Planarian

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Planarian
Dugesia subtentaculata 1.jpg
Dugesia subtentaculata , a dugesiid.
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Platyhelminthes
Subphylum: Rhabditophora
Order: Tricladida
Lang, 1884
Subdivisions [1]
Unidentified planarian

Planarians (triclads) are free-living flatworms of the class Turbellaria, [2] [3] order Tricladida, [4] which includes hundreds of species, found in freshwater, marine, and terrestrial habitats. [5] Planarians are characterized by a three-branched intestine, including a single anterior and two posterior branches. [5] Their body is populated by adult stem cells called neoblasts, which planarians use for regenerating missing body parts. [6] Many species are able to regenerate any missing organ, which has made planarians a popular model in research of regeneration and stem cell biology. [7] The genome sequences of several species are available, as are tools for molecular biology analysis. [7] [8]

Contents

The order Tricladida is split into three suborders, according to their phylogenetic relationships: Maricola, Cavernicola and Continenticola. Formerly, the Tricladida was split according to their habitat: Maricola (marine planarians); Paludicola (freshwater planarian); and Terricola (land planarians). [9]

Planarians move by beating cilia on the ventral dermis, allowing them to glide along on a film of mucus. Some also can move by undulations of the whole body by the contractions of muscles built into the body membrane. [10]

Triclads play an important role in watercourse ecosystems and are often very important as bio-indicators. [11]

Phylogeny and taxonomy

Phylogeny

Phylogenetic supertree after Sluys et al., 2009: [1]

Tricladida

Taxonomy

Sabussowia ronaldi, a Maricola. Sabussowia ronaldi 2.jpg
Sabussowia ronaldi, a Maricola.
Polycelis felina, a planariid. Polycelis felina.jpg
Polycelis felina, a planariid.
Platydemus manokwari, a geoplanid. Peerj-297-fig-1 Platydemus manokwari.png
Platydemus manokwari , a geoplanid.

Linnaean ranks after Sluys et al., 2009: [1]

Anatomy and physiology

Planarians are bilaterian flatworms that lack a fluid-filled body cavity, and the space between their organ systems is filled with parenchyma. [5] [13] Planarians lack a circulatory system, and absorb oxygen through their body wall. They uptake food to their gut using a muscular pharynx, and nutrients diffuse to internal tissues. A three-branched intestine runs across almost the entire body, and includes a single anterior and two posterior branches. The planarian intestine is a blind sac, having no exit cavity, and therefore planarians uptake food and egest waste through the same orifice, located near the middle of the ventral body surface. [5]

The excretory system is made of many tubes with many flame cells and excretory pores on them. Also, flame cells remove unwanted liquids from the body by passing them through ducts which lead to excretory pores, where waste is released on the dorsal surface of the planarian.

The triclads have an anterior end or head where sense organs, such as eyes and chemoreceptors, are usually found. Some species have auricles that protrude from the margins of the head. The auricles can contain chemical and mechanical sensory receptors. [14]

The number of eyes in the triclads is variable depending on the species. While many species have two eyes (e.g. Dugesia or Microplana ), others have many more distributed along the body (e.g. most Geoplaninae). Sometimes, those species with two eyes may present smaller accessory or supernumerary eyes. The subterranean triclads are often eyeless or blind. [14]

The body of the triclads is covered by a ciliated epidermis that contains rhabdites. Between the epidermis and the gastrodermis there is a parenchymatous tissue or mesenchyme. [14]

Nervous system

Planaria nervous system Planaria nervous.png
Planaria nervous system

The planarian nervous systems consists of a bilobed shaped cerebral ganglion that is referred to as the planarian brain. [15] Longitudinal ventral nerve chords extend from the brain to the tail. Transverse nerves, commissure, connect the ventral nerve chords forming ladder-like nerve system. [5] The brain has been shown to exhibit spontaneous electrophysiological oscillations, [16] similar to the electroencephalographic (EEG) activity of other animals.

The planarian has a soft, flat, wedge-shaped body that may be black, brown, blue, gray, or white. The blunt, triangular head has two ocelli (eyespots), pigmented areas that are sensitive to light. There are two auricles (earlike projections) at the base of the head, which are sensitive to touch and the presence of certain chemicals. The mouth is located in the middle of the underside of the body, which is covered with hairlike projections (cilia). There are no circulatory or respiratory systems; oxygen enters and carbon dioxide leaves the planarian's body by diffusing through the body wall.

Reproduction

Planarian reproductive system Planarian reproductive system.png
Planarian reproductive system

Triclads reproduce sexually and asexually, and different species may be able to reproduce by one or both modes. [5] Planarians are hermaphrodites. In sexual reproduction, the mating generally involves mutual insemination.

