Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!princeton!phoenix!pucc!6065833 From: 6065833@pucc.UUCP Newsgroups: sci.bio Subject: Re: Human asymmetry Message-ID: <2168@PUCC.PRINCETON.EDU> Date: Thu, 2-Apr-87 20:15:11 EST Article-I.D.: PUCC.2168 Posted: Thu Apr 2 20:15:11 1987 Date-Received: Sat, 4-Apr-87 18:22:04 EST References: <586@batcomputer.tn.cornell.edu> Reply-To: 6065833@PUCC.PRINCETON.EDU Distribution: sci Organization: Princeton University - Computing and Information Technology Lines: 75 Disclaimer: Author bears full responsibility for contents of this article In article <586@batcomputer.tn.cornell.edu>, plimpton@batcomputer.tn.cornell.edu (Steve Plimpton) writes: says initally there is a symmetrical single celled egg. >guess what I'm >really asking is, that as the fetus cells are busily dividing along, how do >ones on the left know not just that it's time to become heart cells, but >that they're not on the right? Well, first off, while it may seem logical that a cell be symmetrical, this isn't the case at all. Cells are highly asymmetrical, but in ways we don`t understand very well. Which is why there is no answer yet to just HOW cells differentiate. The study of this is a pretty hot topic right now. Gravity may play a role, but I personally suspect the cues may be entirely in the cells' interiors, in the cytoplasm. There needn't necessarily be any direct genetic control. This leads to the business about handedness of physical asymmetries. Left/right handedness is thought to be partly inherited, but whether this is genetic or due to early environment isn't known. Left-handed children are far more likely to have had left-handed mothers. This suggests several things (in descending likelyhood): 1)"Maternal factors"; some developmental characteristics are due to something in the cytoplasm of the initial egg, irrespective of genetic content. This determines the direction in which snail shells coil, for example (a left/right-handedness!). or 2)Genetic control on the sex chromosomes, or 3)Left-handed mothers have a slight tendency (and some other mothers as well) to treat their children as lefties. >Related questions: > >1. How soon is there left/right asymmetry in the growing fetus? I don't really know, but believe it's very soon (first 5 divisions), and maybe even before fertilization. >assumes a gene for right/left See my #3 above. > So how do the chemicals or > proteins or whatever this gene makes know how to travel to the right > brain vs. the left, either before birth or after? They don't. They are just released, and the cell at the receiving end has somehow (this is the mystery!) decided it will receive these chemicals and act on them. >3. Since a small % of people do have their heart on the right side, how did > they get turned around? Are they complete mirror images of "normal" > people? Is this genetic (Republican parents) or just random chance? I know from personal experience that there are all degrees of this inversion. I've met someone with the entire circulatory system turned around, but the dige stive system in the right place. This is apparently still far more rare than the complete inversion case. There is evidence that this is completely random. An aside: I understand that persons with inverted organs were thought to be inviable, and expected to die shortly after birth, or be sterile, etc. This is generally not the case (except perhaps medical care was withheld in anticipation of the child's death). Also, some women have uteruses which curve back instead of forward, and this was supposed to be a cause of infertility (but isn't). What enlightened times we live in. My sources? I'm currently finishing my 4th year in Biology at Princeton; the above information is from recent genetics and evolution (dare I say that word on the net?) courses, and my own investigations. Una Smith 6065833@PUCC I thought signature files were silly until I realized I usually forget to identify myself. No longer. Please forgive my apparent rudeness.