What I taught today: Axis specification

We began today with chocolate. Always a good thing at 8am, I think — so I brought a candy bar to class. Then I told the students that I loved and respected them all equally and that they all had equal potential, but that I was going to mark just one person as special by giving them that candy bar*. So I asked them how I could decide who should get it, telling them right off that dividing it wasn’t an allowed solution, and that yes, this could be an openly unfair process.

There were lots of suggestions: we could do it by random chance. I could throw it into the middle of the room and let them fight over it. We could analyze everyone’s DNA and give it to the most average person…or the most genetically unusual. I could just give it to the first person to raise their hand, or the person closest to me, or the person farthest from me. We could have a competition of some sort, and the winner gets it. I could give it to the person who wants it most, or who needs it most.

The point I was making is that this is a common developmental problem, that you have a potentially uniform set of cells and that somehow one or a few have to be distinguished as different, and carry out a different genetic program than another set of cells. One cop-out is to invoke mosaicism: that is, they aren’t uniform, but inherit different sets of cytoplasmic determinants that make them different from the very beginning, but that even in that case, these determinants aren’t detailed enough to specify every single cell fate in most organisms. Even with an initial prepattern, you’re eventually going to end up with a field of cells, like the dorsal side of the fly wing, and within that uniform field, some cells will have to be programmed to be epithelial, others to be bristles, others to be neurons. And that means that in every organism, even the most classically mosaic, you’ll reach a point where cells have to process information from their environment and regulate to build differential structures.

And with that I went on to talk about some animals that were judged as being mostly mosaic in character: molluscs, tunicates, echinoderms, and nematodes. Even here, these animals all required complex molecular interactions to build their embryos.

For example, I’d earlier used echinoderms as classic examples of regulative development. You can dissociate them at the 4-cell stage and each blastomere can go on to build a complete embryo. But at the 8-cell stage, when the cleavage plane separates an animal half from a vegetal half, that’s no longer true: the top four cells when isolated are animalized, forming only a ciliated ball, while the bottom four cells are vegetalized, only making a static blob with a bit of a skeleton inside. Clever experiments can quantitatively juggle these cells around, removing just the bottom 4 cells (the micromeres) at the 16-cell stage, or assembling composite embryos with different ratios of the different tiers of cells, and get different degrees of development. Even when you’re discussing an organism in which you’d call the pattern of development mosaic, it absolutely depends on ongoing cell:cell signaling at every step, and the final form is a consequence of interactions within the embryo. It’s a mosaic-regulative continuum.

I also described very superficially the work of Davidson and Cameron on specification events in echinoderms. These interactions can be drawn as a kind of genetic circuit diagram, where what you’re seeing is the pattern of genes being switched on and off. We can describe a cell type as the output of mappable gene circuitry, and we can even identify modules of networks of genes associated with a particular kind of cell, and that we can also see a limited number of genes that mediate interactions.

Next week I promised to start going into more detail, when we start talking about early fly development and axis decisions. The next class we’re actually going to switch gears a bit and discuss Sean Carroll’s Endless Forms Most Beautiful.

Slides used in this talk (pdf).

*Yeah, I lied again. I brought enough candy bars for everyone, and after we’d generated a list of ways to share just one, I gave them to everyone. They’ll never trust me again.

What I taught today: Position and Polarity

You really can’t teach a class by lecturing at them…especially not an 8am class. But sometimes there is just such a dense amount of information that I have to get across before the students know what to ask that I have to just tell them some answers. My compromise to deal with this eternal problem is to mix it up; some days are lecture days, others are discussion days. And today was a discussion day.

I’ve been talking at them for the past two weeks, basically working to bring them up to a 1950s understanding of the field of developmental biology, with a glimmering of the molecular answers to come and some of the general concepts, so that they’re equipped to start thinking about the contemporary literature. So I had them read this paper before class:

Kerszberg M, Wolpert L (2007) Specifying Positional Information in the Embryo: Looking Beyond Morphogens. Cell 130(2):205–209.

