First embryonic division of a nematode worm: the DNA (magenta) of the egg and sperm meet each other and initiate the start of a new life.

First embryonic division of a nematode worm: the DNA (magenta) of the egg and sperm meet each other and initiate the start of a new life.

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  1. This is a *C. elegans* zygote. The timelapse is roughly 5 hours. *C. elegans* is great for studying embryogenesis because the whole affair, starting with the very first division seen here all the way up to hatching, can occur outside of the mother and under a microscope. There are a bunch of other reasons too that I’d be happy to nerd out about.

    The bright white spindles are microtubules, which are the cytoskeletal proteins that attach to the chromosomes to pull them apart. They’re probably tagged with green fluorescent protein to visualize them, which is why they glow so brightly.

    The microtubules radiate out from a pair of organelles called centrioles. The oocyte’s own pair of centrioles degrade during meiosis and are already gone by the time this movie starts. Both of the centrioles here are actually supplied by the sperm.

    The mitotic spindle “rocking” up and down is normal, though I’m not sure why it does that. It could just be a byproduct of spindle assembly as the whole apparatus gets organized.

    The bright magenta spots at the left of the image (the anterior end of the cell) are the oocyte’s polar bodies. When an oocyte undergoes meiosis, the cell divisions are extremely asymmetrical, such that the future oocyte gets all of the cytoplasmic goodies, and the other “cell” from the division is basically just a little package of ejected DNA. With spermatogenesis, you start with one diploid stem cell and get four equal, haploid sperm. With oogenesis, you still start with one diploid cell and end with haploid daughters, but the oocyte is huge and the polar bodies get practically nothing.

    You can see one polar body sitting there and the other one drifting in and out of the plane of focus. You might expect four total cells (as one gets with spermatogenesis), but there are only two polar bodies and one oocyte. This is because the first polar body ejected from the oocyte doesn’t contain enough cytoplasmic machinery to undergo a second round of cell division. Both polar bodies will soon be phagocytosed by the developing embryo, though this movie ends before that happens.

    In *C. elegans*, the oocyte’s pronucleus always hangs out at the anterior end of the cell (left here), and the sperm’s pronucleus approaches from the posterior (right). Cue joke about entering through the back door.

    Whew, what else can I ramble on about? The whole process from fertilization to hatching only takes about 14 hours for these worms, which makes them handy for study. They’re self-fertile hermaphrodites, though XO individuals develop as male and will mate with hermaphrodites. A single mother will pump out about 250-300 babies over the course of a few days. Thankfully, *C. elegans* are totally harmless and don’t parasitize humans or anything else.

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