Thus, one of their gametes will combine with the gamete of another planarian. Each planarian transports its secretion to the other planarian, giving and receiving sperm. Eggs develop inside the body and are shed in capsules. Weeks later, the eggs hatch and grow into adults. In asexual reproduction, the planarian fissions and each fragment regenerates its missing tissues, generating complete anatomy and restoring functions. [17] Asexual reproduction, similar to regeneration following injury, requires neoblasts, adult stem cells, which proliferate and produce differentiated cells. [17] Some researchers claim that the products derived from bisecting a planarian are similar to the products of planarian asexual reproduction; however, debates about the nature of asexual reproduction in planarians and its effect on the population are ongoing. [18] Some species of planarian are exclusively asexual, whereas some can reproduce both sexually and asexually. [19] In most of the cases the sexual reproduction involve two individuals; auto fecundation has been rarely reported (e.g. in Cura foremanii ). [14]

Neoblasts

Neoblasts are abundant adult stem cells that are found in the planarian parenchyma across the planarian body. [20] They are small and round cells, 5 to 10 μm, and characterized by a large nucleus, which is surrounded by little cytoplasm. [20] Neoblasts are required for regenerating missing tissues and organs, and they continuously replenish tissues by producing new cells. [17] Neoblasts can self-renew and generate progenitors for different cell types. In contrast to adult vertebrate stem cells (e.g., hematopoietic stem cell), neoblasts are pluripotent (i.e., producing all somatic cell types). [21] Moreover, they give rise to differentiating, post-mitotic, cells directly, [22] and not by producing rapidly-dividing transit amplifying cells. [20] Consequently, neoblasts divide frequently, and apparently lack a large sub-population of dormant or slow-cycling cells. [23]

As a model system in biological and biomedical research

The life history of planarians make them a model system for investigating a number of biological processes, many of which may have implications for human health and disease. Advances in molecular genetic technologies has made the study of gene function possible in these animals and scientists are studying them worldwide. Like other invertebrate model organisms, for example C. elegans and D. melanogaster , the relative simplicity of planarians facilitates experimental study.

Planarians have a number of cell types, tissues and simple organs that are homologous to our own cells, tissues and organs. However, regeneration has attracted the most attention. Thomas Hunt Morgan was responsible for some of the first systematic studies (that still underpin modern research) before the advent of molecular biology as a discipline.

Planarians are also an emerging model organism for aging research. These animals have an apparently limitless regenerative capacity, and asexual Schmidtea mediterranea has been shown to maintain its telomere length through regeneration. [24]

Regeneration

Planarian regeneration combines new tissue production with reorganization to the existing anatomy, morphallaxis. [17] The rate of tissue regrowth varies between species, but in frequently used lab species, functional regenerated tissues are available already 7–10 days following tissue amputation. [17] Regeneration starts following an injury that require the growth of a new tissue. [25] Neoblasts localized near the injury site proliferate to generate a structure of differentiating cells called blastema. Neoblasts are required for new cell production, and they therefore provide the cellular basis for planarian regeneration. [26] Cell signaling mechanisms provide positional information that regulates the cell types and tissues that are produced from the neoblasts in regeneration. [27] Many signaling molecules that provide positional information to neoblasts, in regeneration and homeostasis, are expressed in muscle cells. [28] Following injury, muscle cells throughout the body can alter the expression of genes that encode molecules that provide positional information. [28] Therefore, the activities of neoblasts and muscle cells following injuries are essential for successful regeneration. [29]

Historically, planarians have been considered "immortal under the edge of a knife." [30] Very small pieces of the planarian, estimated to be as little as 1/279th of the organism it is cut from, can regenerate back into a complete organism over the course of a few weeks. [31] New tissues can grow due to pluripotent stem cells that have the ability to create all the various cell types. [32] These adult stem cells are called neoblasts, and comprise 20% or more of the cells in the adult animal. [33] They are the only proliferating cells in the worm, and they differentiate into progeny that replace older cells. In addition, existing tissue is remodeled to restore symmetry and proportion of the new planaria that forms from a piece of a cut up organism. [33] [17]

The organism itself does not have to be completely cut into separate pieces for the regeneration phenomenon to be witnessed. In fact, if the head of a planarian is cut in half down its center, and each side retained on the organism, it is possible for the planarian to regenerate two heads and continue to live. [34] Researchers, including those from Tufts University in the U.S., sought to determine how microgravity and micro-geomagnetic fields would affect the growth and regeneration of planarian flatworms, Dugesia japonica . They discovered that one of the amputated fragments sent to space regenerated into a double-headed worm. The majority of such amputated worms (95%) did not do so, however. An amputated worm regenerated into a double-head creature after spending five weeks aboard the International Space Station (ISS) – though regeneration of amputated worms as double-headed heteromorphosis is not a rare phenomenon unique to a microgravity environment. [35] In contrast, two-headed planaria regenerates can be induced by exposing amputated fragments to electrical fields. Such exposure with opposite polarity can induce a planarian with 2 tails. Two-headed planaria regenerates can be induced by treating amputated fragments with pharmacological agents that alter levels of calcium, cyclic AMP, and protein kinase C activity in cells, [36] as well as by genetic expression blocks (interference RNA) to the canonical Wnt/β-Catenin signalling pathway. [27]

