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  4. How many times must a fertilised egg divide to form a normal term baby?
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How many times must a fertilised egg divide to form a normal term baby?

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Offline thedoc (OP)

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How many times must a fertilised egg divide to form a normal term baby?
« on: 12/05/2016 19:50:02 »
Donald piniach asked the Naked Scientists:
   How many times must a fertilized 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?
What do you think?
« Last Edit: 12/05/2016 19:50:02 by _system »
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Online evan_au

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Re: How many times must a fertilised egg divide to form a normal term baby?
« Reply #1 on: 14/05/2016 11:31:59 »
Since no-one has tried to answer this question, I'll do a rough back-of-the-envelope estimate...
Quote from: Donald Piniach
How many cells are in a newborn?
One estimate puts the number of human cells in a 70kg male at 37 trillion cells, or 37x1012.
See Dr Karl's summary: http://www.abc.net.au/science/articles/2015/11/10/4346790.htm

Assuming that individual adult cells are roughly similar mass to the cells in a newborn, the number of cells in a 4kg newborn is this number times 4/70, or 2x1012 cells.

This ignores the huge number of commensual bacteria which live in adults (and it seems, a smaller number which have already colonized a newborn).
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How many times must a fertilized ovum divide to form a normal term baby?
The minimum number of divisions to reach this number is LOG2(2x1012) = LOG10(2x1012)/LOG10(2) ≈ 41 divisions.

This does not account for apoptosis, which will require additional divisions to produce the cells which subsequently die.

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Does this account for the placenta?
Part of the placenta forms from the fertilized egg, and part grows from the mother's tissue. So it partly includes the placenta. 

Quote from: https://en.wikipedia.org/wiki/Placenta
The placenta functions as a fetomaternal organ with two components: the fetal placenta (Chorion frondosum), which develops from the same blastocyst that forms the fetus, and the maternal placenta (Decidua basalis), which develops from the maternal uterine tissue

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How fast is the cell division curve in each month?
41 divisions in 9 months averages out at 0.9 per week (or a bit faster, if you account for apotosis).

A blastocyst at 6 days after fertilization has 200-300 cells; this represents about 8 divisions.
So in the first week, divisions happen more than once per day.
See: https://en.wikipedia.org/wiki/Blastocyst
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Online evan_au

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Re: How many times must a fertilised egg divide to form a normal term baby?
« Reply #2 on: 21/05/2016 02:38:44 »
Quote from: evan_au
The minimum number of divisions to reach this number is LOG2(2x1012) = LOG10(2x1012)/LOG10(2) ≈ 41 divisions.
The podcast used a slightly different method for arriving at the same answer.

Most people don't have access to a calculator that can do LOG2(x)
  • Most scientific calculators can do LOG10(x)
  • Many scientific calculators can do the "natural logarithm" ln=LOGe(x), where e=2.718281828....
  • You can simulate it in a spreadsheet by calculating 2^n=x, and try different values of n until you hit the target number for x.
  • ...or even by using the "Goal Seek" function in a spreadsheet
  • ...or (even easier) just type into Google: "What is log2(2e12)?", and the answer is "40.8631371386" ≈ 41 divisions.

You can convert logarithms in one base to logarithms in another base.  The method described in the podcast was:
LOG2(2x1012) = LOGe(2x1012)/LOGe(2)
= ln(2x1012)/ln(2) ≈ 41 divisions.
« Last Edit: 21/05/2016 03:07:19 by evan_au »
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