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Author Topic: How Do Cells Know Where To Position Themeselves To Make An Organ ?  (Read 6728 times)

Offline neilep

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Dearest Cellolgosts,

As a sheepy I am of course made up from lots of cells...quite a lot actually....about 137 and that is loads !

look here are some cells:


Some Cells Being Cells Earlier


Nice eh ?...being delivered next Tuesday to form the lining of my tummy.

Cells are like..well klevur...cos they know exactly where they should go and how to orientate themselves in such a way that they make things.

How does a cell know where to position itself to make a lung or a heart ?  How do lots of cells know to not fight for the same place or location ?


Take my hand for instance !


How do the cells know to make each finger and that one particular cell needs to be placed at the top of a finger and the other next in line...In this case they have also sprouted a terrible disfigurement too !

Ewe see, I just do not know..I want to know !...I need to know !


help me know !


Hugs & shmishes


mwah mwah mnwah !!




Neil
I Luff Tainted Luff By Soft Cell
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« Last Edit: 30/08/2009 19:37:23 by neilep »


 

Offline RD

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You'll be wantin' morphogenesis and/or Embryogenesis.

Chemical gradients are (in part) used to determine what grows where.
« Last Edit: 30/08/2009 19:59:13 by RD »
 

Offline Andrew K Fletcher

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Gravity is a good start Neil.

Take the kidneys for example. Their location is conveniently placed to detoxify the blood of dense solutes. Then the bladder, coincidence that it is next inline downstream? And of course the exit point for urine just happens to be right at the end of the tubes that excrete the urine on demand. The Heart conveniently placed right between the vascular and arterial networks. As I have stated before, the density changes from the respiratory tract and from the lymphatic system provide a regulated boost for the circulation by releasing pulses of solutes into the blood stream, where they flow according to the direction of gravity down towards the excretion point mentioned above.

Andrew K Fletcher
 

Offline Nizzle

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Gravity is a good start Neil.

Take the kidneys for example. Their location is conveniently placed to detoxify the blood of dense solutes. Then the bladder, coincidence that it is next inline downstream? And of course the exit point for urine just happens to be right at the end of the tubes that excrete the urine on demand. The Heart conveniently placed right between the vascular and arterial networks. As I have stated before, the density changes from the respiratory tract and from the lymphatic system provide a regulated boost for the circulation by releasing pulses of solutes into the blood stream, where they flow according to the direction of gravity down towards the excretion point mentioned above.

Andrew K Fletcher


But.. but.... This flaws with animals walking on 4 legs..
 

Offline BenV

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But.. but.... This flaws with animals walking on 4 legs..

That's exactly what I was thinking.  I'm no developmental biologist or even an anatomist, but almost all animals that walk on 4 legs have their kidneys above the heart, in a strikingly similar place to ours when compared to the spine.

Andrew, any comment?
 

lyner

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I hope we all appreciate the gravity of this situation.
 ???
 

Offline Andrew K Fletcher

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I hope we all appreciate the gravity of this situation.
 ???

Ever considered that the bladder requires to be below the level of the kidneys in order to make sure that urine and it's toxins are not refluxed back into the blood stream? And that a dogs midrift is rather narrow so in order for there to be a gravity assisted flow from the kidneys there has to be a drop from production to excretion.

 

Offline BenV

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Ever considered that the bladder requires to be below the level of the kidneys in order to make sure that urine and it's toxins are not refluxed back into the blood stream? And that a dogs midrift is rather narrow so in order for there to be a gravity assisted flow from the kidneys there has to be a drop from production to excretion.

Fair point.  So it's only this bit that's flawed...


Take the kidneys for example. Their location is conveniently placed to detoxify the blood of dense solutes. Then the bladder, coincidence that it is next inline downstream? And of course the exit point for urine just happens to be right at the end of the tubes that excrete the urine on demand. ... As I have stated before, the density changes from the respiratory tract and from the lymphatic system provide a regulated boost for the circulation by releasing pulses of solutes into the blood stream, where they flow according to the direction of gravity down towards the excretion point mentioned above.

Andrew K Fletcher

 

Offline Andrew K Fletcher

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Not really a worthy point Ben. Salts dissolved in fluid will cause a flow whether you accept it or not. The solutes still arrive at the kidneys and still flow out into the urine and by doing so drag on all of the other fluid molecules and therefore still assist circulation.

