Zooid

Zooids of subsequent generations perforated the creeping base of the tubes forming the network of the colony (Fig. x).

From: Advances in BioChirality , 1999

Gastrulation: From Embryonic Blueprint to Form

Konner G. Winkley , ... William C. Smith , in Current Topics in Developmental Biology, 2020

4.ane Colonial tunicates

A subset of ascidian species are colonial and grow by repeated rounds of asexual reproduction (budding) that eventually give rise to a colony of genetically identical individuals, called zooids, arranged within a mutual tunic. Each zooid can be sexually mature, but colonial reproduction has several notable differences when compared to solitary species, and these accept hampered embryological studies. First, all colonial ascidians are brooders, and fertilization and development occur within the adult zooids, ofttimes with maternal contributions, such equally a brood pouch, which can be difficult to replicate in vitro. Development is too significantly slower, on the guild of days to a week or more. Finally, the eggs and developing embryos are opaque, hindering high-resolution studies of cell movement. Thus little is known about gastrulation in these species, and in that location are no recently published studies.

More germane to this review is the potential role of a gastrulation-like process during asexual development. During the budding process, colonial ascidians are regenerating all tissues and organs from a population of pluripotent stem cells. An important question is whether asexual evolution is controlled by novel genes and gene regulatory networks, or if embryonic pathways and networks are redeployed (Tiozzo, Brown, & De Tomaso, 2008). Interestingly, there are many colonial species scattered throughout the class Ascidiacea, including different monophyletic orders, suggesting that coloniality has arisen independently multiple times. Comparison of these species reveals a diversity of budding modes, including the source of the new body, and the timing of various developmental landmarks (Tiozzo et al., 2008). However, in all species studied to date, asexually developing zooids go through an initial blastula-like phase. This is followed by invaginations and evaginations of the epithelium to form various tissues and organs. While in a sense this resembles gastrulation, these movements practise not upshot in segregation of cells into presumptive germ layers, as they practise in embryogenesis. The only species in which asexual budding has been studied at a molecular level is Botryllus schlosseri (Manni et al., 2019). In an elegant written report by Ricci, Cabrera, Lotito, and Tiozzo (2016), information technology was found that there is regionalization of germ layer specific transcription factors in the blastula-similar stage and during these epithelial folding events, merely they practice not correspond to the regions that are folding (i.e., it does not appear that the movement is correlated to germ layer specification). In this study, information technology was found that germ layer restricted transcription factors, such as Otx (ectoderm), Fox-A1 (endoderm) and Gsc (mesoderm) are expressed in distinct regions of the blastula-similar structure and could exist followed through organogenesis. Importantly, the expression of each of these germ layer markers corresponded to the source of mature organs during embryogenesis. For instance, the endoderm marking Fob-A1 was initially expressed in a region of the blastula-similar vesicle that somewhen became the gut, and results were equivalent for ectodermal and mesodermal markers.

Given that coloniality has arisen multiple times, it would hardly be viable that completely novel mechanisms for regeneration of every tissue and organ also evolved multiple times, and a more than parsimonious explanation is that this is due to the power to coopt embryonic pathways at unlike times during regeneration. Interestingly though, this does not appear to include a clear recapitulation of embryonic gastrulation.

Read full chapter

URL:

https://world wide web.sciencedirect.com/science/article/pii/S0070215319300742

What is a Part?

Daniel W. McShea , Edward P. Venit , in The Character Concept in Evolutionary Biological science, 2001

2 Parts in Two Bryozoan Species

Eminooecia carsonae is a calcified, branching bryozoan in the form Gymnolaemata (order: Cheilostomata). Cinctipora elegans is also calcified and branching just in the class Stenolaemata (order: Cyclostomata). Function-type lists for two of the zooid types in E. carsonae (autozooids and avicularia) and for the one zooid type present in C. elegans (autozooids) appear in the Appendix. The lists were derived by applying the key (Fig. 4) to anatomical descriptions and drawings of Hippadenella carsonae (reclassified every bit E. carsonae by Hayward and Thorpe, 1988), mainly from Rogick (1957), and of C. elegans, from Boardman et al. (1992). Figure six shows a C. elegans autozooid with many of its structures labeled; many, simply not all, of the labeled structures qualify as parts (see Appendix).

The following give-and-take illustrates how the protocols were used to make decisions about function condition in certain cases, some of them problematic.

Eminooecia carsonae. As is mutual in the cheilostomes, E. carsonae autozooids accept a trapdoor-similar device called an operculum, which closes over the orifice when the lophophore is retracted. The operculum attaches to, and is thus contiguous with, a prepare of occulsor muscle fibers which provide the closing strength. The ii structures differ in composition: the operculum is composed of cuticle, while the muscle fibers are made of muscle cells. Thus, each is classified as a singled-out function.

Zooids in Due east. carsonae are arranged in longitudinal series forth the colony branches. Side by side longitudinal columns are showtime past one-half of a zooid length such that the proximal half of any given zooid is flanked on the right and left by the distal halves of its neighbors (Fig. vii, left). Advice between zooids occurs through four openings in the walls of each zooid, two on each side. The two distal openings are sieve-similar structures chosen rosette plates (Fig. 7, correct). The two proximal openings are elementary holes ringed by a raised annulus. On business relationship of the offset between zooids and their neighbors, the rosette plate from one zooid aligns with the proximal opening of an adjacent zooid, forming a connexion between zooids.

