Starfish, or sea stars, are radially symmetrical, star-shaped organisms of the phylum Echinodermata and the class Asteroidea. Aside from their distinguishing shape, starfish are most recognized for their remarkable ability to regenerate, or regrow, arms and, in some cases, entire bodies. While most species require the central body to be intact in order to regenerate arms, a few tropical species can grow an entirely new starfish from just a portion of a severed limb. Starfish regeneration across species follows a common three-phase model and can take up to a year or longer to complete. Though regeneration is used to recover limbs eaten or removed by predators, starfish are also capable of autotomizing and regenerating limbs to evade predators and reproduce.
Due to their wide range of regenerative capabilities, starfish have become model organisms for studying how the regenerative process has evolved and diversified over time. While the overall morphological processes have been well documented in many starfish, little is known regarding the underlying molecular mechanisms that mediate their regeneration. Moreover, some researchers hope starfish may one day serve as inspiration for therapeutics aiming to expand the extent to which humans can repair and replace damaged cells or tissues.
Degrees of regeneration
Regenerative ability differs greatly among starfish species, but can generally be classified within three categories: unidirectional regeneration, disk-dependent bidirectional regeneration, and disk-independent bidirectional regeneration. In each case, regenerative capacity is enabled by the uniquely simple body plan of starfish.
The typical starfish has five or more arms, or "rays", radiating from a central disk. Each arm contains a copy of vital organs and is equipped with eyespots, an eye-like structure that helps the starfish differentiate between light and darkness, and tube feet, which enable locomotion. All organs connect to the digestive system in the central disk, which also contains the starfish mouth and stomach. This replication and delocalization of vital organs makes starfish especially resilient to the loss of appendages. In addition to being able to replicate organs, starfish are also capable of regenerating neuron cells. Other cells that are left over from the injury are able to become neural cells and take up functions they would not normally do. Following injury or amputation, a star fish can survive with its remaining organ copies during the period of regeneration, which ranges from a few months to over a year.
Unidirectional regeneration
Though the different Asteroidea species show a great range of variation in regeneration capabilities, an overwhelming number of them have the ability to regenerate lost limbs and tube feet. Star fish that exhibit unidirectional regeneration, or regeneration that is restricted to a single direction, are capable of regenerating multiple lost limbs from a disk containing half or more of the original starfish. Unidirectional regeneration is the simplest form of regeneration as the majority of the disk is intact, allowing the starfish to eat, move, and escape predators during the regeneration period. Unidirectional regeneration is also the most common form of regeneration exhibited by starfish as single arms are often removed by predators or shed through autotomy.
Crown-of-thorns starfish (Acanthaster planci), which feed on large swaths of coral reefs in the Indo-Pacific, are notable unidirectional regenerators. Starfish of this invasive species are extremely difficult to eradicate because of their ability to regrow when half or more of the original starfish is intact. Thus, initial population control efforts championed by fishermen and conservationists in the 1960s, which involved sectioning and releasing caught starfish, may have unknowingly exacerbated population outbreaks in the western Pacific coral reefs.
Disk-dependent bidirectional regeneration
Bidirectional regeneration is a robust form of regeneration defined by the ability to regrow the main body axis after whole body severance. Starfish that exhibit disk-dependent bidirectional regeneration are capable of regenerating a full starfish when less than half of the original starfish is intact, given that all or part of the central disk is present. The presence of the central disk gives the detached limb access to its original digestive system and mouth, allowing the starfish to move to find food, eat, and hide from predators during recovery.
Disk-independent bidirectional regeneration
The most extensive form of regeneration exhibited by starfish species is disk-independent bidirectional regeneration. In this rare case, a detached starfish limb with no remnants of the central disk is capable of regenerating a full starfish, referred to as a comet form. In the absence of a mouth or digestive system, the detached arm survives on nutrients stored in the arm until it can regenerate a disk. Without the ability to feed during recovery, disk independent bidirectional regeneration is difficult to execute and requires that the detached arm is in a relatively healthy form. This vigorous form of regeneration has been identified in Linckia species to a very high degree.
Regeneration phases Regeneration is an intrinsically conservative post-embryonic developmental process that repairs and replaces cells, tissues, organs, and body parts of a given organism. Furthermore, the arm regenerative process of all starfish species studied to date can be subdivided into three distinct phases: a repair phase, an early regenerative phase, and an advanced regenerative phase. Although diversity exists among starfish in terms of their physiology, morphology, and amputation susceptibility, a generalized regenerative process can be appreciated. Throughout the regeneration process, the coelomic epithelium is vital. This is a tissue that plays a large role in the formation of new limbs and new organs. Note, the following section details the regrowth of a starfish's arm following amputation in a unidirectional manner of regeneration.
