Reading this thread on “how plant vines know where the next leaf should go,” http://boards.straightdope.com/sdmb/showthread.php?t=577243 , brought to mind a question I have wondered about from time to time. How does a human embyro/fetus in the uterus come to orient itself. I imagine that with all the cell divsions, one cell becoming two, then four, and so on, just turns the embyro into a roughly ball-shaped thing kind of like a bunch of soap bubbles joined together. Obviously, over time, this turns into something that looks like a person, and I am wondering how that happens. That is, can we tell what part of this collection of bubbles is going to become the head, the feet, the arms, etc.? Is it pretty much random (and if so, how), or is some sort of orientation process inherent in the ways that the cells divide to begin with?
Thanks for your answers to these questions. The Dope has already put to rest many of these idle questions floating around in my head, sometimes for years.
It’s an interesting subject. Basically, cells will begin expressing growth factors that set up polarity - one factor will define front vs back; another will define right vs left, etc. One concrete example is involved in situs inversus, the condition in which all of a person’s organs are swapped right to left and left to right. It was discovered fairly recently that this is caused by defective cilia. At one point early in development, hair-like cilia begin beating and set up a current. This current sweeps a specific growth factor that’s being expressed in a specific direction. The factor gets concentrated on a specific clump of cells, and that’s how right and left are defined at that particular stage. When the cilia are defective, the process breaks down, and right vs left is determined pretty much randomly.
I was just wondering this last night, and meant to start a thread.
Up to a point, all the cells are identical, right? And then something happens, and the next generation are no longer identical. Not only that, but they’re in a specific order. So am I correct in thinking that whatever causes the cells to specialize is normally linked to their placement in the body? So one question is: how common is it that they are not in the order they’re supposed to be . . . and this interferes with viability, so the embryo aborts?
You aren’t going to get a complete answer on this because it is past the current boundary of known science. It is a great question though and much more will be known in upcoming decades. A good understanding of of it could mean anything from being able to regenerate organs for individuals to a greatly extended lifespan.
I don’t know how much you know about it already but the basics of stem cell research is a good place to start if you just want some basics.
I hate to be the guy that comes in and just posts another wiki link, but I couldn’t find a better primer on the topic. Here’s their article on embryogenesis, which discusses most of the major events and what we know about them (with plenty of links for further exploration.
Developmental biology, though, is a *huge *topic. We know a lot of amazing details on certain parts of development, but still we’ve only scratched the surface and scientists will be busy for many decades (or centuries!) trying to figure it all out. Most undergrad biology programs offer several courses on the topic, and that still just covers the basics.
I’m trying to figure out a suggestion for you that’s between “poke around wikipedia” and “go check out a big stack of advanced textbooks”… There aren’t a whole lot of books on the topic that are written for a lay audience, but here’s one you might want to check out: Coming to Life: How Genes Drive Development.
Again, I wish I could tell you more, but I’ve only encountered the topic tangentially, so I doubt I could give a good coherent explanation of what’s going on.
I don’t think it’s that big of a mystery. As noted in my previous post, you have a whole category of genes that exist only (primarily) to guide development. If you actually look at the link I provided, there is a diagram of a common fly showing which sections are governed by which homeodomains.
Part of the function of some of the genes switched on or off is to create chemical gradients that act as sign posts for new cells. So if protein x is more prevelant than protein y, a partially differentiated cell may know to become epithelial tissue rather than part of the basement membrane.
It is certainly true that we don’t understand everything, but I think that we have a good enough overview to handle this sort of question adequately.
Thanks for all the links. They’ve been very helpful, although the technical language is getting to me now and again. Before all this, I had thought that the cell just divides over and over again, more or less willy-nilly, and that, somehow, from all that, you somehow get a person. If I have learned anything, it’s that the process of cell division and whatnot is directed from the very beginning.
There is something I don’t quite understand, though. At first, it seems that the cell creates a sphere of many cells, with a whole bigger bunch of cells which then becomes the point of attachment to the uterine wall. Then it seems, if I’m not mistaken, that at some point the zygote kind of folds over itself, eventually making a sort of tube, and then, if I’m still not mistaken, this tube becomes the digestive tract, which seems to me to be an adequate framework for building the rest of the body.
