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출처: Wikipedia
Trichoplax feed by absorbing food particles—mainly microbes—with their underside. They generally reproduce asexually(무성생식[無性生殖]하다), by dividing(이분법[二分法]) or budding(출아법[出芽法]), but can also reproduce sexually(유성생식[有性生殖]하다). Though Trichoplax has a small genome in comparison to other animals, nearly 87% of its 11,514 predicted protein-coding genes are identifiably similar to known genes in other animals.
Discovery
Trichoplax was discovered in 1883 by the German zoologist Franz Eilhard Schulze, in a seawater aquarium(바다 수족관) at the Zoological Institute in Graz, Austria. The generic name(속명[屬名]) is derived from the classical Greek θρίξ (thrix), "hair", and πλάξ (plax), "plate". The specific epithet(종소명[種小名]) adhaerens comes from Latin "adherent", reflecting its propensity(성향) to stick to the glass slides and pipettes used in its examination [2].
Although from the very beginning most researchers who studied Trichoplax in any detail realized that it had no close relationship to other animal phyla, the zoologist Thilo Krumbach published a hypothesis that Trichoplax is a form of the planula larva(플라눌라[포배기에 이어지는 자포동물에 공통인 유생형] 유충) of the anemone-like hydrozoan(아네모네와 유사한 히드로충) Eleutheria krohni in 1907. Although this was refuted in print by Schulze and others, Krumbach's analysis became the standard textbook explanation, and nothing was printed in zoological journals about Trichoplax until the 1960s. In the 1960s and 1970s a new interest among researchers led to acceptance of Placozoa as a new animal phylum. Among the new discoveries was study of the early phases of the animals' embryonic development and evidence that the animals that people had been studying are adults, not larvae. This newfound interest also included study of the organism in nature (as opposed to aquariums) [3].
Trichoplax generally has a thinly flattened, plate-like body in cross-section around half a millimetre, occasionally up to two or three millimetres. The body is usually only about 25 µm thick. These colorlessly gray organisms are so thin they are transparent when illuminated from behind, and in most cases are barely visible to the naked eye. Like the amoebae, which they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form. These may facilitate movement to new habitats.
Trichoplax lacks tissues and organs; there is also no manifest body symmetry, so it is not possible to distinguish anterior(앞) from posterior(뒤) or left from right. It is made up of a few thousand cells of four types in three distinct layers: dorsal and ventral epithelia cells each with a single cilium ("monociliate"), ventral gland cells(배쪽 분비세포[分泌細胞]) and syncytial fiber cells(합포체 섬유세포[纖維細胞]). It does not possess sensory or muscle cells; it moves using cilia on its external surface.
Both structurally and functionally, it is possible to distinguish a back or dorsal(동물의 등 혹은 어류의 지느러미와 연결되는) side from a belly or ventral(복부) side in Trichoplax adhaerens. Both consist of a single layer of cells coated on the outside with slime(점액[粘液]) and are reminiscent of epithelial tissue(상피조직[上皮組織]), primarily due to the junctions(연접[連接])—belt desmosomes(데즈모솜 혹은 접착반[接着斑])—between the cells. In contrast to true epithelium(상피[上皮]), however, the cell layers of the Placozoa possess no basal lamina(기저층판[基底層板]), which refers to a thin layer of extracellular material underlying epithelium that stiffens it and separates it from the body's interior. The absence of this structure, which is otherwise to be found in all animals except the sponges(해면동물[海綿動物門] 혹은 Porifera), can be explained in terms of function: a rigid separating layer would make the amoeboid changes in the shape of Trichoplax adhaerens impossible. Instead of an epithelium, therefore, we speak of an epithelioid(상피모양[上皮模樣]) in the Placozoa.
A mature individual consists of up to a thousand cells that can be divided into four different cell types. The monociliated cells(단일섬모세포) of the dorsal epithelioid are flattened and contain lipid bodies(지방체[脂肪體]). The cells on the ventral side likewise beat just a single cilium, but their elongated columnar form(길게 뻗은 기둥형태) of small cross section(횡단면) at the surface packs them very close together, causing the cilia to be very closely spaced on the ventral side and to form a ciliated "crawling sole(발바닥)". Between them are found unciliated gland cells(분비세포[分泌細胞]) thought to be capable of synthesizing digestive enzymes.
Between the two layers of cells is a liquid-filled interior space(내강[內腔]), which, except for the immediate zones of contact with the central and dorsal sides, is pervaded by a star-shaped fiber syncytium(합포체 섬유[合胞體 纖維]): a fibrous network that consists essentially of a single cell but contains numerous nuclei that, while separated by internal crosswalls(격막[隔膜] 혹은 septum, 복수형은 septa), do not have true cell membranes(세포막[細胞膜]) between them. Similar structures are also found in the sponges (Porifera) and many fungi.
