I just attended a conference in Oberwolfach in an area rather far away from my usual interests, although it was about mathematical biology. I did meet some interesting (known and unkown) people and encountered some new ideas. Here I will just discuss two talks which particularly caught my attention.
In a talk of Takashi Hiiragi I learned a number of interesting things about embryology. The specific subject was the mouse embryo but similar things should apply to the human case. On the other hand it is very far away from what happens in Drosophila, for instance. There is a stage where the first eight cells are essentially identical. More precisely there is a lot of random variation in these cells, but no systematic differences. The subsequent divisions of these cells are not temporally correlated. By the time the number of cells has reached thirty-two an important differentiation step has taken place. By that time there are some of the cells which belong to the embryo while the others will be part of the placenta. If the first eight cells are separated then each one is capable of giving rise to a complete mouse. (The speaker did seem to indicate some restriction but did not go into details.) In order to understand the development process better one of these cells is studied in isolation. The cell contains a clock and so it ‘knows’ that it is in the eight cell stage. It then develops into a group of four cells in the same way that the eight cells would normally develop into 32. Differentiation takes place. The key symmetry-breaking step takes place when one end of the cell (at the eight-cell stage) develops an area at one end where actin is absent. This polarization then influences the further motion of the cells. It is interesting that the interaction between the cells in these processes seems to have more to do with mechanical signals then with chemical ones.
There was a talk of Fredric Cohen about cholesterol. His claim was that the concentration of cholesterol as usually measured is not a useful quantity and that the quantity which should be measured is the chemical potential of cholesterol. This has to do with the fact that cholesterol is hardly soluble in water or in hydrophobic liquids. I must say that the term ‘chemical potential’ was something which was always very opaque for me. As a result of this talk I think I am beginning to see the light. The cholesterol in a cell is mainly contained in the cell membrane. However it is not simply dissolved there as single molecules. Instead most of the molecules are interacting with proteins or with other cholesterol molecules. The chemical potential has to do with how many molecules get transferred when the system is connected to a reservoir. Only those molecules which are free are available to be transferred. So the issues seem to be the relationship between the amounts of free and bound molecules and what the real significance of the concentration of free molecules is for understanding a system.
Hydrobates 
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