View Full Version : A question about cells
12-03-2011, 01:16 AM
I had asked this a long time ago, but can't find the thread now.
We all begin as an impregnated ovum. Then mitosis begins, and there are 2 cells, then 4, then 8, etc. All the cells are identical, up to a certain point at which they become more specialized cells. When does this happen, i.e. how many are there in the last division in which they're identical? And in the next stage, how do the individual cells "know" how to change into a different type of cell, and which type they will become?
It can't be something within each cell, or they'd all become the same type. And it can't be something external, or it would affect them randomly. Or am I missing something?
12-03-2011, 01:29 AM
I believe it is done by chemicals. The cells give off various hormones and depending of the location of the cell in the group, it receives different levels of the hormones which trip different parts of the genome that influence that cell's own production of hormones. At least, that's what I remember from reading The Red Queen (http://en.wikipedia.org/wiki/The_Red_Queen:_Sex_and_the_Evolution_of_Human_Nature).
The wiki on cellular differentiation (http://en.wikipedia.org/wiki/Cellular_differentiation) may help you.
12-03-2011, 07:29 AM
Very early on, cells begin secreting various chemicals that essentially do the job of saying "I'm the front!" "I'm the back!" etc. Cilia on the cells set up a current that swishes these chemicals around, causing some of them to accumulate in one region. That region then becomes different from the other regions. That's the very very start of the whole process. That's also why the disease "situs inversus", where left and right are switched, is caused by defective cilia. From there, it gets really complicated, with lots and lots of signaling molecules setting up gradients of various types, which then control patterns of gene expression.
12-03-2011, 03:11 PM
Well to start out with, no, they are never exactly identical. The egg, even before fertilzation, has different amounts of different mRNA and proteins spread out throughout its cytoplasm. The physiological term to express this is to state that the unfertilized ovum has "polarity (http://molehr.oxfordjournals.org/content/3/10/863.full.pdf)". As the fertilized ovum subdivides each new cell is partitioned with different amounts of those maternal mRNAs and proteins, which began to further reorganize within the cell upon fertilization even before the first divisions take place.
The paternal pronucleus seems to impose or collaborate in
the formation of polar axes in mammalian eggs. Chromatin
located near the sperm tail becomes polarized facing the oocyte
interior as the differentiating paternal pronucleus rotates. This
occurs even while the sperm head lies distant from the
maternal pronucleus (Van Blerkom et al., 1995). Early maternal
pronuclei remain relatively ﬁxed in pericortical ooplasm, as
their juxtaposition with the paternal pronucleus is co-ordinated
in space and time. Chromatin in maternal pronuclei also
polarizes, and rotates to face the paternal pronucleus.
As that article continues in explaining, the maternal mRNA and proteins, so distributed, controls the initial set of further differentiation between cells created by the next several sets of subsequent cleavage:A period of tight maternal control over transcription and
differentiation after fertilization characterizes virtually all
vertebrates and invertebrates. Fundamentally similar from ﬂies
to mammals ... Maternal mRNA stores are translated over the ﬁrst few
cleavage cycles in mammalian embryos. ...
That article will give you more detail than you likely desire but by the 8 cell stage individual cells are apparently already allocated to being part of the trophoblast (which becomes the placenta and supporting tissues) or the inner cell mass, or "ICM" (which becomes the embryo-fetus-baby). By the 4th to 5th cleavage, with about 60 or so cells, the fertilized egg changes from a mass into a hollow sphere and expands. The oct-4 and hox families of genes/protein products then seem to be what drives further differentiation of the ICM including movements of the cells.
12-04-2011, 01:25 AM
For a wonderful overview of the development of complex organisms, I highly recommend "Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom" by Sean Carroll
When I was a child, cellular differentiation was a complete mystery. The work described in layman's language in this book shows that we are well on the way to understanding how a single cell can develop into a complex organism. It also provides tremendous insight into evolution.
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