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One goal of cell biology is to synthesize whole cells, hoping that this will reveal the principles of cell spatiotemporal organization. Natural cells are coated by lipid bilayer membranes and stabilized by cell walls and cytoskeletons. Typically, researchers prepare protein mixtures in batches and then use oil-water emulsions, giant monolayer vesicles, or micromatogenesis chambers to segment them into cell-sized vessels for cell synthesis. The eggs of Xenopus laevis are huge and very easy to obtain. Scientists used cytoplasmic extracts from Xenopus laevis eggs to assemble a complex of cytokinesis in vitro [1].
On November 1, Science published the results of Xianrui Chen and James Ferrell of Stanford University: Spontaneous emergence of cell-likeorganization in Xenopus egg extracts. Studies describe the ovoid extract of Xenopus laevis that organizes itself into fixed cell-like units/compartments and can perform cell division.
The discovery comes from Cheng's observations. While studying a molecular process of programmed cell death, he noticed something unusual about the cytoplasmic extract taken from the egg: after about 30 minutes, the distance between the two nuclei was almost equal. When he looked at the cytoplasmic extract on a microscope, he found that it formed different compartments, similar to a single piece of cell. In these cell-like units, with the added sperm nucleus as the center, the microtubules form a stellate microtubule structure; the mitochondria and endoplasmic reticulum are distributed along the microtubules. Microtubule connections are visible between adjacent cell-like units.
The sperm nuclei added to the experiment have centrioles that can be tissue-induced to the formation of stellate microtubule networks. The authors further explore whether nucleus addition is necessary. Without the addition of sperm nuclei, the formation of cell-like units becomes slow, with no visible stellate microtubules, but eventually roughly identical cell-like units can still be formed. With the concentration of the added nuclei, the cell-like units formed vary to a certain extent.
To understand the mechanisms of cell-like unit formation, the researchers tested whether compartment formation was influenced by chemical inhibitors such as cytoskeletal proteins, locosins, and kinases. These cellular bone proteins, locomotional proteins, and kinases activate other proteins in tissues that regulate the formation of compartments. The results showed that ATP, the main source of energy in cells and microtubules, provide structural support and are necessary for the formation of compartments. Dynein, a sports protein. Dynein inhibitors do not affect the formation of cell compartments, but the aggregation of the endoplasmic reticulum is greatly affected.
The Xenopus cells that prepare cytoplasmic extracts are usually in the intercellular phase, during which the microfilament polymerization inhibitor cytochalasin B will be added. This suggests that cell-like cell-like cell formation does not require regulation of microfilament networks; the addition of another microfilament inhibitor, Lathrunculin A, does not affect cell formation either. Interestingly, if these microfilament inhibitors are removed, cell-like units form, and over time, microfilaments will form networks distributed around the periphery of the cell-like compartments, as shown in Figure D below.
Finally, these cell-like compartments don't just look like cells; They also divide like cells. The researchers will add a chemical when preparing the egg extract that prevents the cells from entering the cell cycle. When this chemical is removed, sperm nuclei are added and the egg extract forms compartments; and further divisions are divided into smaller compartments. The researchers found that these compartments can undergo more than 25 rounds of splitting, suggesting that the process is very powerful. Since the total amount of cytoplasm remains constant, each cycle is divided into smaller and smaller compartments.
All of these findings suggest that the Xenopus omecytoplasm has an intrinsic ability to produce the basic spatial tissues of the cell, and even some functions. There are still questions to be answered about whether this phenomenon is the normal physiological function of the egg; another question is whether this ability to self-organize is specific to the egg or shared by other types of cells.
Original link:
https://science.sciencemag.org/content/366/6465/631
bibliography
1. P. A. Nguyen et al., Science 346, 244 (2014)