bodies and symmetry/asymmetry

[susan]

So I'm hanging around the house wondering about this: Why is the outside of the body symmetrical, and some of the internal organs symmetrical, but others of the organs are not? I.e., why one off-center liver rather than a centered organ, or two of them?

[Darwin's Finch]

The organs that aren't symmetrical are typically those associated with the gut and digestion. Developmentally, the gut starts out as a simple tube, with subsidiary organs later forming from pockets off of that tube. A similar question was pondered over in IMHO (http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=143456) last month.

Since my post was the last one in that thread, I either gave a decent explanation, or my breath stinks. I'm not sure which....

[susan]

Thanks. It's a great pleasure to know other people are thinking about the same questions that I find fascinating, but which, when posed, have historically been met with vacant or hostile stares.

I'm still not sure if your explanation covers the liver or pancreas, however.

[Darwin's Finch]

Gastrointestinal Development (http://www.geocities.com/medinotes/GI_Devt.html):
* The liver, pancreas and gallbladder develop from endodermal diverticula, which bud from the duodenum, eventually forming the glandular/duct epithelia of these organs, whilst the connective and vascular tissue forms from the surrounding mesenchyme.

* The liver develops from the ventral part of the duodenum as the hepatic bud on day 22, growing into the caudal part of the septum transversum that will later become ventral mesogastrium.

* The pancreas develops from 2 buds: the dorsal and ventral pancreatic buds, on opposite sides of the duodenum.

Both the liver and pancrease develop off of the main gut tube.

[Darwin's Finch]

Here (http://www.med.unc.edu/embryo_images/unit-digest/digest_htms/digest008.htm) is an image which may help in visualizing all this duodenum talk.