If I want to freeze the cells, should I wait for a certain number of subculturing? Or can I do it after just one passage of cell?
It depends on the nature of your cell line, whether it is primary or immortalised/transformed cell line. In case of of your precious primary culture, depending upon the number of passage it can survive, you may freeze at an earlier passage. But in case of immortalised/transformed cells, you can grow them to log phase in a large culture and freeze to 10 million cells/ml. However in both cases please make sure that you freeze sufficient cells ( in freezing medium) to enable better cell survival upon thawing.
Generally, primary cells can be directly frozen after being isolated or sorted; and immortalised cells can be frozen after subculturing or waiting for one passage of them.
I think if you want to preserve the properties/ characteristics of cell lines its better to freeze them at earlier passages like before 2-3. This is because cell lines tend to differ in their properties at late passages (in general). But, if for some reason you couldn't make at earlier passages its better to freeze at earliest available passage.
I recommend you to freeze after having a selection of the cells that you want, because in m experience selecting them then it is more complicated, because a problem of survival. Maybe 3-4 passage depending of the type of cell you are trying to culture and the rate of selection. For example bovine fibroblast takes more time to select and therefore passages than murine fibroblast. So it depend of source and type of cells!
Is a commercial cell line or a tissue explant? In commercial cell lines it is recommended the freezing at earlier passages. In tissue explants you have to be sure that you have the desired clone. Another important thing is the nature of cells. Are finate? In this case you must freeze them as soon as possible.
Just be sure that you have enough cells before you freeze them and they were not cultured as a confluent cells too long (this will change their metabolic profile and decrease their survival rate during freezing).
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