How many cell divisions make a baby?

17 May 2016

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Question

How many times must a fertilised ovum divide to form a normal term baby? Does this account for the placenta and apoptosis? How many cells are in a newborn? How fast is the cell division curve in each month?

Answer

Chris Smith got out a paper and pen to work through this one...

Chris - Well, actually, this comes down to pretty simple maths. Because, if we think about it, a single egg divides into two, and one cell becomes two, and then those cells each divide in half again. So two splits to make four and those four cells split themselves in half and they become eight. And then those eight cells split in half and you get 16 and then 32 and this grows exponentially; and that means we can write an equation to represent that growth where you say 2 to the power of n, which is the number of divisions, equals X cells. So, if we want to know what n is (how many divisions), we need to know how many cells. Well a baby contains, probably, a couple of trillion cells. So, therefore 2 to the power of n divisions must equal about 2 trillion.

So how do we find out what n is? Well you can do this using logarithms. So if we take the natural logarithm of both sides of the equation, you actually then get n x ln2 = ln(2 trillion). If we then divide both sides by the natural logarithm of 2 (ln 2) you get:

ln (2 x 1012) / ln2 and that's actually 28.34 divided by 0.693. That's about 41.

So, what that tells you is that a single cell only has to divide about 41 times to end up with 2 trillion cells that would make up a baby! It's not as many as you might think, is it?

Comments

Medical science says life starts at first breath. That would mean 1 egg and 1 sperm multiply to reach about 2 trillion cells. How does a dead cell do anything but decompose? Every dead cell I know about, all life activity stops and does not start again. We are to believe, 2 alive cells, one egg one sperm, become dead and multiply to 2 trillion dead cells. A fetus with first breath, 2 trillion dead cells magically become alive. Medical science is wrong.

Paul Florance assumes that somehow a fertilized egg dies and then comes back to life to grow and become a baby. Perhaps he's conflating a religious resurrection story with embryogenesis. Or perhaps a few of his developing neuronal cells died. You might consider medical science wrong, but it's pretty effective.

the computation is absolute non-sense. Cells divide not uniformly, but some delay for a very long periods, and some stop their division at all.
Also, a large amount die in the process (it is normal).
When process is going it should be quicker than once per day. And I would like to know the number of generations (in its maximum numbers) till a baby is born.

The gestation period is about 270 days. This means cells need to divide every 6.5 days, on average. Sounds slow.

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