It’s pretty much the same in any vertebrate.
by Emily Willingham
The governor of North Dakota recently signed a law making abortions illegal if a heartbeat is detectable in the embryo. Perhaps the emphasis on this beating organ isn’t a surprise. The heart carries strong emotional connotations, hence its use in anti-abortion campaigns. It connotes love. It symbolizes compassion, as in “have a heart.” It symbolizes health and life, and without the successful beating of this muscle, we are, of course, dead. If you’re pregnant with a wanted, longed-for pregnancy, I can attest, the first time you hear that galloping horse thump of a fetal heartbeat will become enshrined in your own heart as a turning point in your life.
But how does a heart–the real one, in you or a frog or a fish or a sea squirt–become the rhythmic beating muscle that it is? You’ve probably seen the anti-abortion bumper stickers that say, “Abortion stops a beating heart.” But what, exactly, is a beating heart in the early human embryo?
It starts early. In fact, the heart is the first organ to form to completion in vertebrates–in any animal with a backbone. If you’ve ever learned anything about your own heart, you’re probably aware that it has four chambers–two upper chambers called the atria, and two lower chambers called the ventricles. Contractions of the heart muscle keep blood moving, and the rate and sequence of these contractions requires exquisite timing within the heart itself. Depending on the vertebrate, other hearts may have only three chambers, relying on the power of only a single contracting ventricle. In still other animals, such as the invertebrate Ciona intestinalis (the sea squirt), the heart is simply a tube that forms a U-shape.
Regardless of these ultimate differences, the hearts among vertebrates all form in a pattern that is so similar, they are scarcely distinguishable. The early frog heart is like the early turtle heart is like the early human heart. Before any of them become the chambered, beating muscle we know of as a heart, however, they start out as a tube.
The tube forms in the early embryo from cells whose destiny is heart formation. Some of the cells aggregate to create the inner lining of the heart, while others form the muscular tissues. As the cells collect together, they develop two tubes. In humans, these two are supposed to fuse at about day 21 or 22 in embryonic development, forming a single tube. This fusion triggers the rhythmic beating of the heart as the cells start to communicate. In human development, the embryo at this point is 2 to 3 mm in length, about the height of a letter on this screen. The tube looks like this:
Imagine a little tiny group of cells beating there in the tube. Their meeting triggers signaling among themselves through special junctions so that they pulsate rhythmically in what we call a “beat.” A transvaginal ultrasound, which involves inserting a large, long plastic wand into the vagina, can detect this pulsating group of cells at about six weeks of pregnancy (based on last menstrual period dating, so about four weeks of actual embryonic development).
Each of the steps of heart formation is under direction from molecules that operate with refined timing at specific concentrations to ensure correct heart development. Our understanding of how these molecules operate has led to the usual tricks of developmental biologists, who just aren’t happy if they can’t find ways to derail development, just to show how it’s supposed to work normally. So, they have, for example, taken cells that weren’t intended to become heart, exposed them to these molecules that manage heart formation, and caused beating heart tubes to grow in places where there shouldn’t be any heart. Lest anyone be experiencing palpitations over ethics, this work was done in frogs.
It’s also possible to form a beating heart–or at least, beating cells–in a lab dish, as you can see in the video:
Does an abortion stop a beating heart? Yes, it does. But the above is what goes through my mind every time I see a bumper sticker that says that. Because when it comes to beating hearts, the process that gets the heart to that point is exquisite, but it’s not something that is uniquely human.
[Evolution of the heart image credit: Credit: Zina Deretsky, National Science Foundation after Benoit Brueau, the Gladstone Institute of Cardiovascular Disease. Grey's Anatomy embryonic heart sketches, public domain, via Wikimedia Commons. Homepage and thumbnail image credit, Denise Chan, via Flickr. A version of this post appeared previously at The Biology Files.]