And then today they got into small groups and tried to explain it to each other. I primed them by suggesting that they try to define the terms positional information, gradient, morphogen, and polarity, and mentioned that I was playing a dirty trick on them, giving them a paper to introduce a basic concept that at the same time was pointing out some of the difficulties and problems of the idea, so I expected them to also do some critical thinking and question the concepts.

So they went at it. It went well; they had some lively conversations going on, which I always worry won’t happen with early morning classes. I find it helpful to ask students to try to poke holes in an idea, rather than just recite by rote what the paper says — it sends them hunting rather than gathering.

Several students noted that having a simple continuum of a molecule begs the question of how that gets translated into many discrete cell types; why does having one concentration of a morphogen make a cell differentiate into a thorax, while a very slightly lower concentration means it differentiates into an abdomen? It’s all well and good to suggest that a couple of overlapping gradients can specify position, like laying out a piece of graph paper with coordinates on it, but it doesn’t explain how that gets translated into position-specific tissues. I was most pleased that several of them, while groping for an answer, related it to the lac operon in E. coli, and brought up the idea of thresholds of gene activation. Yay! That so sets up future discussions about early fly embryogenesis, where that is exactly the answer.

I think they also got the idea that an explanation for general specification of body parts, for example, may not apply for explaining polarity within a body part; we may have to think about some kind of hierarchy of regulation, where we progressively partition the embryo into smaller and smaller units, with different mechanisms at different scales. They might be catching on to the depths of the problems to come.

Another way the paper primed the students was that it very briefly introduces a whole bunch of specific molecules: dpp, bicoid, sonic hedgehog, activin. They got a very general idea of the broad roles these molecules play, all part of my devious grand plan. When we start talking about the details of how animals set up dorsal-ventral polarity, for instance, and dpp/BMP start coming up in more specific contexts, I want them to be familiar old friends — molecules they already knew casually and informally, and now see doing very specific things, and interacting with another set of molecules, which now also joins their circle of pals. Before I’m done with them, they’re all going to regard these developmental signals and regulators as part of their family!

What I’m also going to teach today: a little image processing

My development class also has a lab. The last few weeks have all been teaching them how to get good optics on a research scope, and how to take photomicrographs with my PixeLink camera system. Today I’m going to show them how to appropriately process an image for publication, so they’ll learn a few digital enhancement tricks and a few ethical rules. I lay down a few laws about using image processing on scientific data:

What you must do to your image:

  • You must archive the original data and work with a copy. If I ask to see the original after you’ve enhanced the image up the wazoo, you better be able to show it.

  • You must document every step and every modification you make. You’re going to describe everything either on the image itself or in a figure legend; if this were to be published, you’d probably include it in the Methods section.

  • You must explain the scale and orientation of the image. The scale is usually shown by including a scale bar; orientation may be shown either by including annotations (text describing landmarks in the image) or an explanation in the figure legend, such as that it is a sagittal or horizontal section.

  • You must save the image in a lossless format, such as .png or .psd or .tiff. Do not save it in a lossy format like .jpeg, which can add compression artifacts.

What you may do to your image:

  • You may crop and rotate the image.

  • You may adjust the contrast and brightness for the whole image.

  • You may carry out simple enhancements, like applying a sharpening filter or unsharp masking, to the whole image…but remember, document everything!

  • You may splice multiple images together to produce a photomontage; you can also insert panels with enlarged or otherwise enhanced regions of the image, as long as it is absolutely clear what you’ve done.

What you may not do to your image:

  • You must not carry out selective modifications of portions of the image; you cannot sharpen the cell you care about and then reduce the contrast for other regions, for instance. You should not burn or dodge regions of the image.

  • No pixel operations or retouching: you are not allowed to go into the image and paint your data into existence!

We do a lot of this preliminary basic stuff because I run the course out of my research lab, rather than a student lab. I want to make sure they’re not going to break anything, and also that they know how to do good imaging, a skill they’ll find useful in other courses and in research (years ago when I taught this stuff, we’d also do black&white darkroom work — nobody does that any more, so now it’s all photoshop). The goal is to get them all able to churn out lovely photographic data, so later I can just hand them some nematodes or fruit fly embryos and tell them to do their own experiments and observations, just show me the pretty pictures when they’re done.