Repair phase Immediately following amputation, all starfish must seal their coelomic cavities, particularly the perivisceral coelomic canal, to prevent fluid loss and the entrance of foreign pathogens. This is initially achieved by an emergency mechanism in which the entire arm wall contracts swiftly and powerfully to form a 'hemostatic ring' of sorts.
Additionally, in a process analogous to mammalian platelet clot formation, a morphologically heterogenous population of coelomocytes help prevent the loss of body fluid by forming a clot of cells at the injured perivisceral coelomic canal. Coelomocytes are free-wandering cells that circulate the coelomic fluid, possessing phagocytic, clotting, and cytotoxic functions in most echinoderms. These coelomocytes not only form clots at starfish amputation sites but also help clear the wound site of debris and foreign microorganisms via phagocytosis.
Re-epithelialization occurs within the first 48 hours post-amputation, in the middle of the repair phase. Interestingly, in contrast to most mammals, starfish accomplish re-epithelialization without any immediate proliferation of epidermal progenitor cells at the wound edge or wound epithelium. Rather, epidermal cells are stretched inwards from the wound edge, expanding centripetally until a continuous layer is formed. Of note, these stretched epidermal cells maintain their cell-cell junctions in starfish, whereas in mammals, junctional complexes are disrupted to allow the migration of keratinocytes over the wound. Subsequently, the wound epithelium becomes increasingly differentiated, thicker, and permanent. Moreover, in some starfish species, such as Echinaster sepositus and Acanthaster planci, a phagocytic syncytium transiently supports the migration of epithelial cells while protecting injured stump tissue from fluid loss and foreign entities.
Finally, the end of the repair phase is marked by the formation of a temporary edematous area below the newly established epithelial layer. Overall, this provisional tissue matures over time, to ultimately provide a scaffold for regenerative growth. In many ways, the edematous area resembles the granulation tissue of mammals, possessing a disorganized mix of fibroblasts, phagocytes, nervous elements, differentiating myocytes, and undifferentiated cells. Early on, phagocytes clear the edematous area of foreign material and degrade leftover debris. Meanwhile, fibroblasts develop the extracellular matrix (ECM) and pockets of collagen fibril. The area progressively matures over the span of about a week, ultimately containing a more organized extracellular matrix, dispersed collagen fibril bundles, nerve elements, early pigment cells, and other differentiated or undifferentiated cells.
Early regenerative phase The early regenerative phase begins once the injury has healed, and is characterized by an exodus of dedifferentiating myocytes from various anatomical structures towards the regenerating tip. Throughout this phase, the regenerating coelomic cavities serve as a physical driving force of regrowth. Notably, excess fluid secretion from the coelomic epithelia produces a hypertrophic appearance in the regenerating tip of the coelomic cavities. This hypertrophic state, in turn, produces a pressure that supports the regrowth of canals, particularly the perivisceral coelom and the radial water canal. In addition, the pressure creates a turgidity that physically supports the regenerate's shape until skeleton and muscle formation can occur.
The early regenerative phase is marked by a large mobilization of various cytotypes from different locations (like the coelomic cavities) towards the edematous regenerating region. This proximal to distal migration of cells supports the outgrowth of the radial nerve cord from any existing nerve cord remaining post amputation. Intriguingly, the radial nerve cord and radial water canal (the only two structures that run continuously along the arm) occur in tandem and potentially include an inductive cross-talk relationship. However, it is not currently known which structure induces regrowth and differentiation of the other. Furthermore, initial regeneration of the radial nerve cord results from proliferation of existing structure as well as the differentiation of supporting cells that create cell 'niches' for future neuronal growth. Specifically, the supporting cells (believed to be glial cells) acquire a bipolar shape, implanting opposing cytoplasmic extensions containing regenerative intermediate filament bundles. Niches result from these extensions and house interspersed neurons over time.
A blastema-like region also appears during this phase composed of undifferentiated and barely differentiated cells amongst the epidermal tissue and coelom outgrowths (radial water canal and radial nerve cord). Moreover, the perivisceral coelom funnels undifferentiated cells to the blastema-like formation. Unlike a true blastema, this blastema-like area lacks localization, contains an abundant ECM, and houses organized fiber bundles of collagen. As such, while starfish generally follows a morphallactic process of regrowth, the regenerative mechanisms fall somewhere in between a true morphallactic and epimorphic model, in reality.