development
purepedantry | January 9, 2007Evidence has been found for the stem cell theory of cancer development. For those of you not aware of this theory, it holds that cancers originate from cells that have inadequately differentiated from their stem cell origins. This would contrast with more standard theories of cancer development that have cells de-differentiating from a more mature state.
Evidence for this theory has been fleeting, but researchers at USC may be on to something. Laird and colleagues, publishing in Nature Genetics, look at the DNA methylation state of certain genes in colorectal tumors in comparison to normal…
pharyngula | January 6, 2007Everyone can stop now, my brain is full. Seriously, this is a painful meeting: my usual strategy at science meetings is to be picky and see just a few talks in a few sessions, to avoid burnout…but at this one, I go to one session and sit through the whole thing, and at the breaks I look at the program and moan over the concurrent sessions I have to be missing. I have to come to SICB more often, that's for sure.
I do have one major complaint, though: PowerPoint abuse. The evolution of slides has continued apace since my graduate school days, when one slide was one photograph, developed in a…
pharyngula | January 4, 2007Here's what I heard this morning. Wonderful stuff, all of it, and I'm having a grand time. This is a quick summary, and now I have to rush back to the meeting for more.
S. Kuratani: Craniofacial evolution from a developmental perspective. This was a lamprey and hagfish talk, comparing them to vertebrates. Hox gene expression patterns in lamprey, which assign anterior-posterior positional information, are very similar to those in vertebrates, but there is no temporal colinearity—timing is all over the place. There is no apparent dorsal-ventral patterning of Dlx gene expresion. They've…
pharyngula | December 23, 2006Almost ten years ago, there was a spectacular fossil discovery in China: microfossils, tiny organisms preserved by phosphatization, that revealed amazing levels of fine detail. These specimens were identified as early animal embryos on the basis of a number of properties.
The cells were dimpled and shaped by adjoining cells, suggesting a flexible membrane—not a cell wall. This rules out algae, fungi, and plants.
The number of cells within each specimen was usually a power of 2. This is something we typically see in cleaving embryos, the sequence from 1 to 2 to 4 to 8 to 16 cells.
They were…
pharyngula | December 19, 2006Once upon a time, in Paris in 1830, Etienne Geoffroy St. Hilaire debated Georges Léopole Chrétien Frédéric Dagobert,
Baron Cuvier on the subject of the unity of organismal form. Geoffroy favored the idea of a deep homology, that all animals shared a common archetype: invertebrates with their ventral nerve cord and dorsal hearts were inverted vertebrates, which have a dorsal nerve cord and ventral hearts, and that both were built around or within an idealized vertebra. While a thought-provoking idea, Geoffroy lacked the substantial evidence to make a persuasive case—he had to rely on fairly…
pharyngula | December 11, 2006Today, I gave my final lecture in developmental biology this term. We have one more class session which will be a final discussion, but I'm done yapping at them. Since I can't possibly teach them everything, I offered some suggestions on what to read next, if they're really interested in developmental biology. They've gotten the fundamentals of the dominant way of looking at development now, that good ol' molecular genetics centered modern field of evo-devo, but I specifically wanted to suggest a few titles to shake them up a little bit and start thinking differently.
For the student who is…
pharyngula | December 4, 2006
One of my favorite signal transduction pathways (what? You didn't know that true nerds had favorite molecular pathways?) is the one mediated by the receptor Notch. Notch is one of those genes in the metazoan toolkit that keeps popping up in all kinds of different contexts—it's the adjustable wrench of the toolbox, something that handles a general problem very well and therefore gets reused over and over again, and the list of places where it is expressed in Drosophila is impressive.
The general problem that Notch solves is the resolution of a binary decision in cell fate, one where a few…
pharyngula | November 30, 2006I found these on youtube, a couple of nice cartoony animations of the development of the urogenital system. This is one of the weirder modules in organogenesis, I think; many strange things go on that are relics of ancestral states. We actually build three pairs of kidneys—pronephros, mesonephros, and metanephros—and throw each one away in succession, except the last. Both sexes form paramesonephric (or Müllerian) ducts, in blue in the animation, and these form the core of the female plumbing, but again, males basically throw it away and use a more primitive duct (the mesonephric or Wolffian…
purepedantry | November 24, 2006If you remember back from when I was at the Society for Neuroscience, I saw a talk by Bruce Appel where he showed videos of oligodendrocytes migrating and myelinating in the zebrafish.
Oligodendrocytes are the myelin forming cell in the central nervous system of vertebrates -- the cells that coat axons in a sheet of fat called myelin that helps the axons conduct action potentials more quickly. At a point in oligodendrocyte development the oligodendrocyte precursor cells (OPCs) have to migrate out from the ventral part of the spine to cover the axons in the spinal cord. However, this…
pharyngula | November 23, 2006
The BBC is going to be showing a program with images of developing embryos (there are some galleries online) generated from ultrasound, cameras inserted into the uterus, and largely, computer-generated graphics. It's all very pretty, and I hope it will also be shown in my country, but…these pictures violate all the rules of scientific imaging. The images are clearly generated by imposing artistic decisions derived from the conventions of computer animation work onto the data that was collected—I can't tell what details in these embryos were actually imaged, and which were added by the CGI…
pharyngula | November 20, 2006
I've been writing a fair amount about early pattern formation in animals lately, so to do penance for my zoocentric bias, I thought I'd say a little bit about homeotic genes in plants. Homeotic genes are genes that, when mutated, can transform one body part into another—probably the best known example is antennapedia in Drosophila, which turns the fly's antenna into a leg.
