The occurrence of lampbrush chromosomes in early diplotene oocytes of Xenopus laevis

The Nuclear-to-Cytoplasmic Ratio: Coupling DNA Content to Cell Size, Cell Cycle, and Biosynthetic Capacity

Though cell size varies between different cells and across species, the nuclear-to-cytoplasmic (N/C) ratio is largely maintained across species and within cell types. A cell maintains a relatively constant N/C ratio by coupling DNA content, nuclear size, and cell size. We explore how cells couple cell division and growth to DNA content. In some cases, cells use DNA as a molecular yardstick to control the availability of cell cycle regulators. In other cases, DNA sets a limit for biosynthetic capacity. Developmentally programmed variations in the N/C ratio for a given cell type suggest that a specific N/C ratio is required to respond to given physiological demands. Recent observations connecting decreased N/C ratios with cellular senescence indicate that maintaining the proper N/C ratio is essential for proper cellular functioning. Together, these findings suggest a causative, not simply correlative, role for the N/C ratio in regulating cell growth and cell cycle progression.

The effect of heat shock on the morphology of amphibian lampbrush chromosomes

Isolated oocytes of Triturus cristatus carnifex cultured in Barthe's medium respond to a sudden increase in temperature of 35 or 37 degrees C by the rapid condensation of their lampbrush chromosomes. The degree of condensation depends on the severity of the heat shock and is accompanied by the retraction of the transcriptionally active lampbrush loops. The oocytes were kept at 20 degrees C for 12 h after the period of heat shock, before the chromosomes were isolated. The minimum period required to effect loop retraction in immature oocytes of 0.7 mm in diameter was 10 min at 35 degrees C; although more extensive and complete retraction occurred if the oocytes were incubated for periods of up to 40 min. Chromosomes from more mature oocytes (1.4 mm in diameter) require longer periods of heat shock before undergoing condensation. The oocytes themselves do not show any obvious morphological changes after heat shock, although the germinal vesicles are initially too fragile to isolate manually for several hours. When the oocytes are returned to ambient temperatures, they can be cultured in Barthe's medium for several days, enabling the chromosomes to be isolated and studied. Generally, the chromosomes and loops from immature oocytes remain in a condensed state for about 48 h but then the chromosomal axis begins to relax and the loops begin to re-form. Depending on the severity of the initial heat shock, complete recovery of the normal lampbrush morphology is attained after a few days. The re-formed loops are morphologically indistinguishable from untreated loops in control preparations and are of the same length on average. This response of lampbrushes to heat shock is a reliable and repeatable process and should therefore become a valuable model system for the study of chromatin and chromosome structure during changes in transcriptional activity. The time taken for the lampbrushes to recover their normal morphology, is discussed in terms of the return to normal cellular transcription.

Soft X-ray contact imaging of nucleolar chromatin using synchrotron radiation: a comparative scanning and transmission electron microscope study

Contact images (CI) of dehydrated, nucleolar chromatin from amphibian oocytes have been produced by soft X-rays from a synchrotron radiation source. These CI have been compared with the morphology of the original chromatin as seen in scanning and transmission electron microscopes. The quality and informational content of the CI depend very much on certain preparative procedures. The following factors have a marked effect on image quality and need to be carefully controlled: the total X-ray dose, the time and nature of development and the distance of the specimen from the photoresist. The preparation of the chromatin itself, providing that it is critically point dried, is less important. By following a regime of high X-ray dose, sufficient for penetration of the rather thick chromatin rings, and gentle development so that fine detail is not dissolved from the resist surface, it has been possible to obtain images which closely resemble the original chromatin, although the detailed resolution of the CI is not as clear. The smallest biological structures clearly resolved in the CI are ribonucleoprotein granules, which vary in size from 200 to 800 nm. However, by further refinement of preparative conditions it should be possible to improve on the informational content of these images.