*For medical professionals only
I've got another movie to watch!
This time, scientists from the Okinawa Institute of Science and Technology developed a new spindle imaging method, so we were able to see the entire process of eukaryotic mitosis in ultra-high definition today
The scientific community has long questioned the process of assembly of the spindle during mitosis, and this new method provides a glimpse of it.
Diagram of the title of the dissertation
The medaka fish selected by the researchers have many advantages, their genome is small and easy to manipulate, the fertilized eggs do not require high temperature and can be observed at room temperature, and the spawning volume is also very considerable. The most important thing is that the early division of the medaka embryo is single-celled lamellar and easy to observe.
The researchers labeled chromatin condensation factor RCC1 with red fluorescent protein and tubulin with green fluorescent protein. Under the microscope, chromosomes fluoresce magenta, while the microtubules that form spindles fluoresce green.
Under the conditions of 24°C-25°C, the researchers photographed medaka fertilized eggs every 3 minutes, and continuously carried out live-cell imaging for about 10 hours, during which 82% of the fertilized eggs developed normally, which was comparable to that of the control group, indicating that the imaging operation itself had little effect on the fertilized eggs.
10 h time-lapse photograph of zygote division
The first division of a medaka fertilized egg takes only about 12 minutes, and it is further shortened to about 9 minutes by the 256-cell stage, which is faster than that of mammals (usually starting at 1 hour).
However, at such a fast speed, the cell division is still very stable and precise, and there are no chromosome segregation errors in the first 4 divisions. It is not until the late stage of the mulberry embryo to the early stage of the blastocyst that the embryo has some abnormal division problems such as prolonged cell cycle and polyploidoid nuclei.
Some abnormal divisions can be seen in the late stage of the mulberry embryo
During embryonic division, the cell size gradually decreases, and the spindle length also decreases, shrinking from about 70 μm to 10 μm.
The spindle length varies with the cell size
Interestingly, the researchers found that in the central part of the spindle of the early embryo, there was a special area of dense microtubules. The microtubule network in the center of the spindle can also be observed after treatment with nocodazole, which reduces the stability of the microtubules, suggesting that it is more stable than the peripheral spindles.
Areas of special microtubule density persist after nocodazole treatment (bottom left)
This particular structure comes from Ran-GPT, a pathway that is essential for spindle assembly of cells in the early stages of embryonic division. The researchers modified the corresponding gene in medaka fish and found that Ran mutant embryos were very susceptible to chromosomal abnormalities, and the injection of mutant mRNA would cause all embryos to die.
However, the key role of Ran is only reflected in the early embryonic stage, and its importance is greatly diminished in the late stage. Researchers speculate that this may be due to the fact that the size of the cells in the late stage has decreased, and the structure of the spindle has also undergone some changes.
RAN mutations cause abnormal cell division
The process of spindle assembly in an early embryo
After watching the little movie, it really feels amazing, the formation of the spindle is so symmetrical, and it is always located in the very center of the cell, it is simply mysterious.
Resources:
[1]https://www.nature.com/articles/s41467-024-45251-w
The author of this article丨 Dai Siyu