Cell line status criteria: an “established” (continuous) mammalian cell line is traditionally recognized after extensive subculture. Which benchmark (commonly cited in classic lab practice) indicates establishment?

Introduction / Context:
“Established” (continuous) cell lines are distinct from primary cultures and finite cell strains. Historically, labs used operational criteria—such as many consecutive subcultures without senescence—to denote that a culture has become continuous, typically accompanied by transformed characteristics.

Given Data / Assumptions:

• Question refers to classic, widely taught benchmarks rather than a formal universal law.
• Established lines show indefinite propagation and often altered karyotypes.
• Intervals of ~3 days between passages are common in teaching examples.

Concept / Approach:
Although transformation/immortalization is the biological basis, many training manuals cite a conventional threshold (around 70 serial passages at typical intervals) to claim “establishment.” The option that matches this teaching convention is the one with 70 subcultures at ~3-day intervals.

Step-by-Step Solution:

Verification / Alternative check:
Many continuous lines (e.g., HeLa, certain CHO derivatives) propagate indefinitely and quickly surpass dozens of passages, fitting the “established” concept.

Why Other Options Are Wrong:

• B/D: Lower passage counts are less stringent; used for strains rather than continuous status.
• C: Interval detail is atypical; not the commonly quoted pair.
• E: Far too few passages to infer establishment.

Common Pitfalls:
Assuming there is a strict universal passage-number definition; the essence is indefinite growth with transformed traits, but the 70-passage benchmark is a standard test answer.

Final Answer:
At least 70 subcultures at ~3-day intervals