Overview

Cell proliferation is the increase in cell number through coordinated growth and division. In research and bioprocessing, it is a core concept because it affects how efficiently cells can be expanded, characterized and maintained for applications such as therapeutic protein production, vaccine development, disease modeling and drug screening.

What is cell proliferation and why does it matter?

Cell proliferation is the process of expanding a cell population through growth and division. It is a fundamental biological process that influences early development, tissue and organ formation, wound healing and, in the case of uncontrolled cell proliferation, cancer.

​​​Cell proliferation plays a key role during cell line development, which is carefully monitored and regulated. Cell line development is a critical step during the bioproduction of therapeutic proteins, gene therapies and vaccines in culture. Additionally, proliferative cell lines constitute a platform for disease research, drug discovery and cytotoxicity assessment.

How does cell proliferation support cell line development?

Cell proliferation plays an essential role in various biological processes, from embryonic development¹'² to tissue regeneration³ and cancer development.⁴'⁵ However, it also has transformative significance in life sciences and biotechnology.

​​​In biotherapeutic production, cell line development begins by transfecting a construct of interest into a cell line, followed by clonal selection based on successful DNA integration and product yield. Subsequently, proliferation becomes an important criterion for selecting and expanding top-ranked cells.

Cell proliferation remains fundamental throughout the maintenance and characterization of cell lines. Especially for longitudinal workflows used in disease modeling, drug evaluation and cytotoxicity assessment, cells must be able to proliferate continuously to achieve consistency and continuity in research applications.

To that end, researchers employ immortalized cell lines that either naturally harbor the capacity to proliferate indefinitely (e.g., HeLa derived from cervical cancer cells) or are genetically modified to display continuous proliferation (e.g., Human embryonic kidney 293 (HEK 293) cells or Chinese Hamster Ovary (CHO) cells).

Aberrant proliferation is observed in many cancers and is often targeted by anticancer drugs. Thus, cancer cell lines with highly proliferative phenotypes are convenient for examining genetic mutations associated with proliferation and for screening drugs.

What regulates cell proliferation during cell line development?

Understanding the mechanisms that regulate cell proliferation is necessary to maintain candidate cell lines during cell line development.

The cell cycle is influenced by internal signals from various pathways, initiated when growth factors, such as epidermal growth factor (EGF) and vascular endothelial growth factor (VEGF), bind their specific receptors. Such binding events trigger signaling cascades that ultimately activate transcription factors involved in cell cycle progression.

The cell cycle is tightly controlled by cyclins and cyclin-dependent kinases (CDKs), proteins that determine whether the cell is ready to progress from one phase to the next. Cyclin-CDK complexes regulate the synthesis of cell cycle-associated proteins during phase transitions, also known as checkpoints⁶.

The regulation is strengthened by the activation of tumor suppressors, such as p53 and Rb, that prevent uncontrolled cell growth and division. Finally, small non-coding RNAs and miRNAs can strongly influence cell proliferation by modulating gene expression involved in protein synthesis, cell cycle progression and cell death.

Cell proliferation is also influenced by external factors, such as nutrient and oxygen levels, which the cell needs to generate the energy necessary for growth and division. Considerations of cell type and culture conditions should inform the choice of bioreactor for optimal proliferation.

Which bioreactor approach is best for cell proliferation?

Bioreactors are functional platforms integral to the controlled proliferation of mammalian cell cultures for industrial applications. Bioreactors can be operated in three modes: batch, fed-batch and perfusion.

Batch

A batch process is a closed system in which cells are suspended in a pre-defined culture medium without additional media and nutrients. Although batch processes are cost-effective and easy to operate and reproduce, they are susceptible to batch-to-batch variability and require precise control over parameters such as pH, temperature and even distribution. Nevertheless, batch bioreactors provide a controlled environment and a flexible design that enable cell proliferation for various cell lines.

