Primary Culture Primary culture refers to the stage of the culture after the cells are isolated from the tissue and proliferated under the appropriate conditions until they occupy all of the available substrate (i.e., reach confluence).  At this stage, the cells have to be subcultured (i.e., passaged) by transferring them to a new vessel with fresh growth medium to provide more room for continued growth.		Cell Line After the first subculture, the primary culture becomes known as a cell line or subclone.  Cell lines derived from primary cultures have a limited life span (i.e., they are finite; see below), and as they are passaged, cells with the highest growth capacity predominate, resulting in a degree of genotypic and phenotypic uniformity in the population.		Cell Strain If a subpopulation of a cell line is positively selected from the culture by cloning or some other method, this cell line becomes a cell strain.  A cell strain often acquires additional genetic changes subsequent to the initiation of the parent line.

Fibroblastic (or fibroblast-like) cells are bipolar or multipolar, have elongated shapes, and grow attached to a substrate.		Epithelial-like cells are polygonal in shape with more regular dimensions, and grow attached to a substrate in discrete patches.		Lymphoblast-like cells are spherical in shape and usually grown in suspension without attaching to a surface.