From Myth to Science: The Pioneering Regeneration Discoveries of Abraham Trembley

The concept of regeneration has its roots in Greek mythology, with notable examples including the punishment of Prometheus for stealing fire and Hercules' Twelve Labors, particularly the slaying of the Lernaean hydra, where severed heads were replaced with two new ones. More subtle accounts of regenerating lizard tails can also be found in the works of ancient Greek scientists, including Aristotle.

Up until the eighteenth century, the field of biology primarily revolved around observing and documenting the natural world. One of the pioneers in experimental biology was the Swiss naturalist Abraham Trembley. While he was a tutor for a prominent Dutch family, Trembley stumbled upon polyps (Chlorohydra viridissima) in a freshwater pond. What fascinated him was the varying number of arms on individual polyps. Upon cutting a polyp in half, he observed it regenerating into two complete organisms. Further experiments showed that when the polyp was divided into multiple pieces, multiple organisms would emerge. He even managed to create a seven-headed polyp, which he named a hydra, inspired by the Greek mythological creature. In other experiments, grafting two polyps together resulted in a fused, single individual. Trembley meticulously documented these experiments and their outcomes in a book published in 1744. Initially, he believed polyps were plants, but their movement led him to revise this classification. At the time, Trembley was unaware that Antonie van Leeuwenhoek had previously described these creatures in 1702–1703 as one of his "animalculum."

While Trembley's discoveries were celebrated by much of the scientific community, not everyone readily accepted them. The ability of dissected hydras to regenerate into complete replicas of the original organisms challenged the prevailing belief in preformation, which held that embryos developed from pre-existing parts. One of the early skeptics was Trembley's cousin, Charles Bonnet, also a Swiss naturalist. However, Bonnet changed his mind in 1745 when he witnessed similar regeneration in worms. 

 

The hydra, known for its legendary regenerative abilities, is featured in the painting "Hercules and the Hydra" created by the Italian artist and sculptor Antonio del Pollaiolo (c. 1429–1498) around 1475.

What Happens In the Process of Regeneration?

Regeneration is a biological process where an organism is able to regrow or replace lost or damaged body parts, tissues, or organs. It is a remarkable ability found in various animals, plants, and even some microorganisms. The process of regeneration involves several key steps, which can vary depending on the species and the specific body part being regenerated.

Injury or damage: The regeneration process is triggered by an injury or damage to the organism, which can be caused by various factors such as physical trauma, disease, or natural wear and tear.

Inflammation and cell death: After the injury, inflammation occurs at the site of damage. This is a normal immune response that helps to clear away any debris and damaged cells. In some cases, damaged cells may undergo programmed cell death, known as apoptosis, to make room for new cells.

Cell proliferation and migration: Once the inflammation subsides, the process of cell proliferation and migration begins. Nearby cells often called blastemal cells, start dividing and multiplying to form a mass of undifferentiated cells at the site of injury, known as a blastema. These undifferentiated cells have the ability to differentiate into various cell types depending on the cues from the surrounding environment.

Cell differentiation: The blastemal cells start differentiating into specialized cells, such as muscle cells, nerve cells, or skin cells, depending on the type of tissue or organ being regenerated. This process is regulated by various signaling pathways and genetic factors that control cell fate and tissue development.

Tissue and organ formation: As the blastemal cells differentiate into specialized cells, they start organizing themselves into functional tissues and organs. This may involve the formation of complex structures, such as blood vessels, nerves, and other components of the original tissue or organ that was lost or damaged.

Functional recovery: Once the regenerated tissue or organ reaches a certain level of structural complexity, it starts to regain its normal function. This may take time depending on the complexity of the regenerated structure and the species involved. In some cases, the regenerated tissue or organ may fully restore its original function, while in other cases; it may partially recover or function differently.

The process of regeneration is a complex and highly regulated biological phenomenon that allows certain organisms to repair and replace lost or damaged body parts. It involves a series of cellular events; including inflammation, cell proliferation, and migration, differentiation, and tissue/organ formation, ultimately leading to functional recovery. The specific details of regeneration can vary greatly among different organisms and tissues, and research in this area continues to uncover new insights into the remarkable ability of regeneration in the natural world.

Remarkable Regeneration: From Invertebrates to Humans

Throughout the lifespan of an organism, it is possible for certain parts to become damaged or lost. Most organisms possess the ability to replace these defective or missing parts, a phenomenon referred to as regeneration.

Regeneration in Animals

The capacity for regeneration is observed in various animals, both invertebrates and vertebrates.

Invertebrates

Sponges: Sponges can regenerate their entire body from a conglomeration of their cells.

Regeneration In Planarian 

Hydra: Similar to sponges, hydra also possesses this regenerative ability.

When a Planarian worm is severed, regeneration initiates at each cut surface, resulting in the formation of a blastema. Cell division and differentiation continue within the blastema until a sufficient number of replacement cells are generated. Consequently, two complete worms are formed from the original one.

Earthworm: The earthworm can regenerate the first four or five segments of its head and even longer sections of its tail. If one of its large pincer claws is lost, it begins to regrow. In the case of a sea star, new arms are formed if one or more of its existing arms are removed.

Vertebrates

Among vertebrate animals, the most remarkable regenerative capacity discovered thus far exists in tailed amphibians such as newts and salamanders, which can fully regenerate their limbs when they are lost.

Adult frogs do not exhibit any regenerative ability; only their larvae possess the capacity to regenerate lost limbs.

Although most lizards can regenerate their tails, they are unable to regenerate entire organs. However, they can regenerate tissues, thereby repairing damaged or missing parts.

Regeneration in Humans

In humans, examples of regeneration include the healing of skin wounds, bone regeneration, and blood replacement. Organs such as the tongue, liver, and pancreas demonstrate extensive regenerative capabilities after being damaged.

The ratio between nerve cell processes and those of limbs plays a critical role in regeneration. For instance, in salamanders, if nerves leading to a limb are severed before amputation, regeneration does not occur. Conversely, if an additional nerve is surgically implanted at the base of a frog's leg, regeneration can take place.