On both sides of the septa are liquid-filled capsules that cause the mentioned separating structures to resemble synapses(시냅스), i.e. nerve-cell junctions that occur in fully expressed form only in animals with tissues (Eumetazoa 혹은 진정후생동물[眞正後生動物]). Striking accumulations of calcium ions, which may have a function related to the propagation of stimuli, likewise suggest a possible role as protosynapses(프로토시냅스). This view is supported by the fact that fluorescent antibodies against cnidarians(자포동물[刺胞動物]) neurotransmitters(신경전달물질[神經傳達物質]), i.e. precisely those signal carriers that are transferred in synapses, bind in high concentrations in certain cells of Trichoplax adhaerens and thus indicate the existence of comparable substances in the Placozoa. In addition, the fiber syncytium contains molecules of actin and probably also of myosin, which occur in the muscle cells of eumetazoans. In the placozoans, they ensure that the individual fibers can relax or contract and thus help determine the animals' shape.
In this way, the fiber syncytium assumes the functions of nerve and muscle tissues. Moreover, at least a portion of digestion occurs here. On the other hand, no gelatinous extracellular matrix(세포외기질[細胞外基質] 혹은 세포간질[細胞間質]) exists of the kind observed, as mesoglea(중교[中膠]), in cnidarians and ctenophores(빗해파리 혹은 comb jellies).
Pluripotent cells, which can differentiate into other cell types, have not yet been demonstrated unambiguously in T. adhaerens, in contrast to the case of the Eumetazoa. The conventional view is that dorsal and ventral epithelium cells arise only from their own kind.
The Trichoplax genome contains about 98 million base pairs and 11,514 predicted protein-coding genes [4].
All nuclei of placozoan cells contain six pairs(?) of chromosomes that are only about two to three micrometres in size. Three pairs are metacentric(중부동원체[中部動原體]의), meaning that the centromere(동원체[動原體]), the attachment point for the spindle fibers(방추사[紡錘絲]) in cell division, is located at the center, or acrocentric(말단동원체형[末端動原體型]의), with the centromere at an extreme end of each chromosome. The cells of the fiber syncytium can be tetraploid(4배체[倍體]), i.e. contain a quadruple complement of chromosomes.
A single complement of chromosomes in Trichoplax adhaerens contains a total of fewer than fifty million base pairs and thus forms the smallest animal genome; the number of base pairs in the intestinal bacterium Escherichia coli is smaller by a factor of only ten.
The genetic complement of Trichoplax adhaerens has not yet been very well researched; it has, however, already been possible to identify several genes, such as Brachyury and TBX2/TBX3, which are homologous to corresponding base-pair sequences in eumetazoans. Of particular significance is Trox-2, a placozoan gene known under the name Cnox-2 in cnidarians and as Gsx in the bilaterally(좌우양측의) symmetrical Bilateria(좌우대칭동물[左右對稱動物]). As a homeobox or Hox gene it plays a role in organization and differentiation along the axis of symmetry in the embryonic development of eumetazoans; in cnidarians, it appears to determine the position of mouth-facing (oral) and opposite-facing (aboral) sides of the organism. Since placozoans possess no axes of symmetry, exactly where the gene is transcribed in the body of Trichoplax is of special interest. Antibody studies have been able to show that the gene's product occurs only in the transition zones(전이지역, 전이대[轉移帶]) of the dorsal and ventral sides, perhaps in a fifth cell type that has not yet been characterized. It is not yet clear whether these cells, contrary to traditional views, are stem cells, which play a role in cell differentiation. In any case, Trox-2 can be considered a possible candidate for a proto-Hox gene, from which the other genes in this important family could have arisen through gene duplication and variation.
Initially, molecular-biology methods were applied unsuccessfully to test the various theories regarding Placozoa's position in the Metazoa system(후생동물계[後生動物界]). No clarification was achieved with standard markers such as 18S rDNA/RNA: the marker sequence was apparently "garbled(왜곡된, 혼동된)", i.e. rendered uninformative as the result of many mutations. Nevertheless, this negative result supported the suspicion that Trichoplax might represent an extremely primitive lineage of metazoans, since a very long period of time had to be assumed for the accumulation of so many mutations.
Of the 11,514 genes identified in the six chromosomes of Trichoplax, 87% are identifiably similar to genes in cnidarians and bilaterians. In those Trichoplax genes for which equivalent genes can be identified in the human genome, over 80% of the introns (the regions within genes that are removed from RNA molecules before their sequences are translated in protein synthesis) are found in the same location as in the corresponding human genes. The arrangement of genes in groups on chromosomes is also conserved between the Trichoplax and human genomes. This contrasts to other model systems such as fruit flies and soil nematodes that have experienced a paring down of non-coding regions and a loss of the ancestral genome organizations [5].
