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Ernst Stelzer, Jan Huisken, and col-leagues at the European Molecular

Biology Laboratory (Germany) havedeveloped an inexpensive microscopymethod that allows researchers to seedeep inside living embryos (Science2004, 305, 1007–1009).Called selective plane il-lumination microscopy(SPIM), the new techniquepenetrates more deeplyinto large embryos thanconventional methods,such as multiphoton andconfocal laser scanningmicroscopy.

SPIM uses a focusedsheet of laser light to illumi-nate a single slice through afluorescently labeled sample.Emitted light is detected by an objectivethat is perpendicular to the light sheet.The sample can be imaged, rotated, andimaged again from the opposite side inan optional multiview reconstructionprocedure, which merges the individualdata sets into a single high-resolution3-D image.

James Pawley at the University of Wis-consin says, “The light sheet is a very clev-er idea.” He notes that SPIM is basedon the same principles as the slit lampsused by ophthalmologists, althoughSPIM works on a much smaller scale.

After crafting the SPIM instrumentfrom common, off-the-shelf parts, Stel-zer and Huisken struck up a collabora-tion with developmental biologists attheir institution who were studyingmedaka fish (Oryzias latipes). Sampleswere mounted in a 0.5% agarose cylin-der. “The agarose just acts as a scaffoldthat spatially fixes the sample so that itdoesn’t move anymore, and we canmove it through the light sheet,” saysHuisken. He adds that time-lapse

movies show that medaka and fruit fly(Drosophila melanogaster) embryos re-main alive and develop normally overseveral days in the agarose cylinder.

The muscles of the medaka embryosthat the researchers chose to examine

were labeled with green fluorescent pro-tein. Sections of the fish were viewed ata resolution of ~6 µm and at a depth of500 µm. Multiview reconstruction canextend the depth that SPIM can imageto ~3–5 mm, says Huisken.

Depth is the big advantage of SPIM,say experts. According to Pawley, al-though multiphoton microscopy canalso illuminate and image a single sec-tion of an embryo, it cannot penetrateas far as SPIM. “In addition, because[SPIM] is a parallel image technique, itcan image each plane quite quickly, andthis is important when imaging livingembryos,” says Pawley. Jeff Hardin atthe University of Wisconsin says, “Ithink [SPIM] is going to be really goodfor looking at big embryos, and by big,I mean things that are on the order of amillimeter.” Hardin was particularly im-pressed with the images the researchersacquired of medaka hearts and says this“would have been really hard with anyother technique in living embryos.”

Like any new analytical technique,

SPIM has limitations. SPIM’s resolutionis not yet as good as it could be, say ex-perts. Hardin says that although thecurrent resolution is fine for the largermedaka embryos, it is not optimal forimaging small fruit fly and nematode

embryos. “But this is justthe first generation of thedevice, so I fully expectthat this will get betterover time,” he says. Mount-ing samples in agarose alsocould be a limitation insome cases. “If your em-bryo is changing shapeduring the course of film-ing in such a way that theagarose becomes a me-chanical constraint tomorphogenesis, that would

be a problem,” says Hardin.Stelzer plans to improve the technique

and combine it with other microscopymethods, such as fluorescence recoveryafter photobleaching or fluorescence res-onance energy transfer. “The nice thingis that we have an excellent signal-to-noise ratio that is really a dramatic differ-ence in comparison to other technolo-gies, and we have a relatively high speed[of detection],” says Stelzer. These prop-erties make SPIM an ideal partner forother visualization methods.

Because SPIM provides informationat both the organism and cellular levels,Stelzer says SPIM may help scientistssee things in ways they never dreamedpossible. “SPIM should be regarded as atool that can be used for true 3-D cellbiology,” he says. “I really think that[looking at] flat specimens, such as fi-broblasts on coverslips, is not really thekind of cell biology people should dobecause in our body, the cells are simplynot flat; they are 3-D objects.” a

—Katie Cottingham

A multiview reconstruction of (a) a medaka embryo head and (b) a medakaheart cut open virtually by computer. (Adapted with permission. Copyright2004 American Association for the Advancement of Science.)

(a) (b)

Peering into the depths of embryos