There are two basic systems for growing cells in culture, as monolayers on an artificial
substrate (i.e., adherent culture) or free-floating in the culture medium (suspension
culture). The majority of the cells derived from vertebrates, with the exception of
hematopoietic cell lines and a few others, are anchorage-dependent and have to be
cultured on a suitable substrate that is specifically treated to allow cell adhesion and
spreading (i.e., tissue-culture treated). However, many cell lines can also be adapted for
suspension culture. Similarly, most of the commercially available insect cell lines grow
well in monolayer or suspension culture. Cells that are cultured in suspension can be
maintained in culture flasks that are not tissue-culture treated, but as the culture volume
to surface area is increased beyond which adequate gas exchange is hindered (usually
0.2–0.5 mL/cm2
), the medium requires agitation. This agitation is usually achieved with a
magnetic stirrer or rotating spinner flasks