[gary_lankford (OP)]

Can the center of zygotic cell division be identified with a specific organ in the embryo or fetus?  If, as I am guessing, that organ is the brain, can the "center" be identified with a specific structure?

Gary Lankford

[chris]

Hi Gary

I'm not really sure whether I'm answering your question or not here, and I may well have the wrong end of the stick, but I'll start simple anyway; just ignore anything patronising.

An embryo arises through the fusion of a sperm and egg. Each, carries half the normal number of chromosomes so that when they mix their genetic material the correct chromosome number (46 in humans) is restored.

Each normal mature adult cell contains 2 copies of each of our chromosomes, one from the father (inherited via the sperm), the other from the mother (from the egg) so there are 2 chromosome number 1s, two chromosome number 2s and so on. The exception is the x and Y chromosomes of which there is only one copy each.

Immediately that fertilisation takes place the egg / sperm combo becomes known as a diploid zygote and it begins to divide. One cell becomes two, these two then split yielding four, these four divide into eight and so on. Eventually a ball of cells is produced comprising an outer cell layer (that will give rise to the placenta and the membranes surrounding the developing foetus) and an 'inner cell mass' that begins to produce the embryo proper.

As such there is no single centre of division. At early stages of embryogenesis each cell retains the ability to divide in two producing two copies of itself. As the embryo becomes more advanced some of these cells begin to become more specialised and turn in specific tissue types, but they keep on dividing, in fact until the day you die.

Here's an article about fertilisation and embryogenesis that goes into slightly more detail.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[gary_lankford (OP)]

Duh!
Thank you Chris for sweeping the cobwebs out of my brain.