It’s a good life, being able to sit back and let students bring me gifts of biological beauty. I think they’ll also be posting some of these to their blogs.

What I taught today: gene regulation and signaling

Today was more context and a bit of a caution for my developmental biology course. I warned them that we’d be primarily talking about animals and plants (and mostly animals at that), but that actually, all of the general processes we’re describing are found in bacteria and other single-celled organisms — that in a lot of ways, microbiology is actually another developmental biology course. Yes, I went there: developmental biologists tend to be imperialists who see all the other sciences as mere subsets of the one true science. Of course, you could also take that as developmental biology being a synthetic discipline that steals bits and pieces from everywhere…

So the primary lessons today were reviews of stuff these students should have gotten in cell and molecular biology, with bits of biochemistry and microbiology thrown in. We talked about genes getting switched on and off, and the example I used was the classic: the lac operon in E. coli. What? You don’t know about it? It’s a beautiful system, in which the bacterium switches on the genes needed for digesting the sugar lactose only when the sugar is available in the environment. I showed this nice little 3 minute video:

Switching genes on and off? That’s development! It gets a little more intricate in multicellular animals, but all of the fundamental logic is right there in E. coli: activators and repressors, positive and negative feedback, the boolean logic of gene regulation. I also mentioned that when we’re reading Carroll’s Endless Forms Most Beautiful, he’s going to make a big deal out of exactly this kind of regulation, but I want them to remember that it’s not unique to butterflies or fruit flies or frogs, it’s a common theme in all kinds of diverse cells.

We then talked about cell signaling. How does a cell know which genes are supposed to be off and on? It interacts with its environment (as in the lac operon) or its neighbors to make decisions about activity. To illustrate that, we went through quorum sensing and biofilms — again, cell signaling is not unique to animal development, it was all worked out in principle in single-celled organisms. I gave them a little foreshadowing and mentioned that we’d be discussing Sonic Hedgehog and Notch and Delta later in the course, classic examples of signaling in multicellular systems, but in bacteria we have things like hapR and AHL signaling.

Finally, I raised the issue of a phenomenon we’ll be talking about on Wednesday: patterning. Why aren’t your arms growing from your hips, why don’t you have fingers on your feet instead of toes, why are your eyes paired and on the front of your head? Because there is positional information in the embryo that can be read by cells and tissues and lead to development of appropriate structures in their proper places. But once more, this is not unique to multicellular animals. A paramecium, for instance, is not a generic blob, but has a definite shape and orientation; it has organelles in predictable places, and is covered with a nearly crystalline lattice of cilia with specific axes of orientation. I showed them choanoflagellates and pointed out that these protists, representing a multicellular precursor, had a specific shape and a collar organ in a specific functional location: how do they know how to do that?

That’s the question we’ll be asking next. I warned them too that I won’t be lecturing at them on Wednesday, so they’d better have their morning coffee. I’m expecting them to read a review paper on positional information in embryos (pdf), and I’m going to make them explain it all to me for a change.

Slides for this talk (pdf)

For Wednesday:

Kerszberg M, Wolpert L (2007) Specifying Positional Information in the Embryo: Looking Beyond Morphogens. Cell 130(2):205–209.

New bloggers for Science!

As is my custom, my upper level courses have an expectation that students will do this blogging thing. They’re just now getting set up so there isn’t a lot of content yet, but here’s the current list of student web pages. Cruise on by and talk to them!

What I taught today: a little old-school history of embryology

This is an abbreviated summary of my class lecture in developmental biology today. This was the first day of class, so part of the hour was spent on introducing ourselves and going over the syllabus, but then I gave a lightning fast overview of the history of developmental biology.