Biochemical memory experiments

In 1955, Robert Thompson and James V. McConnell conditioned planarian flatworms by pairing a bright light with an electric shock. After repeating this several times they took away the electric shock, and only exposed them to the bright light. The flatworms would react to the bright light as if they had been shocked. Thompson and McConnell found that if they cut the worm in two, and allowed both worms to regenerate each half would develop the light-shock reaction. In 1963, McConnell repeated the experiment, but instead of cutting the trained flatworms in two he ground them into small pieces and fed them to other flatworms. He reported that the flatworms learned to associate the bright light with a shock much faster than flatworms who had not been fed trained worms.

This experiment intended to test whether memory could be transferred chemically. The experiment was repeated with mice, fish, and rats, but it always failed to produce the same results. The perceived explanation was that rather than memory being transferred to the other animals, it was the hormones in the ingested ground animals that changed the behavior. [37] McConnell believed that this was evidence of a chemical basis for memory, which he identified as memory RNA. McConnell's results are now attributed to observer bias. [38] [39] No blinded experiment has ever reproduced his results of planarians scrunching when exposed to light. Subsequent explanations of this scrunching behaviour associated with cannibalism of trained planarian worms were that the untrained flatworms were only following tracks left on the dirty glassware rather than absorbing the memory of their fodder.

In 2012, Tal Shomrat and Michael Levin have shown that planarians exhibit evidence of long-term memory retrieval after regenerating a new head. [40]

Planarian species used for research and education

Several planarian species are commonly used for biological research. Popular experimental species are Schmidtea mediterranea , Schmidtea polychroa, and Dugesia japonica, [5] which in addition to excellent regenerative abilities, are easy to culture in the lab. In recent decades, S. mediterranea has emerged as the species of choice for modern molecular biology research, due to its diploid chromosomes and the availability of both asexual and sexual strains. [7]

The most frequently used planarian in high school and first-year college laboratories is the brownish Girardia tigrina . Other common species used are the blackish Planaria maculata and Girardia dorotocephala .

See also

Related Research Articles

Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of stem cells in the adult organism.

<span class="mw-page-title-main">Regeneration (biology)</span> Biological process of renewal, restoration, and tissue growth

Regeneration in biology is the process of renewal, restoration, and tissue growth that makes genomes, cells, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. Regeneration can either be complete where the new tissue is the same as the lost tissue, or incomplete after which the necrotic tissue becomes fibrotic.

<i>Planaria</i> Genus of flatworms

Planaria is a genus of planarians in the family Planariidae. Due to its excellent ability to regenerate, species of Planaria has also been used as model organisms in regeneration studies. When an individual is cut into pieces, each piece has the ability to regenerate into a fully formed individual. When decapitated, they retain their memories.

Biological immortality is a state in which the rate of mortality from senescence is stable or decreasing, thus decoupling it from chronological age. Various unicellular and multicellular species, including some vertebrates, achieve this state either throughout their existence or after living long enough. A biologically immortal living being can still die from means other than senescence, such as through injury, poison, disease, predation, lack of available resources, or changes to environment.

<span class="mw-page-title-main">Blastema</span> Mass of cells capable of enacting growth and regeneration

A blastema is a mass of cells capable of growth and regeneration into organs or body parts. The changing definition of the word "blastema" has been reviewed by Holland (2021). A broad survey of how blastema has been used over time brings to light a somewhat involved history. The word entered the biomedical vocabulary in 1799 to designate a sinister acellular slime that was the starting point for the growth of cancers, themselves, at the time, thought to be acellular, as reviewed by Hajdu. Then, during the early nineteenth century, the definition broadened to include growth zones in healthy, normally developing plant and animal embryos. Contemporaneously, cancer specialists dropped the term from their vocabulary, perhaps because they felt a term connoting a state of health and normalcy was not appropriate for describing a pathological condition. During the middle decades of the nineteenth century, Schleiden and Schwann proposed the cell theory, and Remak and Virchow insisted that cells can only be generated by division of existing ones. Consequently, the conception of the blastema changed from acellular to cellular. More specifically, the term came to designate a population of embryonic cells that gave rise to a particular tissue. In short, the term blastema started being used to refer to what modern embryologists increasingly began calling a rudiment or Anlage. Importantly, the term blastema did not yet refer to a mass of undifferentiated-looking cells that accumulates relatively early in a regenerating body part. For instance, Morgan (1900), does not use the term even once in his classic book, “Regeneration.” It was not until the eve of World War 1 that Fritsch introduced the term blastema in the modern sense, as now used by contemporary students of regeneration. Currently, the old usage of blastema to refer to a normal embryological rudiment has largely disappeared.