Fed-batch

Fed-batch bioreactors combine elements of batch and continuous processes to ensure optimum growth, cell density and product yield. Instead of adding the entire culture medium from the beginning, fresh medium is added throughout the process in a controlled manner, which mitigates nutrient depletion and starvation. Adding nutrients at specific time intervals also prevents the accumulation of metabolic byproducts that might interfere with cell proliferation. These advantages make fed-batch processes suitable for industry-scale cell line development.

Perfusion

Cell perfusion involves a continuous exchange of medium in a bioreactor. In addition to the continuous infusion of fresh media containing nutrients, perfusion also involves the removal of cell waste and nutrient-deficient medium. The constant exchange enables a steady state with higher cell proliferation and productivity than fed-batch processes. However, perfusion is often more complex than fed-batch due to the need for sterilization and for optimizing culture storage volume.

How does cell proliferation work at the molecular level?

Cell proliferation progresses through the following cell cycle phases:

• The G1 Phase is associated with cell growth and metabolic activity, the synthesis of mRNA and enzymes required for DNA replication. Organelle duplication occurs

• The S Phase involves DNA replication

• The G2 Phase is the final growth phase before cell division starts. The replication errors arising during the S phase are repaired

• The Mitotic (M) Phase involves four phases:

  • Prophase: The nuclear membrane breaks down and releases chromosomes while the mitotic spindle forms
  • Metaphase: The released chromosomes align along the central plane of the cell, while the mitotic spindle binds to the centromere that links the sister chromatids
  • Anaphase: The sister chromatids are segregated and pulled to the opposite sides of the mitotic spindle
  • Telophase: The chromatids reach the poles of the mitotic spindle. A nuclear envelope forms around the chromatids on each pole. The mitotic spindle disintegrates
  • Cytokinesis: Cytoplasmic division gives rise to two daughter cells⁷

Understanding where in the cell cycle cultures are crucial for measuring productivity, viability and sustainability. Factors such as free nutrients, metabolic waste, dissolved oxygen and time between feeds can influence cell culture growth.

How is cell proliferation measured?

Measuring cell proliferation is essential for evaluating the achievement of industry-scale cell lines. Furthermore, it is indispensable when screening drugs for efficacy and cytotoxicity.

A plethora of assays are employed in proliferation measurements by monitoring hallmarks of proliferation, such as DNA synthesis and metabolism, using a variety of staining techniques. While some assays are suitable for directly monitoring cell proliferation, others measure it indirectly via cell viability.

DNA Synthesis-based Assays;

5′-bromo-2′-deoxyuridine (BrdU) assay

This assay uses the thymidine analog BrdU to detect cells undergoing DNA replication in the S phase. The assay works by introducing BrdU to the cell culture and incorporating it into the cellular DNA. As cells prepare for proliferation, BrdU replaces thymidine and is integrated into the replicated DNA. It is then detected using anti-BrdU antibodies, which can be visualized or quantified by fluorescence microscopy, flow cytometry or enzyme-linked immunosorbent assay (ELISA).

5-ethynyl 2´-deoxyuridine (EdU) assay

EdU is a nucleoside analog that works similarly to the BrdU assay. Instead of fluorophores, the newly synthesized DNA is labeled using click chemistry for fluorescent detection. EdU is more advantageous than BrdU, as it does not require DNA denaturation. The EdU assay is also faster and less toxic than BrdU.

Nuclear Proteins for Cell Proliferation Measurements

​​​Cell proliferation can also be directly measured by detecting cell cycle biomarker proteins. Antigen Kiel 67 (Ki-67) and phospho-histone H3 (PHH3) are the most common markers. Ki-67 is expressed during all cell cycle phases of proliferating cells (G1, S, G2 and M), while the phosphorylation of PHH3 on its serine-10 residue mainly takes place in the G2 mitotic phases. These proteins can be detected by immunohistochemistry, immunofluorescence and flow cytometry using monoclonal antibodies and can be used to evaluate proliferation in cancer cell lines.⁸

ATP Cell Viability Luciferase Assay

ATP is a crucial marker of metabolic activity in live cells. This bioluminescence-based assay indirectly measures ATP by the amount required to convert Firefly luciferase to D-luciferin, which emits a flash of luminescence. Luciferase/luciferin assays are particularly useful for real-time measurement of cancer cell proliferation⁹.