This is not the first time 4 legged animals and indeed fish, bats and Sloth have been brought up and quite rightly so.

In fact, a long discussion with a veterinary surgeon who was truly fascinated resulted in him stating that the spinal cord of a dog is tilted in order to assist the distribution of markers / die introduced into the cord. He added, we literally see the flow you are talking about as the fluids migrate due to the posture change.

Again, I have mentioned that 4 legged animals often use the direction of gravity to alter their body temperature, cattle and sheep for example sleep facing uphill when there is a raised area in the field. A dog will drag bedding under itís chest to raise it and ironically when a dog is asleep and I sincerely hope you dog owners out there will test this, the heart rate and respiration rate decrease markedly over that of the same dog sleeping horizontal.

Interestingly enough the reduction in heart and respiration rate is identical to that in sleeping adults and children on an incline compared to sleeping flat.

A beanbag might therefore be a better sleeping aid than a flat dog bed.
 

Offline John Chapman

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This is very interesting but what you are discussing are the merits of anatomical form and function, which was not the original question. How does a stem cell in any one location know what type of tissue it should grow into? Or is it a matter of a cell of particular tissue type knowing the precise location to position itself. Serous membranes such as mesentery tissue are in the form of a thin sheet of cells, a bit like cellophane(no pun intended). How do the individual cells know how to orientate themselves like this?
 
 

Offline BenV

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Good point, we should keep discussion relevant.  It's been said before that chemical gradients play a role - should we assume that chemical concentration activates/switches off certain genetic pathways?
 

Offline Nizzle

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Well, let's start from the very beginning:

When a sperm cell and an egg fuse together, we call it a zygote, which is still a round cell with no orientation whatsoever. After a few rounds of dividing, the first differentiation we see is one between the outer edge of the clump of cells, which receive signals in the form of hormones or other molecules, like mRNA, coming from the mother, and the inner clump of cells, which do not receive these signals (initially). The outer edge of the cells will become the placenta, and the embryo is entirely made up of the clump of inner cells.

The second differentiation we see is the development of the anterior-posterior axis, or in layman's terms, the round clump of cells becomes an elongated bar of cells. This differentiation is triggered under the influence of mRNA from the mother, that penetrates the embryonic cells and acts as activators that "switch on" the transcription of gap genes and pair-rule genes. Since the mRNA only enters the embryonic clump from one side (which will later become the umbilical cord), there is a difference in gradient of mRNA throughout the embryonic clump, which (partially) determine the amount of gap and pair-rule genes will be activated. When these genes are translated into proteins, the gap and pair-rule proteins themselves act as transcription activators for the Hox-genes. The Hox-proteins are again transcription activators for the Homeobox genes, but they also divide the embryo in segments. Embryonic segments will later give rise to all sorts of differentiated tissue. For example. The most anterior positioned segment will become the head, the second and third will become the throat structures, the 4th will become the upper torso + arms, etc etc.
Further differentiation within the segments occurs gradually, for example: all hormone producing glands in our body are derived from a single 'hormone producing ancestor'. In other words, the pancreas, hypofyse, hypothalamus, adrenal glands all had a semi-hormone producing ancestor, just like our brain, sensory nerves, motor nerves all had a semi-nerve cell ancestor...
The topology of the cells in the embryo determine what organ they will evolve into, which was proven by engineering chimaeric embryos of chicken and turkey. In these experiments, investigators took a chicken embryo that was in a stage as early as possible while still producing a life chick, and they replaced some cells by turkey embryonic cells. Later they dissected the chick and saw that some had a turkey liver, other had turkey kidneys, still other had a turkey intestine etc etc.

To make a long story short:
Q: How Do Cells Know Where To Position Themeselves To Make An Organ ?
A: By different gradients of transcription factors that activate gene transcription, starting with the first transcription factors being maternal, that activate the first embryonic transcription factors.

For a better understanding, i would like to add to the explanation above that there are very few different maternal mRNA strands penetrating the embryo who activate only a very few different gap and pair-rule genes. Then there are more different hox-genes than gap/pair-rule genes, and in term, there are more different homeobox genes than hox genes. Further down the road, there will be an ever increasing number of different transcription factors until we arrive at a completely differentiated cell (nerve, skin, liver, ...).
 

Offline Nizzle

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Ofcourse this is only a partially satisfying answer, but I can give more information if more specific questions are asked ;)
 

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