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.  And if you want to see more from us, there’s  always a subscribe button somewhere nearby.