The phylogenetic relationship(계통 발생 관계) between Trichoplax and other animals has been debated for some time. A variety of hypotheses have been advanced based on the few morphological characteristics of this simple organism that could be identified. More recently, a comparison of the Trichoplax mitochondrial genome suggested that Trichoplax is a basal metazoan—less closely related to all other animals including sponges than they are to each other [6]. This implies that the Placozoa would have arisen relatively soon after the evolutionary transition from unicellular to multicellular forms. But an even more recent analysis of the much larger Trichoplax nuclear genome instead supports the hypothesis that Trichoplax is a basal eumetazoan, that is, more closely related to Cnidaria and other animals than any of those animals are to sponges [4]. This is consistent with the presence in Trichoplax of cell layers reminiscent of epithelial tissue (see above).
Trichoplax was first discovered on the walls of a marine aquarium, and is rarely observed in its natural habitat [7]. Trichoplax has been collected, among other places, in the Red Sea, the Mediterranean, and the Caribbean, off Hawaii, Guam, Samoa, Japan, Vietnam, Brazil, and Papua New Guinea, and on the Great Barrier Reef off the east coast of Australia [8].
Field specimens tend to be found in the coastal tidal zones(해안의 조간대[潮間帶]) of tropical and subtropical(아열대[亞熱帶]) seas, on such substrates(기저부 기판) as the trunks and roots of mangroves, shells of mollusks(연체동물[軟體動物]), fragments of stony corals(돌산호) or simply on pieces of rock. One study was able to detect seasonal population fluctuations, the causes of which have not yet been deduced.
Trichoplax adhaerens feeds on small algae, particularly on green algae (Chlorophyta 혹은 녹조류 [綠藻類]) of the genus(속[屬]) Chlorella, cryptomonads(암색편모충류[暗色鞭毛蟲類]) of the genera Cryptomonas(크립토모나스) and Rhodomonas(로도모나스), and blue-green bacteria (Cyanobacteria) such as Phormidium inundatum, but also on detritus(암설[岩屑], 시체 혹은 배설물) from other organisms. In feeding, one or several small pockets form around particles of nutrients on the ventral side, into which digestive enzymes are released by the gland cells; the organisms thus develop a temporary "external stomach", so to speak. The enclosed nutrients are then taken up by pinocytosis(음세포작용[飮細胞作用]) ("cell-drinking") by the ciliated cells located on the ventral surface.
Entire single-celled organisms can also be ingested through the upper epithelioid (that is, the "dorsal surface" of the animal). This mode of feeding could be unique in the animal kingdom: the particles, collected in a slime layer, are drawn through the intercellular gaps (cellular interstices 틈새) of the epithelioid by the fiber cells and then digested by phagocytosis(식균작용[食菌作用] 혹은 식세포활동[食細胞活動]) ("cell-eating"). Such "collecting" of nutrient particles through an intact tegument(외피[外皮] 혹은 피막[被膜]) is only possible because some "insulating" elements (specifically, a basal lamina under the epithelioid and certain types of cell-cell junctions) are not present in the Placozoa.
Not all bacteria in the interior of Placozoa are digested as food: in the endoplasmic reticulum(소포체[小胞體]), an organelle of the fiber syncytium, bacteria are frequently found that appear to live in symbiosis with Trichoplax adhaerens.
Placozoa can move in two different ways on solid surfaces: First, their ciliated crawling sole lets them glide slowly across the substrate; second, they can change location by modifying their body shape, as an amoeba does. These movements are not centrally coordinated, since no muscle or nerve tissues exist. It can happen that an individual moves simultaneously in two different directions and consequently divides into two parts.
It has been possible to demonstrate a close connection between body shape and the speed of locomotion, which is also a function of available food:
- At low nutrient density, the spread-out area fluctuates slightly but irregularly; speed remains relatively constant at about 15 micrometres per second.
- If nutrient density is high, however, the area covered oscillates with a stable period of about 8 minutes, in which the greatest extent reached by the organism can be as much as twice the smallest. Its speed, which remains consistently below 5 micrometres per second, varies with the same period. In this case, a high speed always corresponds to a reduced area, and vice versa.
The following is a qualitative explanation of the animal's behavior:
- At low nutrient density, Trichoplax maintains a constant speed in order to uncover food sources without wasting time.
- Once such a source is identified by high nutrient density, the organism increases its area in regular increments and thereby enlarges the surface in contact with substrate. This enlarges the surface through which nutrients can be ingested. The animal reduces its speed at the same time in order to actually consume all of the available food.
- Once this is nearly completed, Trichoplax reduces its area again to move on. Because food sources such as algal mats(지형[紙型]) are often relatively extensive, it is reasonable for such an animal to stop moving after a brief period in order to flatten out again and absorb nutrients. Thus Trichoplax progresses relatively slowly in this phase.
