Biomedical Engineering Reference
In-Depth Information
6.1.5 Clinical Considerations: Important Questions
What are clinically meaningful numbers of cells?
Currently, the cells estimated to be needed for clinical practice and experimental cell
therapy protocols fall into the range of “a few tens of millions” to “a few billion”
(Table 6.2). Since tissue-like cultures require densities above 10 million cells per milliliter,
the sizes of the cell culture devices for cell therapies appear to fall into the range of 10 to
a few hundred milliliters in volume.
What are the fundamental limitations to the production of normal cells?
The number of divisions a cell can undergo is dependent on its maturational
lineage
stage, with the key stages being (1) stem cells (diploid, pluripotent), (2) diploid somatic
cell subpopulations (unipotent), and (3) polyploid cell subpopulations. Stem cells can self-
replicate and undergo unlimited numbers of divisions (stem cells are also a potential source
of immortalized cell lines and of tumor cells). Diploid cells are limited in the number
of times they can divide and are subject to the so-called Hayflick limit. Normal, somatic,
diploid human cells can undergo about 30 to 50 doublings in culture. Therefore, a single
diploid cell can theoretically produce 10
10
to 10
15
cells in culture. Given the requirements
for cell therapies, the Hayflick limit is not a major issue for cell therapies that utilize diploid
cells, either stem or somatic phenotypes. However, the expansion potential is minimal or
negligible for subpopulations of polyploid cells, which are found in all tissues. Conversely,
expansion potential is very high in various quiescent tissues such as the liver and heart.
Regenerative stimuli can cause polyploid cells to undergo DNA synthesis with negligible
capability to undergo
cytokinesis
. This results in an increase in their level of ploidy, an
increase in cell volume, and a phenomenon called
hypertrophy
. Thus, the regeneration of
tissues is a combination of
hyperplasia
(the diploid subpopulations) and of hypertrophy
(the polyploid subpopulations) activities.
How rapidly do normal cells grow in culture?
Normal cells vary greatly in their growth rates in culture. Hematopoietic progenitors
have been estimated to have 11- to 12-hour doubling times, which represent the minimum
cycle time known for adult human cells. Dermal foreskin fibroblasts grow with doubling
times of 15 hours, a fairly rapid rate that may be partially attributable to the fact that they
are isolated from neonatal tissue containing a high proportion of stem/progenitor cells.
In contrast, adult chondrocytes grow slowly in culture, with doubling times of about 24
to 48 hours.
How are these cells currently produced?
Expansion of cells can be performed in a variety of culture containers (plates, flasks,
roller bottles, bags), and in some cases also in suspension cultures (e.g., certain hemopoietic
cells or cell lines). Novel scaffolding material is permitting ex vivo expansion of cell
types that previously have proved difficult to culture. The ability to maximize expansion
potential for specific cell types requires precise culture conditions. These conditions may
comprise specific forms of extracellular matrix, defined mixtures of hormones and growth
