(Narrator) Can a single cell be sophisticated enough
to be capable of making a complex decision?
Over a century ago, the American Zoologist Herbert Spencer Jennings
conducted an experiment suggesting that this might indeed be the case.
(Gunawardena) The hierarchy of behaviors which Jennings
observed I felt was very important
because it indicated the organism was in some sense, quote unquote, changing its mind as
to what to do.
And very few other instances where we have such clear indication that
individual cells are capable of that kind of change of mind.
(Narrator) Jennings was studying Stentor roeselli, a
member of a group of single celled organisms called ciliates.
Trumpet shaped and unusually large for a single cell, Stentor like to stick to debris via
a holdfast,
and have little hairs called cilia, which they use to swim and to create a vortex around
their oral cavities to feed.
Jennings experiment was fairly straightforward, but the behaviors he observed were not.
(Gunawardena) He used a fine capillary pipette to squirt
Carmine dye at their oral cavity.
And that was the stimulation he used to annoy them.
What Jennings observed was a sequence
in which the organisms first bent away from this irritation.
They also altered the direction of their ciliary rotation,
so that instead of ingesting the particles they would expel them.
And then when that wouldn’t work they contracted sharply down onto their holdfast.
And finally, they would get completely fed up, pull up their holdfast and swim off.
To me what was fascinating about it is the suggestion that individual cells,
if you know how to ask them right questions, or annoy them in the right way,
will reveal that they are more sophisticated and autonomous than we realize.
(Narrator) However, subsequent efforts to replicate Jennings
experiments failed,
and his findings were discredited and largely forgotten to science.
(Gunawardena) It kept bugging me, so eventually I tracked
down the paper and I discovered
that the experiments had been undertaken on a different kind of ciliate.
(Narrator) So, with then graduate student Joseph Dexter
and then postdoctoral fellow Sudhakaran Prabakaran,
the three Harvard Medical School systems biologists set out to replicate Jenning’s original
experiment.
Over the course of several years, they located stentor roeseli test subjects
and set up an experimental apparatus to repeatedly deliver an irritant,
in this case microplastic beads, and then they recorded the organisms’ responses.
(Gunawardena) What we saw was every single one of the avoidance
behaviors,
but the behavior we found was actually much more variable than Jennings had reported.
And Most difficult part I think of the analysis was finding how to look at this from a statistical
point of view.
(Narrator) Where observation failed, math triumphed.
(Gunawardena) What came out of the statistical analyses
was exactly what Jennings had claimed.
There is a hierarchy of behaviors.
So If you ask whether alteration and bending happen before contraction,
then the statistical evidence across this across many organisms is extremely compelling.
Stentor roseli in this case is deciding whether to alter its cilia or bend, or contract,
or to keep on contracting or detach.
And that gives it a quality of autonomous behavior which is in fact Jennings was so
interested in himself.
(Narrator) So why would scientists at a medical school
be interested in these unusual organisms?
(Gunawardena) We tend to think of a process of development,
of say a human from a single fertilized egg,
as being programed, which tends to give the idea that the cells sort of know what to do.
What Jennings experiments to me suggest is that part of that program might
actually be giving cells a lot more autonomy to make up minds about what to do
in the context they find themselves in.
And I think that’s becoming clearer from the evidence we have about the way cancer
grows,
and the way in which say the immune system does its work, but I think it is also relevant
to how an organism constructs itself from a single cell.
I think this alerts us to the possibility
of much more complex behaviors at the level of the single cell.