What is a beating embryonic heart?

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:

In vertebrates, the beating tube of cells, once formed, begins a process called looping that in some ways remains a mystery. The tube folds on itself, showing handedness, so that one side will consist of the right atrium and ventricle (in humans) and the other side will consist of the left atrium and ventricle.

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.]

19 thoughts on “What is a beating embryonic heart?

  1. Pingback: When your heart was just a tube | CfpDir.com

  2. Great post, and it got me thinking about my recent miscarriage and my feelings about abortion rights. A miscarriage is a devastating loss, but the experience makes me feel more strongly pro-choice. I didn’t know that my pregnancy wasn’t viable until 10 weeks, but the embryo probably stopped developing around 5, so it may have never had a beating heart. Who’s to say that life was worth less to me or anyone else than one that made it to 10 weeks of development? Regardless, that embryo that came and went was a personal joy and loss to us. I personally wouldn’t have chosen the loss, but I also wouldn’t want anyone else making decisions about that life for me and my family.

    • Alice, I lost a pregnancy that was first detected with a transvaginal ultrasound at about 6 weeks (which I had because I was bleeding), and so I saw that little silver pulse. Then at 8 weeks, it was gone (missed abortion), as confirmed by another transvaginal ultrasound, even though HCG was skyrocketing, which makes me suspect a triploid conceptus or hydatidiform mole (?). A misoprostol abortion attempt turned out to be incomplete, and I ultimately had a DNC at what would have been 12 weeks. My experience with transvaginal ultrasounds and the trauma I associate with them makes me firmly against ever *requiring* a woman to have them for any reason. And had that been a triploid conceptus or molar pregnancy that had progressed, I wouldn’t have wanted North Dakota telling me I couldn’t have an abortion because it was “genetic.”

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  4. All very good information. But the essential question for abortion is simply “what is it that an abortion kills?” Very simple. If it’s not a human then no one would argue that you don’t have a right to kill it (No problem killing that frog, hit it with a car and no body feels more than a moments remorse). If it is a human than of course no one would argue that you should be allowed to indiscriminately kill it. (it’s never ok to kill a baby just because it annoys you).
    But simply because the question is clear doesn’t mean that we can’t reasonably disagree on whether or not, at a given point in development it is a human baby. Laws that some developmental point at which we will give the baby the benefit of the doubt, seem entirely reasonable, and even if that point of development seems like it is very similar to that of other animals, it still comes down to where should we put the benefit of the doubt at that point?

  5. David – I’m not entirely clear what you are saying, but I can say as a friend to a woman who just had to terminate her pregnancy at 21 weeks and did not want to, that I don’t believe these decisions should be left to politicians or religious zealots. The choice (and many women don’t believe they have a choice at this point) should be left between a woman, her faith and her doctor. Even if you’re super puritanical and extreme, you have to believe there is some logic God had behind allowing the life of the baby and the mother to be inextricable at this point. She will make the best choice. The rest of us can just F*** off and leave them alone at worse or support them at best.

    • Nancy,
      My comment had nothing to do with religion at all . In fact I leave open the possibility that even if there were not extenuating circumstances then your friend may have had reason to believe that it was ok to have an abortion. But if there are extenuating circumstances and you suggest there were then those clearly have weight in the situation.
      My comment was purely about a case where abortion may be a choice when in a world (potentially imaginary) where extenuating circumstances may not apply. where all that matters is what is it that you are trying to “kill”? purely from a scientific standpoint I leave open the question as to if it is a human child or not. However, if the science (which is not clear) were clear, then the reason for the killing had better be very good (and I still hold out that in some cases the reason may be good enough).
      I agree that supporting someone making a tough decision is vital, but I think in general, making sure they fully understand the decision the consequences of the decision they are about to make is part of that support. If they don’t fully understand there there is room for doubt, then they may be filled with even more regret later.

  6. Pingback: DoubleXScience co-founder declares embryo hearts “not uniquely human” « The Firewall

  7. I worked as a postdoc in a lab that studied heart development in chick embryos. Here’s a link to a great video (and experiment) from a colleague of mine at the time named Cheng Cui, which shows the actual cells (fluorescently labeled) forming the heart tube. You can even see the heart begin to beat (as it enters the c-looping phase):
    http://www.youtube.com/watch?v=LNTGgDk1Yck&feature=youtu.be

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