Cells are the fundamental unit of life and disease; therefore, many avenues of research converge on cells, making images of cells prominent in research and teaching. Besides, cells are beautiful, and a gorgeous photo of a cell can spice up an otherwise dreary talk. Much of the progress of modern biomedical science can be tied to advances in our ability to better visualize the functional morphology of cells, including higher resolution imaging, informative molecular tags, and techniques for making observations in living cells. Given the microscopic scale that cells inhabit, and the variety of imaging techniques used to visualize them, it's not surprising that students are both inspired and confused by cells. How big are they? What shapes can they exhibit? How do cells perform their myriad functions? How do scientists produce all those pretty colors? This Feature describes websites that would help answer these and other important questions about cells, as well as sites that would help address some common sources of difficulty in the presentation of cellular information.

Let's begin with the issue of scale. A common problem for many students in understanding the sizes of cells is photomicrographs without scale bars, or unlabeled scale bars, or changing scale bars to make matters worse. On the other hand, scale bars can mar a beautiful image, and students often ignore the information they provide. Scale in the natural world is important and interesting enough to be worthy of explicit discussion with students. Many readers may be familiar with the classic film Powers of 10 by Charles and Ray Eames. I first saw the film when it came out in 1977 and I was in high school. The film made a lasting impression on me, in part because the setting is a park in my native Chicago. The film has been paid homage in many creative ways over the years, including a beautiful IMAX version and a hilarious animated feature of a child on a pogo stick jumping by powers of 10, over the hedge, over the rooftops, and then out into the universe. The official Eames Powers of 10 website (www.powersof10.com) makes the original film available, along with some other useful features (Figure 1). Sample images at various powers of 10 are provided, along with links to websites concerned with any given scale. You click on 10⁻⁵ or 10⁻⁶, for example, and see cellular images and can then follow links to other websites addressing the world at that scale. It is a wonderful idea, but unfortunately the links appear to be somewhat haphazardly chosen and some are out of service. To fully use the site, active X controls and the Shockwave plug-in are required.

One of the best websites for finding detailed information on various optical methods, digital enhancements, and fluorescent reagents is Michael Davidson's Molecular Expressions (http://micro.magnet.fsu.edu) affiliated with Florida State University and the National High Magnetic Field lab (Figure 4). The site makes extensive use of JAVA for demonstrations and tutorials. The section on “Live-Cell Imaging Techniques,” for example, has detailed information on keeping cells alive during observation, special optical pathway and detector requirements, automated observation, and vital fluorescent probes. The movies in this section are very good as well. The coverage of imaging technology and techniques is comprehensive, from the physics of standard light microscopy to how to set up a near-field scanning optical microscope. To explain all those pretty colors we see, here's just a sample of the topics covered in the section on Fluorescence: Introductory Concepts, Fundamentals, Microscope Anatomy, Filters, Light Sources, Focusing and Alignment, Electronic Imaging Detectors, Fluorophores, Fluorescent Proteins, Indirect Immunofluorescence, Photomicrography, Multiphoton Excitation Microscopy, Fluorescent Resonance Energy Transfer, Total Internal Reflection Fluorescence Microscopy, Laser Systems, Stereomicroscopy, and many others. Although the site has fantastic resources, it is text-heavy and has some navigation and organization problems. For example, I could not find the nice powers of 10 tutorial (http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/) from the homepage or from the site search, though I was successful with a Google search.

Beautiful images can be so captivating that we may forget that pictures of cells provide scientific information; images are data. As an undergraduate, I was dubious about the notion that I could learn anything about cellular functions by looking at fixed and sectioned cells. But scoring and counting cells at various stages of mitosis in a plant root tip to calculate the time a cell spent at various stages in mitosis taught me a lot about sampling, the power of simple calculations, and how to extract functional information from snapshot data. I was hoping to find a version of this classic experiment on the Web and my quest was rewarded by a resource at the University of Arizona. The University of Arizona Biology Project has an excellent set of cell biology tutorials (www.biology.arizona.edu/Cell_bio/cell_bio.html). The tutorials feature good graphics and some photos, covering membranes, signaling, cytoskeleton, cell cycle, cell division, and the intracellular organization of cells. The website has a very nice simulation of the classic cell-cycle calculation exercise. Following an optional tutorial on mitosis, I discovered a series of onion cell micrographs and was tasked to categorize each cell as being in interphase, prophase, metaphase, anaphase, or telophase (Figure 5). I did pretty well. The activity includes creative feedback for both correct and incorrect answers. After classifying more than 30 cells, I was asked to make the calculations to estimate how much time in each phase a cell spends. How well does this method work? How prone is it to sampling errors? Send your students to the Rutgers University website (http://bio.rutgers.edu/~gb101/lab2_mitosis/highmagroot1.html) to see a gorgeous root tip section to calculate an independent sample for comparison. In answer to students who think the exercises are merely academic busy work, have them go to the University of Tennessee website (www.tiem.utk.edu/~gross/bioed/webmodules/tumorgrowth.html) for information on estimating the growth rate of tumors as a biomedical analogous application. For an update on the state of the art for extracting useful quantitative data from cellular images, visit CellProfiler (http://www.cellprofiler.com/) to see sample experiments and images and to download free software.