Trichoplax is a genus placed in the phylum Placozoa. The species Trichoplax adhaerens is one of four Placozoa species, alongside Hoilungia hongkongensis, Polyplacotoma mediterranea and Cladtertia collaboinventa. Placozoa is a basal group of multicellular animals, possible relatives of Cnidaria. Trichoplax are very flat organisms commonly less than 4 mm in diameter, lacking any organs or internal structures. They have two cellular layers: the top epitheloid layer is made of ciliated "cover cells" flattened toward the outside of the organism, and the bottom layer is made up of cylinder cells that possess cilia used in locomotion, and gland cells that lack cilia. Between these layers is the fibre syncytium, a liquid-filled cavity strutted open by star-like fibres.
Trichoplax feed by absorbing food particles—mainly microbes—with their underside. They generally reproduce asexually by dividing or budding, but can also reproduce sexually. 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 at the Zoological Institute in Graz, Austria. The generic name is derived from the classical Greek (), "hair", and (), "plate". The specific epithet adhaerens is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination.
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 of the anemone-like hydrozoan 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).
Morphology
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. Because they are so thin and fragile, and because the cilia which they use for locomotion are only loosely coordinated, they are constantly being split into two or three separate clones when their cilia moves in opposite directions, causing microfractures in the animal's epithelium. One hypothesis is that the larger a motile animal lacking a nervous system is, the less coordinated its locomotion becomes, placing an upper limit on their possible size. 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 single-celled 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 from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers: dorsal epithelia cells and ventral epithelia cells, each with a single cilium ("monociliate"), ventral gland cells, syncytial fiber cells, lipophils, and crystal cells (each containing a birefringent crystal, arrayed around the rim). Lacking sensory and muscle cells, it moves using cilia on its external surface. The collective movements of the cilia are completely coordinated by mechanical interactions.
Signal processing There are no neurons present, but in the absence of a nervous system the animal uses short chains of amino acids known as peptides for cell communication, in a manner resembling the way animals with neurons use neuropeptides for the same purpose. These specialized cells are called peptidergic cells. Unlike neurons, the cells do not use electrical impulses and their messaging is restricted to sending signals to other nearby cells, as they are unable to both send and receive signals. Individual cells contain and secrete a variety of small peptides, made up of between four and 20 amino acids, which are detected by neighbouring cells. Each peptide can be used individually to send a signal to other cells, but also sequentially or together in different combinations, creating a huge number a different types of signals. This allows for a relatively complex behavioural repertoire, including behaviours such as "crinkling", turning, flattening, and internal "churning". The genome of Trichoplax codes for eighty-five neurotransmitter receptors, more than in any other sequenced animal.
Epitheloid Both structurally and functionally, it is possible to distinguish a back or dorsal side from a belly or ventral side in Trichoplax adhaerens. Both consist of a single layer of cells coated on the outside with slime and are reminiscent of epithelial tissue, primarily due to the junctions—belt desmosomes—between the cells. In contrast to true epithelium, however, the cell layers of the Placozoa possess no basal lamina, 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, 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, Placozoa species have an epitheloid.
A mature individual consists of up to a thousand cells that can be divided into four different cell types. The monociliated cells of the dorsal epitheloid are flattened and contain lipid bodies. The cells on the ventral side likewise possess a single cilium, while their elongated columnar shape, with a small cross section at the surface, packs them very closely together, causing the cilia to be very closely spaced on the ventral side and to form a ciliated "crawling sole". Interspersed among these ventral epithlioid cells are unciliated gland cells thought to be capable of synthesizing digestive enzymes.
Fibre syncytium Between the two layers of cells is a liquid-filled interior space, which, except for the immediate zones of contact with the ventral and dorsal sides, is pervaded by a star-shaped fibre syncytium: a fibrous network that consists essentially of a single cell but contains numerous nuclei that, while separated by internal crosswalls (septa), do not have true cell membranes 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 septa to resemble synapses, i.e. nerve-cell junctions that occur in fully expressed form only in animals with tissues (Eumetazoa). Striking accumulations of calcium ions, which may have a function related to the propagation of stimuli, likewise suggest a possible role as protosynapses. This view is supported by the fact that fluorescent antibodies against cnidarian neurotransmitters, 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. The fibre syncytium also 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 fibres can relax or contract and thus help determine the animals' shape.
In this way, the fibre 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 exists of the kind observed, in mesoglea, in cnidarians and ctenophores.
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 epithelioid cells arise only from other cells of the same type.
Genetics The Trichoplax genome contains about 98 million base pairs and 11,514 predicted protein-coding genes.
All nuclei of placozoan cells contain six pairs of chromosomes that are only about two to three micrometres in size. Three pairs are metacentric, meaning that the centromere, the attachment point for the spindle fibers in cell division, is located at the center, or acrocentric, with the centromere at an extreme end of each chromosome. The cells of the fiber syncytium can be tetraploid, 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 symmetrical Bilateria. 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 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. No clarification was achieved with standard markers such as 18S rDNA/RNA: the marker sequence was apparently "garbled", 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.