A. Definitions of Transformation in vitro When normal tissues or organs
are explanted to conditions favoring the growth of cells as individual
units ("cell culture"), the original cell population undergoes a large
variety of modifications. Only a minority of the cells will thrive and
multiply and within a rather short period of time, the complex
composition of the original explant is replaced by a much simplified one
of only a few recogniz- ably different cell types. With most organs
fibroblast-like cells survive longest and outgrow other types. This is
then a stable state of affairs for many gener- ations. This treatise
will not discuss whether this simplification and stabilization
represents selection of certain pre-existing cell types or a
modification of cells into only a few recognizably different categories;
for an excellent review see HARRIS. (1964). Table 1. Terminology
Employed to Describe Transformations in vitro Type of transformation
Essential features Irregular growth Lack of contact inhibition of cell
membrane movement ("ruffled membranes") between juxtaposed cells
Unrestrained growth Deficient inhibition of the cell cycle (mitosis) in
a crowded culture Infinite growth Capacity of cells to undergo an
infinite number of di- visions (formation of established cell lines)
Cells may depart from this typical behavior in numerous ways involving
for instance cellular morphology, immunology, chromosomes or metabolism.
Such changes have, sometimes rather vaguely, been called
"transformations". This is unprecise and the term "transformation" will
here be used exclusively to indicate disturbances in cell growth related
to neoplasia.