Plants also have homeotic genes, and here is a little review of flower anatomy to remind everyone of what 'body parts' we're going to be talking about. The problem I'll be pursuing is how four different, broadly defined…
pharyngula | November 13, 2006
Oh happy day, the Sea Urchin Genome Project has reached fruition with the publication of the full sequence in last week's issue of Science. This news has been all over the web, I know, so I'm late in getting my two cents in, but hey, I had a busy weekend, and and I had to spend a fair amount of time actually reading the papers. They didn't just publish one mega-paper, but they had a whole section on Strongylocentrotus purpuratus, with a genomics mega-paper and articles on ecology and paleogenomics and the immune system and the transcriptome, and even a big poster of highlights of sea urchin…
pharyngula | October 24, 2006
There is a treasure trove in China: the well-preserved phosphatized embryos of the Doushantuo formation, a sampling of the developmental events in ancient metazoans between 551 and 635 million years ago. These are splendid specimens that give us a peek at some awesomely fragile organisms, and modern technology helps by giving us new tools, like x-ray computed tomography (CT), scanning electron microscopy (SEM), thin-section petrography, synchrotron X-ray tomographic microscopy (SRXTM), and computer-aided visualization, that allow us to dig into the fine detail inside these delicate…
pharyngula | October 9, 2006Sometimes a plan just comes together beautifully. I'm flying off to London tomorrow, and on the day I get back to Morris, I'm supposed to lead a class discussion on the final chapters of this book we've been reading, Endless Forms Most Beautiful. I will at that point have a skull full of jet-lagged, exhausted mush, and I just know it's going to be a painful struggle. Now into my lap falls a wonderful gift.
There was a review in the NY Review of Books that said wonderful things about Carroll's work, and in particular about the revolutionary nature of evo-devo. This prompted Jason Hodin, an evo…
pharyngula | October 5, 2006
Ultrasound imaging technology is coming along so fast that you can now get a near-real time, moderate resolution image of a living fetus. Unfortunately, this new technology is also having an unfortunate consequence.
Sophisticated ultrasound scans that show foetuses as early as 12 weeks appearing to "walk" in the womb have had a dangerous impact on the public debate over abortion, leading doctors and scientists said yesterday.
The emotive photographs, taken with new fourdimensional imaging technology, have created a misleading impression that foetuses become viable and potentially self-aware…
pharyngula | September 30, 2006Here's a prediction for you: the image below is going to appear in a lot of textbooks in the near future.
(click for larger image)Confocal image of septuple in situ hybridization exhibiting the spatial expression of Hox gene transcripts in a developing Drosophila embryo. Stage 11 germband extended embryo (anterior to the left) is stained for labial (lab), Deformed (Dfd), Sex combs reduced (Scr), Antennapedia (Antp), Ultrabithorax (Ubx), abdominal-A (abd-A), Abdominal-B (Abd-B). Their orthologous relationships to vertebrate Hox homology groups are indicated below each gene.
That's a technical…
pharyngula | September 29, 2006Forgive me, but I'll inflict a few more zebrafish videos on you. YouTube makes this fun and easy, and I'm going to be giving my students instruction in video micrography next week, so it's good practice.
This is a more detailed look at what's going on in the embryo. Using a 40x objective, we zoom in on a patch of cells near the surface of a 4-hour-old embryo—this is a generic tissue called the blastoderm. We just record activity with an 1800-fold time compression for a few hours to see what the cells are doing. The movie below displays typical, baseline activity: the cells are jostling about…
pharyngula | September 28, 2006Ah, the evils of strong drink. Or weak drink. You all know that you shouldn't drink alcohol to excess during pregnancy, and the reason is that it can affect fetal development. We take zebrafish eggs and put them on a real bender: we soak them in various concentrations of alcohol (which are hard to compare with human blood alcohol levels, I'm afraid, but trust me: these are such gross levels of ethanol that mere humans would be dead and pickled. Fish are tough), and let them stew for hours. Since fish development is much, much faster than human development, it's rather like having a woman…
pharyngula | September 28, 2006By popular request, here's a roughly annotated version of that zebrafish development movie.
Stuff to watch for:
This movie starts at the 8-16 cell stage. The cells of the embryo proper (blastomeres) are at the top, sitting on a large yolk cell.
The pulsing is caused by the synchronous early divisions of all the cells. They lose synchrony at the mid-blastula transition.
Epiboly is the process by which the cells migrate downward over the yolk. An arrow will briefly flash, pointing to about 11:00, in the direction of the animal pole (where the future nose will form, sorta). That happens…
pharyngula | September 28, 2006At my talk on Tuesday, the centerpiece was a short movie of zebrafish development—I was trying to show just how amazingly cool the process was. People seemed to like that part of the show, at least, so I thought I'd try to figure out this YouTube doohickey and upload it for general viewing. So here it is, a timelapse recording of about 18 hours of zebrafish embryology compressed into 48 seconds:
I've got more, and my students will be making videos of their own soon enough, so maybe I'll try uploading some other stuff soon. I'm discovering that YouTube is a little tricky about the aspect…