Acorn Worms and Regeneration of Human Body Parts
Impressive and Potentially Helpful Animals
Acorn worms are marine animals that have an impressive ability to replace lost body parts. Surprisingly, humans have many of the same genes as the animals, including most—and perhaps all—of the ones involved in regeneration. For an unknown reason, the regeneration pathway isn't active in us. If we could find a way to stimulate the correct genes, it might be possible for humans to regrow lost body parts. Scientists are studying worm and human genes with this goal in mind.
Acorn worms belong to a group known as hemichordates. This group is related to another group of organisms known as chordates. Humans and other vertebrates are chordates. Acorn worms are not closely related to earthworms, which are invertebrates, even though the abbreviated term "worms" is sometimes used to refer to the animals.
An Acorn Worm Digs in the Sand
The prefix hemi means half. Hemichordates could he thought of as half way between invertebrates and chordates.
Body of an Acorn Worm
Biologists divide the acorn worm's body into three sections—the proboscis, collar, and trunk. The proboscis is at the front of the worm. It's elongated and often conical in shape. The collar is a fleshy, ring-like structure behind the proboscis. The trunk is the longest section of the animal. The worms range from less than inch in length to as long as seven feet.
Acorn worms are named from the fact that the proboscis and collar sometimes resemble an acorn (the fruit of an oak tree) sitting in its cup. Some people think that the region looks more like a structure found in a male human than an oak tree, however.
Most acorn worms have a dull yellow, pale orange, or pale pink colour. Researchers exploring a deep sea environment recently discovered beautiful purple worms. The animals are shown in the videos below. They have a slightly different appearance as well as a different colour from the worms found at shallower depths.
The Proboscis and Collar
The proboscis is a muscular structure that enables an acorn worm to dig its way through the sand or mud. The worm has no eyes, ears, or other structures that we might expect on the head of an animal. The skin of the entire animal contains sensory receptors, however. These probably enable it to sense light, chemicals, and touch. The skin cells are ciliated. Cilia are tiny hair-like structures that beat to create a current of liquid.
Chordates have a flexible, rod-like structure called a notochord in at least some stage of their lives. In humans, the notochord is replaced by the spinal column during embryonic development. Acorn worms have a similar structure to a notochord called a stomochord, which develops no further. Most of the stomochord is located under the collar.
The mouth is located on the underside of the worm between the proboscis and the collar. The worm has a complete digestive tract that travels from the mouth, through the trunk, and to the anus at the end of the trunk. The mouth leads to the pharynx, which is in turn followed by the esophagus, stomach, and intestine.
The trunk contains many of the worm's organs. Some of the structures described below extend from the trunk into the collar and even into the proboscis, however.
The gill slits are located behind the collar. Water enters the worm via the mouth and then flows over the gills. Oxygen leaves the water and enters the blood vessels of the gills while carbon dioxide moves from the blood into the gills. The water leaves the body and returns to the sea through the gill slits.
A vessel along the animal's back (the dorsal vessel) sends blood to the proboscis. Here a muscular sac acts as a heart. Blood travels backwards through a vessel on the lower surface of the worm (the ventral vessel). The worm has an open circulatory system, which means that the blood is not confined in blood vessels throughout its route. In some places it travels through spaces called sinuses. The blood is colourless and contains dissolved substances but no cells.
The nervous system appears to be quite simple but requires more study. The animal has a dorsal nerve cord along the upper part of its body and a ventral one along the bottom part. It also has a plexus (a collection of branched nerves) under its skin. It has no brain, however.
The excretory organ is located next to the heart and is known as the glomerulus or the kidney. This organ removes waste from the blood.
An Acorn Worm on the Deep Sea Floor
The acorn worm above was discovered by the NOAA (National Oceanic and Atmospheric Administration) Okeanos Explorer team. The animal was found during an expedition known as the 2016 Deepwater Exploration of the Marianas.
Life of an Acorn Worm
Acorn worms live in u-shaped tunnels that they create in the sand or mud of the intertidal area or in areas covered by deeper water. The animals are rarely seen by humans. One end of the tunnel is used for feeding and the other end for defecation. The skin contains glands that secrete mucus, which lines the tunnel. The worms tend in stay in one place once they have dug their burrow, although they are capable of slowly crawling from one spot to another. The proboscis is the most active part of the worm during digging and feeding, but the collar helps the digging process.
Most worms swallow sand or mud and extract detritus from it. Detritus consists of tiny fragments of dead and decomposed creatures as well as particles of their waste material. The sand is swept towards the worm's mouth by cilia on the proboscis and collar. Once the detritus is extracted, the sand is expelled through the anus at the surface of the burrow, produced worm-shaped castings reminiscent of those left by earthworms.
