A heart that beats (almost) like ours


The fruit fly, long the organ of choice for scientists studying genetics and basic biological processes, still holds some secrets.

One of them is what is called the “head-heart” of the fly. This tiny pumping station supplies the fly’s antennae with hemolymph, the insect equivalent of blood. While biologists have long known of the existence of the head-heart fly, no one has explored how it works in detail.

In a new study, biologists at the University of Iowa explain in detail how the fly’s head and heart work. The researchers found that the fly’s head-heart beat much like a human’s: its rhythmic impulses are automatically generated by a small muscle that runs through the middle of the brain, from the front to the back of the head. of the fly, and it is the contraction and relaxation of this muscle which causes the supply of hemolymph in the antennae.

“What we have shown is that the rhythmic contraction of the head-heart is not controlled by the fly’s brain but by the muscle itself”, explains Alan Kay, professor in the biology department of the ‘Iowa. “It’s like his own pacemaker.”

The human heart is also controlled by specialized muscle cells located in the heart. This main muscle in our chest is a wonderfully synchronized metronome, absorbing blood as it relaxes and forcefully expelling blood as it contracts. This blood is then transported throughout our body through a large, complex network of tubes, from major branches like arteries to tiny channels called capillaries.

Insects work differently. There is a main heart in the abdomen which supplies hemolymph, but the action of this pump does not reach the extremities. To compensate, the insects instead have secondary hearts, which ensure that the hemolymph reaches vital external areas, such as the antennae, where smell and hearing take place.

One of these helper hearts is found in the head of the fruit fly. An essential part of this head-heart, Iowa biologists discovered, is an elastic-walled chamber called the bulb. When the head muscle contracts, it stretches this ampullary membrane, opening valves in the wall and allowing the ambient hemolymph – which had been sent to the head by the fly’s main heart – to drain into the wall. ampullary chamber 50 micrometers wide. When the head-heart muscle relaxes, the valves close and the hemolymph is pushed through tubes that connect to the antennae by the tension of the elastic membrane.

Biologists at the University of Iowa have discovered how a “heart” works in the head of the fruit fly. The researchers filmed how a muscle in the fly’s brain triggers rhythmic jets of insect “blood”, called hemolymph, into the fly’s antennae. Video courtesy of Alan Kay.

“When the human heart contracts, blood is pumped from the heart to the body, says Kay, the study’s corresponding author. “With the fruit fly, ‘blood’ rushes in when the muscle contracts and is expelled when the muscle relaxes. “

“It works like our hearts but the other way around,” adds Daniel Eberl, professor in the Iowa Department of Biology and co-author of the study.

The rhythmic contraction of the fly’s head-heart muscle, two to three times per second, not only stimulates the flow of hemolymph, but also causes the fly to shake with each contraction, Iowa biologists have found. Biologists believe this shake is important for the fly’s brain health.

“There is no active circulation in the fly’s brain,” Kay says. “So, we believe that continued head-heart brain agitation could help move metabolites into the extracellular space of the fly’s brain, as well as waste to keep the fly’s brain clean. The human brain also receives rhythmic jogging from our heart, and other scientists have suggested that these are essential for our brain’s vitality.

Much of the information on the fly’s head-heart has come from a technique developed by Kay and Eberl, with help from researchers at the College of Dentistry and the College of Engineering in Iowa. Called goggatomy, the technique involves quickly coating the insect with a light-curing resin, allowing researchers to open up the exoskeleton and study the insect’s organs and cells while it remains alive.

For this study, the researchers used goggatomy to produce videos of a living head-heart fly in action, including:

  • the head-heart muscle in action in a living fly’s brain, seen from above;
  • the real-time hemolymph flow in the bulb as the head-heart muscle contracts in a live fly;
  • the spray of hemolymph flooding the antennae of the fly when the head-heart muscle relaxes in a live fly; and
  • the head-heart muscle contracts and stretches, in isolation.

The study, “Myogenic contractions of somatic muscle fuel the rhythmic flow of hemolymph through Drosophila antennae and generates cerebral pulsations ”, was published online September 29 in the Journal of Experimental biology.

Co-authors include Jing Wang, who received his doctorate in biology from Iowa and is now at the University of California, San Diego.

The US National Science Foundation and the National Institutes of Health funded the research.

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