Cell line

A cell line comprises descendants after a primary cell culture's first subculture. Further division can be finite or continuous; one founder biological cell is not required.^[ref]

The useful continuity is ancestry across successive subcultures: descendants supply the next experiment. Individual biological cells can divide or die while their cell line persists. A common origin helps compare experiments, but does not imply that every descendant has identical DNA, gene expression or cell state.

A human colorectal-carcinoma-derived epithelial cell line used in models including three-dimensional cyst organization. Its behaviour should not be assumed identical to that of a normal embryonic epithelium.

An immortalised cell line can apparently sustain unlimited cell line population doublings under suitable conditions. Immortality describes lineage reproduction, not individual biological cell survival.^[ref]

A subculture transfers part of a cell culture into fresh cell culture medium; each transfer adds one passage.^[ref]

In an original constructed example, seed $300000$ biological cells in $6\mathrm{mL}$ of cell culture medium. Suppose the recoverable count becomes $1200000$ before the next subculture. Transfer a well-mixed quarter of that harvest into a new vessel and bring its final volume to $6\mathrm{mL}$. Assume complete recovery, no loss during transfer and no division during handling. The selected portion containsThe other $900000$ biological cells remain outside the tracked vessel. The lower count after transfer is selection of a portion, not evidence of death. Adding fresh cell culture medium lowers the number density further without changing the selected count.

If the next interval again changes $300000$ to $1200000$, the two growth intervals contribute $2+2=4$ net cell line population doublings. At the second harvest there has been just one intervening passage. Another quarter-transfer would add another passage without adding a growth interval. Thus neither passage number nor the logarithm of the final count divided by the very first seed count records all growth when portions are removed.

For positive recoverable counts $N_0,N_1$ bracketing one growth interval, define its net cell line population doublings byThe formula counts how many factors of two produce the observed net factor. For exactly two divisions per original biological cell, with all daughters recovered, $N_1=2^2{N}_0$, giving $d=2$. Death or loss of biological cells makes this a net estimate of division.^[ref]

Under constant-rate exponential growth, $N(t)=N_0{e}^{rt}$, a growth interval of duration $\Delta t$ givesFor the constructed fourfold increase over $48\mathrm{h}$, $d=2$ and the model doubling time is $24\mathrm{h}$. The proof follows by taking the natural logarithm and solving $e^{r{t}_2}=2$. This is a fitted property of that interval and model: it is not a universal HeLa doubling time, nor the interval between two divisions of each individual biological cell. Across successive growth intervals, add their $d$ values; keep transfer losses separate as in cell culture passage accounting.

Cell counting estimates the number of biological cells in a preparation. A count per calibrated volume gives number density; multiplying by the whole preparation's volume estimates its total count. A trypan blue exclusion test adds an operational distinction between dye-excluding and dye-stained biological cells.

A hemocytometer has a calibrated grid and depth. ATCC describes $1{\mathrm{mm}}^2$ squares $0.1\mathrm{mm}$ deep, each sampling $10^{-4}\mathrm{mL}$.^[ref]

An original constructed hemocytometer count uses four such squares after mixing the biological cell preparation with the same volume of stain. The dye-excluding counts are $36,44,40,40$; the dye-stained counts are $4,6,5,5$. The dilution factor is $D=2$. With $v$ the sampled volume per square and $\bar{n}$ the arithmetic mean count per square,Hence the undiluted preparation has dye-excluding number density $2(40)/10^{-4}=800000{\mathrm{mL}}^{-1}$ and total number density $2(45)/10^{-4}=900000{\mathrm{mL}}^{-1}$. Its dye-excluding fraction is $160/180=8/9\approx 88.9\%$. If the well-mixed preparation contains $3\mathrm{mL}$, it contains an estimated $2400000$ dye-excluding biological cells. The chamber sample itself contains only $160$ counted dye-excluding biological cells.

The relation $c=D\bar{n}/v$ follows from counting material in a known volume, then undoing dilution. It assumes a representative mixed sample and correctly identified single biological cells. Clumps, inconsistent decisions at grid edges, wrong chamber depth and nonrepresentative sampling introduce measurement error. A fixed convention such as including top and left edges while excluding bottom and right edges assigns shared-edge centers to just one square. Adding ten times as much suspension to a correctly filled chamber does not make each square's calibrated volume ten times larger.

The trypan blue exclusion test reads cell membrane integrity through trypan blue exclusion. Prolonged dye exposure can itself increase staining.^[ref]

An unstained biological cell is dye-excluding at the time of this assay. That observation alone does not demonstrate future division, sustained metabolism or long-term renewal of a cell line. Conversely, a smaller unstained count can reflect both staining and loss of biological cells from the recovered preparation. Report the denominator and recovery procedure alongside the fraction.

HeLa is a human continuous cell line originating in cervical cancer biological cells collected from Henrietta Lacks in 1951. Its name combines the beginning of her first and last names.^[ref]

ATCC's CCL-2 HeLa preparation grows attached to a surface. Its product sheet specifies a cell culture medium supplemented with fetal bovine serum, at $37{}^{\circ }\mathrm{C}$ with $5\%$ carbon dioxide in air.^[ref] These are specified conditions for that preparation, rather than defining conditions for every HeLa derivative.

Continuing access to related biological cells makes HeLa useful for comparisons of cell state and responses to interventions. But a continuous cell line is a laboratory model: its history and environment matter, and observations in it do not alone establish responses in an intact organism. Cell counting measures abundance; a trypan blue exclusion test measures a particular membrane-associated readout; neither proves cell line identity.

A published teaching experiment used ATCC CCL-2 HeLa to make the distinction between a cell line and the condition of individual biological cells visible. Cultures were expanded in treated flasks and supplied as adherent dishes. A wash with phosphate-buffered saline preceded trypsin treatment to detach them; added fetal bovine serum stopped the digestion. The dispersed preparation was mixed with an equal volume of trypan blue and loaded into a counting chamber under an optical microscope.^[ref]

For the ethanol comparison, treatment groups and an untreated control were brought to the same final volume; the control received phosphate-buffered saline. Dye-excluding counts were compared across replicates. The paper reports representative student measurements in Figure 3A and documents the untreated control in its discussion. Its alternative temperature comparison used three replicates per condition and a $30\min$ exposure.^[ref]

The paper's disposable chamber had a different count-to-number density calibration from the hemocytometer above. Dye exclusion does not establish future division; recovery and mixing also affect counts. The constructed hemocytometer example is independent of those observations.^[ref]