Small animals are also capable of swimming actively with the aid of their cilia. As soon as they come into contact with a possible substrate, a dorsoventral response occurs: the dorsal cilia continue to beat, whereas the cilia of ventral cells stop their rhythmic beating. At the same time, the ventral surface tries to make contact with the substrate; small protrusions(융기[隆起]) and invaginations(함입[陷入]), the microvilli(미세융모[微細絨毛]) found on the surface of the columnar cells(원주세포[圓柱細胞]), help in attaching to the substrate via their adhesive action.
A notable characteristic of the Placozoa is that they can regenerate themselves from extremely small groups of cells. Even when large portions of the organism are removed in the laboratory, a complete animal develops again from the remainder. It is also possible to rub Trichoplax adhaerens through a strainer in such a manner that individual cells are not destroyed but are separated from one another to a large extent. In the test tube they then find their way back together again to form complete organisms. If this procedure is performed on several previously stained individuals simultaneously, the same thing occurs. In this case, however, cells that previously belonged to a particular individual can suddenly show up as part of another.
The Placozoa normally propagate asexually, dividing in the middle to produce two roughly equal-sized daughters. These remain loosely connected for a while after fission. More rarely, budding processes are observed: spherules(소구[小球] 혹은 구형다핵세포[球形多核細胞]) of cells separate from the dorsal surface; each of these combines all known cell types and subsequently grows into an individual of its own.
Sexual reproduction is thought to be triggered by excessive population density. As a result, the animals absorb liquid, begin to swell, and separate from the substrate so that they float freely in the water. In the protected interior space, the ventral cells form an ovum(알 혹은 난자[卵子]) surrounded by a special envelope, the fertilization membrane(수정막[受精膜]); the ovum is supplied with nutrients by the surrounding syncytium, allowing energy-rich yolk(난황[卵黃]) to accumulate in its interior. Once maturation of the ovum is complete, the rest of the animal degenerates, liberating the ovum itself. Small unciliated cells that form at the same time are interpreted to be spermatozoa(정자[精子]). It has not yet been possible to observe fertilization itself; the existence of the fertilization membrane is currently taken to be evidence, however, that it has taken place. Putative eggs have been observed, but they degrade at the 32–64 cell stage. Neither embryonic development nor sperm have been observed [9].
Usually even before its liberation, the ovum initiates cleavage processes in which it becomes completely pinched(죄어드는, 가늘어지는) through at the middle. A ball of cells characteristic of animals, the blastula(포배[胞胚]), is ultimately produced in this manner, with a maximum of 64 cells. Development beyond this 64-cell stage has not yet been observed.
Trichoplax lack a homologue of the Boule protein that appears to be ubiquitous and conserved in males of all species of animals tested in preparation a 2010 paper published in PLoS Genetics [10]. If its absence implies the species has no males, then perhaps its "sexual" reproduction may be a case of the above-described process of regeneration combining cells separated from two separate organisms into one.
Due to the possibility of its cloning itself by asexual propagation without limit, the life span of Placozoa is potentially infinite; in the laboratory, several lines descended from a single organism have been maintained in culture for an average of 20 years without the occurrence of sexual processes.
Long ignored as an exotic(별난), marginal phenomenon(한계현상[限界現狀]), Trichoplax adhaerens is today viewed as a potential biological model organism. In particular, research is needed to determine how a group of cells that cannot be considered full-fledged(깃털이 다 난, 날아갈 수 있을 만큼 성장한) epithelial tissue organizes itself, how locomotion and coordination occur in the absence of true muscle and nerve tissue, and how the absence of a concrete body axis affects the animal's biology. At the genetic level, the way in which Trichoplax adhaerens protects against damage to its genome needs to be studied, particularly with regard to the existence of special DNA-repair processes. Complete decoding of the genome should also clarify the placozoans' place in evolution, which continues to be controversial.
In addition to basic research, this animal could also be suitable for studying wound-healing and regeneration processes; as yet unidentified metabolic products should be researched. Finally, Trichoplax adhaerens is also being considered as an animal model for testing compounds.
Significant genetic differences have been observed between collected specimens matching the morphological description of T. adhaerens, suggesting that it may be a cryptic(암호문의, 이유를 알수 없는) species complex [11].
Because great genetic differences often occur between representatives of Trichoplax adhaerens, differences that in other taxa(분류군[分類群]) would result in their being spread among different genera, it is currently unclear whether the single species based on morphological criteria does not actually correspond to a group of cryptospecies, i.e. species that are not outwardly(외견상) distinguishable from one another. Distribution of the genetic variants is not a function of geography: some variants are found in multiple regions (e.g. Pacific, Caribbean and Red Sea). At the same time, very different genetic variants can be isolated from the same habitat.
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