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SOP-002: Passaging Adherent Mammalian Cells

1. Purpose

To maintain healthy adherent cell cultures by preventing overconfluency and ensuring cells remain in an optimal physiological state for growth and experimentation.

2. Scope

This SOP applies to routine passaging of adherent mammalian cell lines cultured under standard laboratory conditions.

3. Principle

Adherent cells attach to the culture vessel through cell-surface adhesion molecules. As cells proliferate, they occupy the available surface area. Once cultures become overconfluent, nutrient depletion, waste accumulation and contact inhibition can alter cellular physiology. Passaging detaches cells and transfers a fraction of the population into fresh culture vessels containing new growth medium.

4. Materials and Equipment

Materials​

  • Adherent cell culture flask
  • Prewarmed complete growth medium
  • Phosphate-buffered saline (PBS)
  • Trypsin-EDTA solution
  • Sterile centrifuge tubes

Equipment​

  • Class II Biosafety Cabinet
  • Inverted microscope
  • CO₂ incubator (37°C, 5% CO₂)
  • Pipettes and sterile pipette tips

5. Safety Considerations

  • Wear appropriate personal protective equipment (lab coat, gloves and eye protection).
  • Perform all procedures inside a certified biosafety cabinet.
  • Treat all cell cultures as potentially biohazardous.
  • Disinfect work surfaces before and after use.
  • Dispose of biological waste according to institutional biosafety guidelines.

6. Procedure

6.1 Assess Cell Confluency​

  1. Examine the culture under an inverted microscope.

  2. Passage cells when they reach approximately 70-90% confluency, unless experimental requirements specify otherwise.

💡 Why? Cells maintained within the optimal confluency range exhibit healthier growth and more consistent physiological behavior.

⚠ Important: Avoid allowing cultures to reach complete confluency.

🔍 Check: Cells should appear healthy, evenly distributed and free from contamination.

6.2 Remove Spent Medium​

  1. Aspirate the culture medium completely.

💡 Why? Removing spent medium eliminates metabolic waste products and serum proteins that may interfere with enzymatic detachment.

⚠ Important: Remove as much medium as possible without disturbing the cell monolayer.

6.3 Wash with PBS​

  1. Add sufficient PBS to cover the cell layer.

  2. Gently rinse the culture surface.

  3. Aspirate the PBS completely.

💡 Why? PBS removes residual serum proteins that inhibit trypsin activity.

⚠ Important: Do not allow the cells to dry during the wash.

🔍 Check: Cells should remain firmly attached after washing.

6.4 Add Trypsin-EDTA​

  1. Add sufficient Trypsin-EDTA to completely cover the cell monolayer.

  2. Incubate at 37°C for 2-5 minutes.

💡 Why? Trypsin digests adhesion proteins, while EDTA chelates calcium and magnesium ions that stabilize cell attachment.

⚠ Important: Avoid prolonged exposure to trypsin.

🔍 Check: Cells should gradually round up and begin detaching from the culture surface.

6.5 Monitor Cell Detachment​

  1. Observe the cells under the microscope.

  2. Gently tap the flask if necessary to assist detachment.

💡 Why? Continuous monitoring prevents excessive trypsin exposure and minimizes cellular damage.

⚠ Important: Do not leave cultures unattended during trypsinization.

🔍 Check: Most cells should be detached as single cells with minimal clumping.

6.6 Neutralize Trypsin​

  1. Add complete growth medium containing serum.

💡 Why? Serum contains protease inhibitors that rapidly stop trypsin activity and protect cell surface proteins.

⚠ Important: Neutralize trypsin immediately after cell detachment.

6.7 Collect Cell Suspension​

  1. Transfer the cell suspension into a sterile centrifuge tube.

💡 Why? This prepares the cells for counting, centrifugation or reseeding.

⚠ Important: Pipette gently to avoid mechanical damage.

🔍 Check: The suspension should appear homogeneous with minimal cell clumps.

6.8 Seed New Culture Vessels​

  1. Transfer the desired volume of cell suspension into fresh culture vessels.

  2. Add prewarmed complete growth medium.

  3. Mix gently to distribute the cells evenly.

💡 Why? Subculturing reduces cell density and provides fresh nutrients and growth factors.

⚠ Important: Maintain a consistent split ratio between passages whenever possible.

🔍 Check: Cells should be evenly distributed across the culture surface.

6.9 Incubation​

  1. Place cultures into a humidified incubator.

Standard Incubation Conditions​

ParameterCondition
Temperature37°C
CO₂5%
Relative Humidity>95%

💡 Why? These conditions maintain physiological temperature and stabilize culture medium pH.

⚠ Important: Minimize unnecessary removal of cultures from the incubator.

🔍 Check: Cells should begin attaching within several hours and resume normal proliferation within 24 hours.

7. Quality Control

Following passaging, assess:

  • Cell morphology
  • Cell attachment
  • Growth rate
  • Evidence of contamination

8. Troubleshooting

ProblemPossible CauseCorrective Action
Cells fail to detachInactive trypsinUse fresh Trypsin-EDTA
Poor detachmentResidual serumPerform a thorough PBS wash
Low viabilityOver-trypsinizationReduce incubation time
Slow growth after passageSplit ratio too highSeed cells at a higher density

9. Documentation

Record the following information:

  • Cell line name
  • Date of passage
  • Operator name
  • Passage number
  • Split ratio
  • Culture vessel identification
  • Observations on cell morphology and viability

10. Common Beginner Mistakes

  • Allowing cultures to become overconfluent before passaging.
  • Skipping the PBS wash before adding trypsin.
  • Over-trypsinizing the cells.
  • Pipetting too vigorously.
  • Forgetting to neutralize trypsin promptly.
  • Using inconsistent split ratios between passages.

11. References

  • Freshney RI. Culture of Animal Cells: A Manual of Basic Technique and Specialized Applications.
  • ATCC. Animal Cell Culture Guide.
  • ECACC. Cell Culture Protocols.