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.
Flannery
A V
AV
Department of Genetics, University of Liverpool, United Kingdom.
Hill
R S
RS
D000818:Animals; D002875:Chromosomes / Q000648:ultrastructure*; D006358:Hot Temperature; D007700:Kinetics; D009865:Oocytes / Q000648:ultrastructure*; D014319:Triturus / Q000033:anatomy & histology*