Apr 24, 2011

Happy Easter! So how did the egg get its ellipsoid shape?

Given today is the one day of the year most people in the world appreciate the ellipsoid shape of the egg, my science pick for this week is my own research: on how the egg gets its ellipsoid shape.

Insects and birds are two animal groups that produce ellipsoid-shaped eggs, thought to be an adaptation to laying eggs on land.  But the puzzling part is that developing eggs (a.k.a. egg chambers or follicles) start out as a sphere.  So how does it become ellipsoid?  I figured this out by looking at developing eggs in the common fruitfly, Drosophila melanogaster, directly by keeping them alive for a few hours outside of mom and sticking them under a microscope and recording what happens.

Here's what I saw them do:  they spin around the long axis.  Pretty remarkable, huh?

Developing Drosophila eggs



Developing eggs spin!
(Click the link to play the movie)







So what's the point of spinning eggs?  Spinning around helps the developing egg build polarized fibrils like Collagen around its surface.  It's the oriented Collagen fibrils that helps the egg get its ellipsoid shape as it continues to grow until the eggshell is deposited.  (Collagen is the main component of our tendons and ligaments, so it's a strong molecule to deal with forces and help shape tissues.)

Interestingly, developing chicken eggs have also been reported to spin around the time its ellipsoid shape is being formed.  But because it's hard to study that process as it happens, no description at the cellular or molecular level has been made in the chicken.

Drosophila is one of the most popular animal model systems in biology because scientists can identify the genetic, cellular and molecular basis of many biological processes, and scientists have used findings from Drosophila research to discover that many processes in humans and other animals use the same genes and principles that help them grow, develop, and function.  So we hope that our work may help uncover new insights to thinking how we form organs during our own development and during the development of many animals!

For free, full access to the original paper, visit:
Global Tissue Revolutions in a Morphogenetic Movement Controlling Elongation

Hooray for the Easter egg!

16 comments:

  1. Great post. I like the spinning Drosophila eggs. How fast are they rotating and what is the force applied to the material, collagen or other?

    Thanks,
    Dean

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  2. Thanks. The Drosophila seem to use cilia to rotate. How do chickens do it?

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  3. On average, they spin at 0.5 micron/min, and we estimate each one completes 3 revolutions. We haven't done any direct measurements regarding force, but the force comes from collective migration of the surface follicle epithelium (labelled in green in the movie) using actin-based protrusions to pull against Collagen and the rest of the extracellular matrix surrounding the developing egg.

    Thanks for the comment!

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  4. Hi Daniel. Most cells in Drosophila don't have cilia. We think this movement is driven by actin-rich protrusions on the basal side of the follicle epithelium (see comment above), although this has not been formally tested yet.

    Developing chicken eggs are known to rotate in the oviduct when it's getting its ellipsoid shape. It's thought that the spiral anatomy and contractions of the the muscles surrounding the developing egg help it spin around its elongating axis. No one's tested whether blocking these contractions or affecting the function of these muscles alters chicken egg shape. But it's interesting to think that in both flies and chickens, they use the supporting cells around the egg to help give it its shape. It takes a village to raise an egg!

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  5. Hi, Saori. Popped over from Pharyngula. Didn't find a place for general comments, so I hope you don't mind me saying here that I immediately appreciate your writing in posts which a layman like me can understand and enjoy, and your more technical responses to specific comments.

    Also, Me Talk Pretty One Day is hilarious.

    The developing eggs in the video rotated in different directions. Any idea how direction of rotation occurs or if it makes a difference in egg development?

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  6. I will never be able to see the word drosophila again without thinking of how vehemently Sarah Palin protested "wasteful spending" on fruitfly research while having absolutely not even the faintest clue how important fruitfly research is, has been, and probably will be for a long dang time.

    Anyway, very interesting article. :) I look forward to seeing more.

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  7. Also just popping in from Pharyngula, I've read all the posts you've done so far, subscribed, and I... I think I love you.

    Seriously, though, wonderful stuff. If I had a god, my god would be Learning.

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  8. Neat blog! Nice pick for the background photo, too.

    Thanks for doing this.

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  9. Hi Gary. Great question! We're still trying to figure out how the spinning gets started.

    What I can say is that the opposite left- or right-handedness in spinning direction we see seems to be associated with those actin-based protrusions I mentioned above. That is an active area of research right now by the scientific community. It seems like each developing egg makes that directional choice randomly, because I see in some samples neighbor moving in the same direction, in others the opposite...but each developing egg never changes directions once they get started in the movies I've seen. In the end, it seems to have no obvious effect on the final shape...although we haven't formally tested this yet because we can't get them to grow outside of mom long enough yet for them to become a mature egg (we film for ~3 hrs; it takes 3+ days to make an egg!).

    I'll keep you posted, though, if we figure anything more about this out. Thanks!

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  10. Well, you wanted to write for the lay person and I am he, so please bear with me. If the shape is due to spinning, wouldn't it follow that the egg would elongate on both ends somewhat equally? Thanks for writing. I have a 7yo who loves science and will be trying to read along with his old man. He was really excited about the question alone (how eggs get their shape).

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  11. Hi tuuky. Thanks for tuning in!

    Normal developing eggs with the polarized spin you see in the movie do seem to elongate at both "head" and "tail" ends, but we haven't measured that directly.

    The reason why we know that polarized spinning is required to control egg shape is because when we use genetic mutations in the fly that block egg spinning, we end up with round eggs! This is because when spinning no longer occurs, then the developing egg grows in all directions, becoming a larger sphere and thus, a round egg.

    Polarized spinning helps build polarized Collagen fibrils around the egg. Collagen is part of the extracellular matrix - the external skeleton of the developing egg - that helps shape the egg by limiting where growth happens. Think of tying a bunch of strings around a round ballon in one direction. If you start to blow up the ballon, it won't grow spherically but will be limited in growth in its circumference because of all those strings and end up growing out like a hot dog. Similar concept with an egg. Except it uses cells via spinning to help lay those "strings" around the egg. Neat, huh?

    Please let me know if I can further clarify!

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  12. Two questions about the "on land" hypothesis:

    - It seems that the drosophilia eggs are developing in a neutrally bouyant environment, so I'm not sure what "on land" is explaining. The chicken egg is affected by gravity in the oviduct, the fly egg not so much.

    - Do land dwelling plants have ellipsoidal eggs?

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  13. Hi David,

    Sorry for the delayed response. The "on land" hypothesis is not in reference to effects by gravity; it is based on my own readings of research done by other biologists on the benefits of ellipsoid eggs in various animals. For the fly, ellipsoid eggs provide a fitness advantage because females producing round eggs are sterile because these eggs get stuck in the oviduct on their way out!

    I am no plant biology expert but plants produce seeds (not eggs) of great diversity in size and shape. Click on the link to see a snapshot:

    http://www.tc.umn.edu/~hmuller/Seeds.jpg

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  14. So egg shape is a three-way trade-off between maximising volume, minimising surface area, and not getting stuck? Good grief.

    Which direction does the egg point when it comes out the female fly? (I'm trying to imagine why an ovoid egg would do better than an elliptical egg. It's making my eyes water.)

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  15. The egg points with its "derriere" coming out first out of the mom. Round eggs cause female sterility in the fly; the oviduct muscles that let eggs out from the ovaries can only accomodate ellipsoid eggs but not round eggs, so they get stuck! Hope this helps!

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  16. It's definitely great to know how an egg gets its ellipsoid shape. Thanks for the info.

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