This is a rather eerie result. Two researchers at Stanford report that the often-used model system of Xenopus frog eggs have self-organizing properties. Extracts from homogenized eggs had already been known to be more functional than one might have predicted (the paper has a number of references to such studies), but this paper finds that homogenate from such eggs, if left standing, organizes itself into compartments within about half an hour (see at right). These things are 300 to 400 microns across, and their formation require ATP and involves microtubule polymerization. There doesn’t seem to be a propagation wave moving across the samples; it happens across a large field more or less simultaneously. And this takes place in the presence of cycloheximide (which inhibits protein translation), so it doesn’t seem to be the result of some new program being initiated.
Nuclei, mitrochondria, microtubules, endoplasmic reticulum and other components end up distributed through these compartments in ways that are quite similar to the original cells – for example, there’s a single nucleus or cluster of nuclei found in the center. The clear borders between each of these seem to be “organelle-depleted” cytoplasm. The authors verified that this organization took place in both the presence and absence of frog sperm (which naturally set off profound cytoplasmic changes when combined). It’s faster with added sperm, but happens regardless. The effect needs a certain concentration threshold – diluting the homogenate slowed or abolished it.
Now, these were all interphase-arrested cells. But the paper goes on to use homogenates that are capable of going through the cell cycle (no cycloheximide in there, for one thing). These formed the same sorts of compartments, and adding demembranated sperm nuclei to this mixture caused the compartments to divide as if they were normal egg cells. Each of the new smaller compartments had a similar division of organelles as the “parent”, and in many cases the process ran further, with these dividing in turn.
So it appears that the cell contents, when totally mixed, are able to roughly reconstitute themselves into units that are about the size of cells, contain the appropriate subcellular fractions, and can go through some of the most important cellular processes. Now, these things are surely deficient in many ways; I would not recommend homogenizing your body’s own cells to get a fresh start for the new year. But it’s remarkable that this level of order can be regained. The “moving parts” of the cell fit together more robustly than one might have imagined. Are these compartments alive? What exactly does “alive” mean?