Some acorn worms can obtain nutrients by filter feeding. Sea water enters the body through the mouth and exists via the gills. Suspended particles in the water are trapped on the gills and retained for food.
Acorn worms are either male or female. The female releases a mass of eggs covered with mucus. The male releases sperm. Once the sperm fertlizes the eggs in the sea, the mucus breaks down. The young worm develops while in the ocean. In some species of hemichordate, the youngster looks like a juvenile worm. In others, it looks quite different from the adult and is known as a tornaria larva, as shown in the illustration above. At least some species of acorn worms can reproduce asexually when bits of the worm's trunk break off and grow into new animals.
The larvae of acorn worms closely resemble those of the phylum Echinodermata, which contains starfish, sea urchins, and sea cucumbers. Certain structural features and aspects of embryonic development link echinoderms to chordates. These links between the phyla cause biologists to place echinoderms, hemichordates, and chordates in one group called the deuterostomes.
Regeneration in Acorn Worms
Researchers at the University of Washington (UW) recently published the results of a detailed exploration of acorn worm regeneration. If a worm is cut in half between the head and the tail, each worm grows the missing half in the correct proportion. All of the lost internal organs and structures are replaced and they are each in the correct position and of the correct size and shape. In fact, it's impossible to distinguish the regenerated worms from the original one. If each of the new worms is cut, the regeneration process is repeated.
The researchers found that by day 15 after a worm had been cut into two sections, the damaged pieces had regrown the missing organs, nerves, and body structures. Furthermore, all of these parts were functional.
We share thousands of genes with these animals, and we have many, if not all, of the same genes they are using to regenerate their body structures.— Shawn Luttrell, University of Washington
Application to Human Biology
The UW researchers studied gene expression in the acorn worms as they regenerated. Genes control the construction of a body and the action of body processes by coding for proteins. The phrase "gene expression" means that a gene becomes active. The researchers suspect that a master gene or genes controls the other genes involved in the regeneration of an injured acorn worm.
The scientists hope to find a similar genetic control mechanism in humans. If they do, it may be possible to take a tissue sample from an injured person, trigger the correct genes to become active and then place the sample over the injury as a graft. If all goes according to plan, the missing structure will be regenerated.
Another View of a Deep Sea Acorn Worm
Current Regeneration Abilities in Humans
Humans currently have a very limited ability to regenerate structures in the body. Some examples of natural regeneration locations include:
- the endometrium in the uterus (lost during each menstruation and then regenerated)
- fingertips (under some conditions)
- the liver, provided at least a quarter of the organ is still present
Regenerating entire nerves after they are injured, replacing whole organs after devastating damage, and replacing amputated limbs would be wonderful advances in medical science. Acorn worms may show scientists how to accomplish this.
I really think we as humans have the potential to regenerate, but something isn’t allowing that to happen. I believe humans have these same genes, and if we can figure out how to turn on these genes, we can regenerate.— Billie Swalla, Friday Harbor Laboratories Director
Regeneration via Stem Cells
The UW researchers are trying to discover whether acorn worms use stem cells to produce new body parts or whether other cells are reprogrammed. Stem cells are unspecialized but can be stimulated to form specialized cells under the right conditions. Interestingly, medical scientists have achieved some success in inducing the regeneration of human tissue and structures via stem cells. Perhaps stimulating stem cells and stimulating genes that we share with acorn worms will both be helpful for regeneration in the future.
Why Can't Humans Regenerate Lost Body Parts Naturally?
It's not known for certain why humans lack natural regeneration capabilities beyond a few cases. According to the University of Washington researchers, there are at least two theories that may explain the situation.
When a piece of the body is broken off, our immune system may react so strongly to prevent blood loss and infection that it produces scar tissue that prevents regeneration. Another factor involved may be that since we are so much larger than an acorn worm, the energy required to create a new body part may be too high.
Acorn Worm Genes and Humans
About seventy percent of human genes have a counterpart in acorn worms. It's strange to think that a creature that looks so different from a human being and that is relatively primitive in function could share so many genes with us. Understanding how the genes of the worm work may be very helpful, however. Regeneration of body parts could have a dramatic effect on human lives.
Exploration of the hemichordata and the acorn worm in a virtual lab from Rutgers University
Regeneration in acorn worms and humans: A news release from the University of Washington
The growing scientific interest in hemichordates plus facts about the animals from The Node (The Company of Biologists)
Questions & Answers
How do members of the class Enteropneusta regenerate?
For a more detailed look at how the animals regenerate, you could go the University of Washington news release mentioned in the "References" section of my article and then click on the relevant link in the news release to explore the scientific paper. The research is interesting, but it includes too many details to summarize properly here.
© 2016 Linda Crampton