I learned something new today, and something surprising. I’ve opened up my fair share of bellies and seen intestines doing their slow peristaltic dance in there, and I knew in an abstract way that guts were very long and had to coil to fit into the confined space of the abdominal cavity, but I’d always just assumed it was simply a random packing — that as the gut tube elongated, it slopped and slithered about and fit in whatever way it could. But no! I was reading this new paper today, and that’s not the case at all: there is a generally predictable pattern of coiling in the developing gut, and it’s species-specific.

The midgut forms as a simple linear tube of circular cross-section running down the midline of the embryo, and grows at a greater rate than the surrounding tissue, eventually becoming significantly longer than the trunk. As the size of the developing mid- and hindgut exceeds the capacity of the embryonic body cavity, a primary loop is forced ventrally into the umbilicus (in mammals) or yolk stalk (in birds). This loop first rotates anticlockwise by 90° and then by another 180° during the subsequent retraction into the body cavity. Eventually, the rostral half of the loop forms the midgut (small intestine) and the caudal half forms the upper half of the hindgut (the ascending colon).

The chirality of this gut rotation is directed by left–right asymmetries in cellular architecture that arise within the dorsal mesentery, an initially thick and short structure along the dorsal–ventral axis through which the gut tube is attached to the abdominal wall. This leads the mesentery to tilt the gut tube leftwards with a resulting anticlockwise corkscrewing of the gut as it herniates. However, the gut rotation is insufficient to pack the entire small intestine into the body cavity, and additional loops are formed as the intestine bends and twists even as it elongates. Once the gut attains its final form, which is highly stereotypical in a given species, the loops retract into the body cavity. During further growth of the juvenile, no additional loops are formed, as they are tacked down by fascia, which restrict movement and additional morphogenesis without inhibiting globally uniform growth.

[Read more…]

Have you ever witnessed a cat in heat? Yowling, hissing, screaming, tearing at each other…or at best, moping about the house, trying to get intimate in unseemly ways with you, pressing their butts up against everything and responding to every touch with lordosis. They are tacky and obnoxious. It’s no wonder that pet owners get the randy little beasts neutered — it’s not just to prevent them spawning more of their creepy kind, but to suppress their repulsive sexual demonstrations.

Now molluscs, on the other hand — if we had them as pets, we’d be putting hormones in their food to induce more frequent balletic copulations. We’d want a pair elegantly and silently writhing in a corner of the living room all the time, and we’d applaud in wonder and stroke them when they were done, murmuring “Beautiful boy/girl, lovely boy/girl, well done.”

(Also on Sb)