There are two cell culture techniques to grow cells in culture, as monolayers on an artificial substrate (i.e., adherent culture) or free-floating in culture medium (suspension culture). The majority of the cells derived from vertebrates, with the exception of hematopoietic cell lines, are anchorage-dependent and have to be cultured on a suitable extracellular matrix to allow cell adhesion and spreading (i.e., tissue culture treated). However, many cell lines can also be adapted for suspension culture. Cells that are cultured in suspension can be maintained in culture flasks that are not tissue culture treated.