Chickens Have Outies

I've plowed through 69 pages of a nearly 400-page dissertation, dug up half a dozen cool papers on the role of the gut microbiome in such diverse things as mammalian radiation into diverse habitats and development of metabolic diseases (obesity, diabetes), and started building an enormous spreadsheet listing all of the published poultry flaxseed feeding trials since the mid 1980's along with a brief summary of their outcomes. I also spent half an hour at the barn and five hours in the lab this morning. Time for a break!

I thought I would share an amazing discovery with you: chickens have outies. Okay, I know, birds aren't placental mammals so technically they don't have belly buttons, innie, outie, or otherwise. But they most certainly have the functional equivalent to a structure associated with the belly button.

What is a belly button? The scientific word is omphalos, Greek for navel or center. I love the word omphalos. The navel marks the spot where the placenta attaches to the embryo during gestation. In mammals, the navel is an external feature, an embryological scar if you want to think of it that way. Birds don't have a placenta but they do have a structure that links what eventually becomes the GI tract of the embryo to the yolk, its energy source during development in the egg.

Just before birds hatch, they absorb the remaining bit of yolk into their intestinal cavity. They use the lipids in the yolk remnant for the energy that they need to break the shell then begin their search for food and water. If you've ever cut up day-old chicks (they are fed to snakes, raptors, etc) you might have noticed the runny yellow yolk remnant inside its belly.

As the birds age, they use up the yolk but the small bit that connected their GI tract to the yolk remains. It is called the Meckel's diverticulum and it specifically marks the start of the lower part of the small intestine in birds (the ileum). Meckel's diverticulum resembles a tiny outie belly button hanging off the outside of the chicken's gut. We have a Meckel's diverticulum that hangs off our lower small intestine too; originally it attaches to the placenta but once we are born, it shrivels up a bit, detaches from our belly wall, and becomes vestigial.

Photos after the break. Don't click if guts bother you.

This is a section of a mammal, possibly human, GI tract; even without a scale bar, I can say that it isn't from a chicken. I'm sorry that I don't have a complete source reference for the figure. Meckel's diverticulum is also known medically as the persistent viteline duct. 

This is a chicken GI tract--I know this because I dissected the chicken that owned it. The colon is in the upper left, the paired ceca stretched out on either side just above it (we don't have ceca, that's a bird thing). The ileum part of the small intestine extends from the ceca to Meckel's diverticulum. I have my finger on that structure in the upper right.