We’re starting this episode out with a question that we’re never going to have a good answer for:
how many cells do animals have? I can already hear some of you scoffing some version of
“why is Hank Green asking me such an obviously un-answerable question
while a rotifer chills on my screen?” And you’re not wrong to be skeptical.
There are a lot of animals in the world, and a lot of cells inside those animals.
How could we ever hope to count all of the cells in each of those animals? And how could we even begin
to assume that the amount of cells in one individual is going to be the same
for all the other individuals? And what would even be the point
of treating every animal like a jar full of jellybeans that we need to guess the number of?
Are you waiting for the twist? Cause you know there is going to be a twist.
Maybe you even know what the twist is already. Maybe you remember our video on rotifers all
the way back in 2019, where we said this:
rotifers are eutelic, which means that from
the moment they are born, they will have a fixed number of cells in their bodies for the rest of
their life. For most rotifers, that number is around 1000 cells.
And for the rotifer to grow,
the cells themselves, which cannot divide, have to get bigger.
Think about that for a second. We know how many cells are inside of a rotifer, which means that
at some point in history, someone sat down and counted the cells in a rotifer.
And they didn’t just stop at one rotifer: they kept counting,
going from rotifer to rotifer to rotifer until a simple truth appeared. All of these rotifers
had the same number of cells as each other. So the rotifer is one of the few animals that
we can treat like a jar full of countable beans, and it’s because of this property called eutely.
The number of cells in the rotifer has been set, which means that if you’re looking
at a mature rotifer, you know that you’re looking at about 1000 cells in action. It may be a little
more or a little less depending on the species, but within those species, all the individuals
will be the same in this one very countable way. We had hoped at this point to give you a fun
exercise in historical imagination. We’d ask something like, “What kind of person
would have the patience to count the cells in a rotifer?” And then we’d give you the
name of the scientist who did the counting, and maybe even quote their writings on the subject.
However, science has many dark pasts that weave through history,
binding together our understanding of even the smallest creatures with the political
and social forces that shape humanity.
The word “eutely” was brought into the biological
vernacular because of a zoologist named Erich Martini, who in the beginning of the 20th century,
counted the cells in the rotifer species Hydatina senta and established the female individual
had 958 somatic cells, meaning all of the cells apart from the individual’s eggs.
However, we have to add that in addition to Martini’s contributions to our understanding
of eutely, his legacy also involves a debate between historians over whether
he engineered a Nazi plot to spread malaria among Italian citizens in 1943, which would
be considered an act of biological warfare. Martini was not the first nor was he the
last person to be associated with both important scientific discovery and Nazism.
And while we could explore the question of how many cells are inside of animals without
making reference to him, we think it’s important to not gloss over the ugliness
that often accompanies the history of science. Other scientists were also fascinated by eutely.
In 1932, the scientist Harvey J. Van Cleave published his paper “Eutely or
Cell Constancy in its Relation to Body Size,” confirming the cell counts that Martini had
made in his rotifer studies while also describing the evidence researchers had found for eutely in
animals like gastrotrichs and nematodes. And other scientists would add tardigrades to the list.
Every animal begins its life as a single cell that is full of destiny. It divides and divides
and divides, sending cells tumbling down a path toward their fate as they take on specific shapes
and functions that have been preordained through a combination of chemical and genetic cues.
In most animal bodies, including our own, many of those cells continue to divide,
whether that’s to help us grow or heal or adapt. So while scientists have estimated that humans
have around 37.2 trillion cells, their estimate is, by necessity, based on very restrictive
assumptions about what a human body looks like. Ultimately, most of us will exist somewhere in
a gray area of variation around that number. Some of us will have more, some of us will have less.
Whether there’s a deeper pattern underlying the disparities is hard to pin down, except for the
fact that the heart of nature is diversity. But not for the rotifer. Sure, it started with an
egg that divided and made more cells that followed a set path to their final fate,
eventually growing into the mature wheeled animal we love to watch. But when the rotifer
reaches maturity, it no longer makes any new cells. If it needs to grow, then the cells
will grow, but they will not make new cells. And perhaps not needing to divide and make
new cells creates a sort of peaceful constancy that means the rotifer doesn’t need to waste
energy on making new DNA and duplicating its cells. But it also means that if the
rotifer gets hurt, it doesn’t have the tools to make new cells to heal itself.
We don’t really have an idea of why rotifers or any other animal would become eutelic.
There’s no obvious phylogenetic relationship that exists between eutelic animals that we know of.
And at times, identifying eutely in one species has led us to make incorrect assumptions
about their relatives. That’s because eutely itself
is a diverse phenomenon. Some of those animals that we mentioned were eutelic?
It turns out they are…but they also aren’t. Tardigrades, for example, have storage cells
that help distribute energy but that also sometimes divide to make more storage cells.
And some gastrotrichs have even been documented healing themselves after an injury.
These may be cases of partial eutely, where some parts of the animal do maintain some kind
of constancy while others do not. Or it may be that even within a phylum, the occurrence
of eutely has no obvious logic to it—maybe some gastrotrichs just are eutelic, and some aren’t.
And as we said, those kinds of cases just add to the confusion of eutely. The nematode
Caenorhabditis elegans has been such a popular model organism in part because we know that the
adult C. elegans hermaphrodite has a body built out of 959 somatic cells. And knowing the path
from egg to embryo to juvenile to adult through the story of those 959 somatic cells
has been a tremendous gift to science. It’s a blueprint of the worm’s development,
a pattern that scientists can prove and disrupt to uncover other hidden motifs.
But it turns out that C. elegans’ eutely is not the shared trait it was once thought to be
among nematodes. Free-living marine nematodes were found to not have it And other species
have been found whose skin is dotted with a different number of nuclei, revealing variation
where previous scientists hadn’t expected any. And so even in these unusual cases where
it seems like maybe we can get closer to quantifying the number of cells in an animal,
the simple act of counting paints a much more complicated picture of the world.
Nature loves to change, even if that is a change towards a bizarre case, of constancy.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
And thank you to all of the people whose names are on the screen right now.
They are our patrons. They, in a way,
are like the individual cells that make of the body of Journey to the Microcosmos’s funding.
Look, they’re a bunch of people who decided they want content like this to exist on the internet.
And in order to make sure it happens they support us at Patreon.com/JounreyToMicro.
Which you can find a link to in the description. Check it out, and see
if you would like to join them. If you want to see more from our
master of microscopes, James Weiss, check out Jam and Germs on Instagram.
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