strands are seen within some chromomeres; their diameter corresponds closely to that of the chromonemata forming the loops associated with the same chromomeres. In thin transverse sections of isolated chromosomes, no special structure is visible in the axial region except random profiles of fibrils similar to those seen in the loops of the same preparations. 3. Two strands sometimes connect adjacent chromomeres. Where gaps exist along the axis, after stretching of the chromosomes, a loop occasionally straddles the break and returns to a chromomere on each side.