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Dynamic Measurement of Cell Proliferation

​Fluorescent dyes are powerful tools for capturing cell populations undergoing division. While the agents used in this method are not fluorescent on their own, the products of their reactions in the cytoplasm emit fluorescence. 5(6)-Carboxyfluorescein diacetate N-succinimidyl ester (CFSE) is one such agent, which forms a fluorescent compound that stably conjugates to amine groups on proteins in the cell. The fluorescence is passed on to the daughter cells, although its intensity is halved at each division.

This allows real-time tracking of cells as they divide and the measurement of the number of cell divisions. A similar reagent is Calcein-AM, which interacts with esterase to synthesize a green, fluorescent product. Additionally, staining can be used to detect dead cells to accentuate live cells. Propidium iodide and trypan blue are dyes that permeate only dead cells and emit red fluorescence or appear blue, respectively.

Where is cell proliferation applied in the real world?

• Biomanufacturing: Proliferation supports the expansion of production cell lines used to manufacture monoclonal antibodies, recombinant proteins, viral vectors and vaccines
• Cell and gene therapy development: Robust proliferation is important when expanding cells for upstream workflow development, process optimization and analytical characterization
• Disease modeling: Stable proliferative behavior helps researchers maintain consistent in vitro systems for studying cancer biology, genetic disease mechanisms and treatment response
• Drug discovery and toxicology: Proliferation assays are widely used to evaluate drug efficacy, off-target effects and cytotoxicity across candidate compounds
• Process development and scale-up: Monitoring proliferation helps teams compare media formulations, feeding strategies and reactor conditions as processes move from bench to manufacturing scale

What are the main limitations and tradeoffs to consider?

• Assay-specific constraints: Some methods directly measure DNA synthesis, while others rely on metabolic or protein-based proxies, which can affect interpretation
• Cell type dependence: Assay performance and growth behavior vary by cell line, making method selection highly context dependent
• Culture sensitivity: Small changes in nutrients, oxygen transfer, pH, shear stress or temperature can alter proliferation outcomes
• Scale-up risk: Conditions optimized at small scale do not always translate cleanly to larger bioreactors
• Data interpretation complexity: Increased cell number does not always indicate healthy or productive cultures, so proliferation should be interpreted alongside viability, phenotype and product quality

References

1. Green RM, Lo Vercio LD, Dauter A, et al. Quantifying the relationship between cell proliferation and morphology during development of the face. Preprint 2023.
2. Boehm B, Westerberg H, Lesnicar-Pucko G, et al. The role of spatially controlled cell proliferation in limb Bud Morphogenesis. PLoS Biology 2010;8(7).
3. Jackson LN, Silva SR, Ueda J, Watanabe H, Evers B. PI3K/AKT activation is critical for hepatic regeneration after partial hepatectomy. Journal of Surgical Research 2006;130(2):301.
4. Shapiro P. Ras-MAP kinase signaling pathways and control of cell proliferation: Relevance to cancer therapy. Critical Reviews in Clinical Laboratory Sciences 2002;39(4–5):285–330.
5. Marei HE, Althani A, Afifi N, et al. P53 signaling in cancer progression and therapy. Cancer Cell International 2021;21(1).
6. Malumbres M, Barbacid M. Cell Cycle, CDKs and cancer: a changing paradigm. Nature Reviews Cancer 2009;9:153-166.
7. Schafer KA. The Cell Cycle: A Review. Vet Pathol 1998; 35:461-478.
8. Aziz S, Wik E, Knutsvik G, et al. Evaluation of tumor cell proliferation by Ki-67 expression and mitotic count in lymph node metastases from breast cancer. PLOS ONE 2016;11(3).
9. Teow S-Y, Liew K, Che Mat MF, et al. Development of a luciferase/luciferin cell proliferation (XenoLuc) assay for real-time measurements of GFP-Luc2-modified cells in a co-culture system. BMC Biotechnology 2019;19(1).