Classical embryology began with Aristotle, whose work was surprisingly good: he approached the problem of development with relatively few preconceptions and fairly accurately summarized what was going on in the development of the chick. Most of this old school embryology is descriptive and was really a narrow subset of anatomy, but there were a few major conceptual issues that concerned the old investigators, in particular the question of preformation (the plan of the embryo is laid out in the egg) vs. epigenesis (the plan of the embryo emerges progressively). Aristotle, by the way, was on the right side of this debate, favoring epigenesis.

In the 19th century, development was seen as a progressive process that paralleled the hierarchical organization of nature — that is, developmental biology, what there was of it, was coupled to the great ladder of being. This is not an evolutionary idea, but reflects the view that there was a coherent pattern of greater and lesser development that was part of a coherent divine plan for life on earth. The German ‘Natural Philosophers’ pursued this line of reasoning, often to degrees that now look ridiculous in hindsight. In contrast, there were developmental biologists like Karl Ernst von Baer who wanted nothing to do with a cosmic teleology but instead preferred to emphasize observation and data, and simple minimal hypotheses.

In the late 19th century, developmental biology split into two directions. One was a dead end; Ernst Haeckel basically lifted the explanatory framework of the natural philosophers, replaced divinity with evolution, and tried to present development as a parallel process to evolution. Von Baer had already demolished this approach, and despite a few decades of popularity Haeckelian recapitulation died as a credible framework for studying evolution in the early years of the 20th century. The other direction developmental biology took was Wilhelm Roux’s Entwicklungsmechanik, or experimental embryology. This was an approach that largely eschewed larger theoretical frameworks, and focused almost exclusively on observation and experimental manipulation of embryos. It was a successful discipline, but also divorced mainstream developmental biology from the evolutionary biology that was increasingly influential.

As examples of Entwicklungsmechanik, I discussed Roux’s own experiments in which he killed one cell in a two-cell embryo and saw partial embryos result, an observation that fit with a preformationist model, but more specifically a mosaic pattern of development, in which patterns of development were encoded into the cytoplasm or cortex of the egg. Those experiments were seriously flawed, however, because the dead cell was left attached to the embryo, and could have deleteriously affected development. The experiments of Hans Driesch were cleaner; he dissociated embryos at the four cell stage, cultured each blastomere independently, and discovered that each isolated cell developed fully into a complete, miniature larva.

Driesch, unfortunately, interpreted these results to imply that there was an entelechy, or guiding intelligence outside the embryo, and that the only conceivable explanation was the existence of purpose behind embryology. This was also a dead end; the modern explanation for the phenomena is that they regulated, that is, that cells determine their fate by interacting with one another, rather than some kind of cosmic plan. And that’s really going to be a major focus of this course: how do cells communicate with one another, how are genes regulated to set up coherent and consistent patterns of gene expression that produce the organized cell types we find in an adult multicellular plant or animal?

That set up the next lecture. Entwicklungsmechanik, while representing a solid and productive research program, quickly reached its limits, because what we really needed to examine were those patterns of gene expression rather than trying to infer them from observations of morphology. The big breakthrough was the melding of developmental biology and molecular biology — most of the modern developmental biology literature focuses on examining interactions between genes. So on Wednesday we’ll get another fast overview of the molecular genetics research program, and a bit of evo-devo.

Slide thumbnails (PDF)

Did you have to remind me?

I wake up this morning to discover Doonesbury telling me stuff I already know.


Yep, classes start for me tomorrow at 8am. I have a lighter load than the grueling mess last semester, and I also get to teach my fave class, developmental biology. No new paradigms this time, though — I think it worked fairly well the way I did it last time, with a mix of once weekly lectures and lots of class time dedicated to discussion and analysis. I’ll also be compelling my students to set up blogs and write about science publicly, so I’ll occasionally be linking to a lot of student work.

One thing I’m considering doing differently…I might post summaries of lectures and discussion topics here, if time allows. Public exposure of all the stuff that usually goes on behind the doors of the classroom? I don’t know if the world is ready for that.

I’m including the syllabus for my developmental biology course. Just in case you think I’m totally slacking with just one class, I’m also teaching a course called Biological Communications, a writing course that tries to get students to read and write in the style of the scientific literature, and am also doing individual studies with 5 students.