<i>Dugesia</i> Genus of flatworms

Dugesia is a genus of dugesiid triclads that contains some common representatives of the class Turbellaria. These common flatworms are found in freshwater habitats of Africa, Eurasia, and Australia. Dugesia is best known to non-specialists because of its regeneration capacities.

<span class="mw-page-title-main">Planariidae</span> Family of flatworms

Planariidae is a family of freshwater planarians.

<span class="mw-page-title-main">Geoplanidae</span> Family of flatworms

Geoplanidae is a family of flatworms known commonly as land planarians or land flatworms.

<i>Bipalium</i> Genus of flatworms

Bipalium is a genus of large predatory land planarians. They are often loosely called "hammerhead worms" or "broadhead planarians" because of the distinctive shape of their head region. Land planarians are unique in that they possess a "creeping sole", a highly ciliated region on the ventral epidermis that helps them to creep over the substrate. Native to Asia, several species are invasive to the United States, Canada, and Europe. Some studies have begun the investigation of the evolutionary ecology of these invasive planarians.

<i>Schmidtea mediterranea</i> Species of worm

Schmidtea mediterranea is a freshwater triclad that lives in southern Europe and Tunisia. It is a model for regeneration, stem cells and development of tissues such as the brain and germline.

<i>Bipalium kewense</i> Species of flatworm

Bipalium kewense, also known as the shovel-headed garden worm, is a species of large predatory land planarian with a cosmopolitan distribution. It is sometimes referred to as a "hammerhead flatworm" due to its half-moon-shaped head, but this name is also used to refer to other species in the subfamily Bipaliinae.

<span class="mw-page-title-main">Dugesiidae</span> Family of flatworms

Dugesiidae is a family of freshwater planarians distributed worldwide. The type genus is Dugesia Girard, 1850.

<i>Girardia</i> Genus of flatworms

Girardia is a genus of freshwater planarians belonging to the family Dugesiidae.

<i>Schmidtea</i> Genus of flatworms

Schmidtea is a genus of freshwater triclads. Species of the genus Schmidtea are widely used in regeneration and developmental studies.

<span class="mw-page-title-main">Geoplanoidea</span> Superfamily of flatworms

Geoplanoidea is a superfamily of freshwater and land triclads that comprises the species of the Geoplanidae and the Dugesiidae families.

<span class="mw-page-title-main">Kenkiidae</span> Family of flatworms

Kenkiidae is a family of freshwater triclads. Their species can be found sporadically in caves, groundwater, and deep lakes in Central Asia, Far East and North America.

Girardia tigrina, known as the brown planarian or the immigrant triclad flatworm, is a species of dugesiid native to the Americas. It has been accidentally introduced into Europe and Japan.

<i>Dugesia japonica</i> Species of flatworm

Dugesia japonica is a species of freshwater planarian that inhabits freshwater bodies of East Asia, including Japan, Korea, Taiwan, China and northeastern Siberia. However, molecular studies suggest that Dugesia japonica is polyphyletic and different populations across its area of occurrence constitute distinct species.

Epimorphosis is defined as the regeneration of a specific part of an organism in a way that involves extensive cell proliferation of somatic stem cells, dedifferentiation, and reformation, as well as blastema formation. Epimorphosis can be considered a simple model for development, though it only occurs in tissues surrounding the site of injury rather than occurring system-wide. Epimorphosis restores the anatomy of the organism and the original polarity that existed before the destruction of the tissue and/or a structure of the organism. Epimorphosis regeneration can be observed in both vertebrates and invertebrates such as the common examples: salamanders, annelids, and planarians.

<span class="mw-page-title-main">Neoblast</span> Planarian regeneration proliferative cells

Neoblasts (ˈniːəʊˌblæst) are adult stem cells found in planarian flatworms. They are the only dividing planarian cells, and they produce all cell types, including the germline. Neoblasts are abundant in the planarian parenchyma, and comprise up to 30 percent of all cells. Following injury, neoblasts rapidly divide and generate new cells, which allows planarians to regenerate any missing tissue.

References

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  39. ↑ For a general review, see also Chapouthier G (1973). "Chapter 1: Behavioral studies of the molecular basis of memory". In Deutsch JA (ed.). The Physiological Basis of Memory. New York and London: Academic Press. pp. l–25.
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