If this is true, then I guess if I had some idea about how one of the ends became the head, this question would be answered for me. Is it the end of the tube that’s (say) closest to the uterine wall, farthest from it, or is it more or less random? Is this the result of these homeodomains? (And just out of curiosity, do these areas have to be exact, or can they develop in a fairly wide area, moving into final position later?) Or is it that the head develops in the initial big mass of cells, and the rest develops from there.
Thanks again, and if you ever need any help with moral philosophy, I will answer your questions as quickly and as well as I can.
I don’t know enough to answer your questions, but I do think you are right on about the alimentary canal being the focal point of development. I think it is a feature that has been conserved by evolution for all forms of animal life - certainly vertebrates and I believe invertebrates as well.
This is a topic that is tangential, but I believe related, to the thread I started earlier this week dealing with “phantom limbs”.
In my researching this topic, I was told to check out the author Rupert Sheldrake.
In Googling him, I have found that he seems to address the specific question you are asking.
He has proposed a theory he calls “Morphic Resonance” which, he says, is responsible for cells differentiating into different cell types, and subsequently into different organs.
Since I am now waiting for his book to arrive from Amazon, I don’t know any more than that; however, if you are interested, his book is entitled: “Morphic Resonance: the nature of formative causation.”
Long before the primitive streak, etc., the anterior-posterior axis is set up when the winning sperm hits the egg and starts causing changes in the cytoskeleton. The sperm end becomes the posterior end, and the opposite end becomes the anterior end.
All of the above is interesting but, so far, the discussion has been a qualitative description of observed processes. The answers do not directly answer the question in the original post.
So let me paraphrase what I think the original poster is asking (and which, incidentally, I would also like to know):
Given two cells “A” and “B”, one moment after the first cell division; or even later, for that matter:
What makes cell “A” decide it will be a muscle cell, and cell “B” decide it will be a liver cell.
If the two cells are adjacent to each other, and “developing cells affect the development of nearby cells”, how does cell “A” tell cell “B” that “A” will remain a muscle cell, and that cell “B” should be a liver cell?
What happens if cell “B” disagrees, and wants to be muscle cell? How do the cells reach agreement; and how is that agreement communicated to other cells?
What happens if cell “B” changes its mind and decides it wants to be a brain cell? How would it change its’ mind anyway?
Those are all good questions. While Sheldrake’s idea of psychic cell communication is interesting, I prefer my own theory which revolves around magical unicorn farts.
Part of the answer is chemical signaling. Until you have, I think, 8 embryonic cells, they are all interchangeable. After that they begin to differentiate. One way that is done is that the cells start to secret different chemicals, proteins, peptides and probably other substances. As each cell begins to specialize, different substances are produced. These create chemical gradients that vary depending on location. When the cells divide, they sense the varying concentrations. Depending on the composition of the mix, they will become further specialized.
So if a cell senses one mix it may become an epithelial cell. A slightly different mix causes it become part of an adjacent structure like the basement membrane.
If that sounds far fetched, it’s not. Even bacteria are able do something called “quorum sensing”. This is important in determining, for example, when a colony of E. Coli will start to produce toxins. As long as the concentration of the quorum sensing chemical is low, indicating that there are relatively few total bacteria, the cells will not produce any toxins. But once the chemical reaches a certain level, the entire colony will act as one entity and begin.
I’m sure there are other mechanisms as well - both known and unknown - so this is just one example.
OK, but why do cells “begin to differentiate” in the first place?
In addition, given that every living organism is a highly developed and integrated system, the development of such would require a very high degree of precision and timing.
In contrast, the “chemical signaling” explanation sounds somewhat ad hoc and random.
For the sake of argument, let’s say chemical signaling triggers the development of internal organs.
So, signal “A” triggers the development of a liver. How does signal “B” trigger the development of kidneys; particularly since there are two of them? Why stop at two??
And what about the connecting plumbing??? How does the system know to connect this end of a tube to kidney “A”, and the other end to the bladder? Assuming that the bladder has been formed in the first place.
Clearly, there is a lot of complexity here in command and control that would require more than just the random release of chemical triggers into a great chemical soup.
DNA and chemistry are doing it. DNA is the blueprint; chemical concentrations, lifespans of chemicals, are what mark the order in which the blueprint is read. There is nothing random about the release of the chemicals involved in embrionic development.