Figure 7. Left: system of zooids along a branch in Due east carsonae. Right: two views of the rosette plate.

 (From Rogick, 1957; reproduced with permission.)

The status of the annulus surrounding the proximal opening and of the rosette plate might seem to be somewhat problematic. Both structures present prominent and distinctive gestalts, strongly suggestive of parts. However, the annulus seems to be equanimous of the aforementioned calcium carbonate as the zooid wall, and therefore is not a singled-out function. Too, the rosette plate apparently does not differ in composition from the zooid wall, and therefore is not a part. Finally, the pigsty in the annulus might seem to authorize as a function, in that its composition differs from that of the annulus itself. Yet, the pigsty is not an object, and therefore cannot be a office.

Cinctipora elegans. Effigy 6 shows the C. elegans digestive tract, consisting of a series of candidate parts: oral cavity, pharynx, cardia, caecum (breadbasket), pylorus, rectum, and anus. The cells of the pharynx are inflated and those of the upper pharynx are ciliated (Boardman et al., 1992). The pylorus is also ciliated, while the remaining digestive-tract candidate parts are not.

The rima oris and anus are openings, not objects, and therefore are not parts. Cell inflation was deemed a sufficient compositional departure for the pharynx to qualify as a singled-out office. If ciliation were also deemed sufficient, then the digestive tract would really consist of five distinct office types: (1) ciliated, inflated-celled, upper throat; (2) unciliated, inflated-celled, lower pharynx; (iii) unciliated cardia and caecum; (4) ciliated pylorus; and (five) unciliated rectum. Even so, ciliation was accounted a minor compositional feature, and thus in our scheme, the digestive tract consists of just two parts: (1) inflated-celled pharynx and (2) the remainder of the tract (cardia, caecum, pylorus, and rectum).

Read full chapter

URL:

https://www.sciencedirect.com/scientific discipline/commodity/pii/B9780127300559500227

Comparative Reproduction

Fabio Gasparini , Loriano Ballarin , in Encyclopedia of Reproduction (2nd Edition), 2018

Pyrosomatida

Pyrosomes, common in tropical and temperate seas, owe their proper name to luminescent organs which contain symbiotic bioluminescent bacteria. These leaner enter the eggs and are transmitted to the new colonies. Colonies have the course of hollow cylinders with i blind stop (Fig. half-dozen).

Fig. 6. Full general morphology of pyrosomes colony (right) and colonial zooid (left).

 Modified from Ballarin, 50., Burighel, P., 2003. Tunicata and Cephalochordata. In: Minelli, A., Contrafatto, G. (Eds.), Biological Science Fundamentals and Systematics. Oxford: Eolss Publishers. Available at: http://www.eolss.net.

Their size ranges from few centimetres to more i metre in length. Zooids are located in the wall of the cylinder, embedded in the common tunic; the oral siphon of each zooid faces outwards, whereas the atrial aperture opens into the primal cloacal crenel: water expelled through the cloacal aperture allows colonial locomotion by propulsion. The general structure of pyrosome zooids is quite similar to that of their ascidian counterparts: they are, hermaphroditic (an ovary and a testis lying ventrally, near the gut) and have a wide branchial basket occupying most of the trunk book ( Fig. 6).

Colonies abound by addition of new zooids that bud from a brusque stolon in the heart region of parental zooids. In a colony the oldest zooids are protandric and located near to the closed end; conversely, younger zooids are protogynous and located near the colonial opening, those in the eye are simultaneously hermaphroditic. Self-fertilisation is possible. Eggs, surrounded by both examination and follicular cells, are provided with abundant yolk. Fertilisation is internal, and the zygote undergoes fractional cleavage in a peribranchial pouch. A tadpole larva is missing and, at the completion of development, a goblet-similar oozooid, named cyatozooid, is released from the atrial siphon. A stolon originates from its endostylar pharyngeal wall that buds four master blastozooids. At this point the new colony reaches superficial waters and grows past repeated cycles of asexual reproduction.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128096338206018

Evolutionary Origin of Asymmetry in Early on Metazoan Animals

Jerzy Dzik , in Advances in BioChirality, 1999

Behavioural disproportion

Behaviour is better known than anatomy in many primitive metazoans of the latest Precambrian and early Palaeozoic. This happens owing to the abundance of marks left in the sediment past benthic organisms during their feeding activity (trace fossils) as well as remnants of their shelters built of collected sediment particles or of secreted organic matrix.

Perhaps the oldest case of ordered behaviour in respect to the left and right sides is that represented by the Tardily Cambrian burrows of Treptichnus [49]. These organisms produced U-shaped tubular cavities inside the mud with walls stiffened with mucus. Faecal pellets occurring at the bottom of some burrows betoken that they were empty during life of the animal. Scratches on their walls approximately in the middle of their length show the presence of about 16 hooks arranged around the trunk. Its likely earliest Cambrian antecedent Trichophycus had much less regular manner of burrowing, with densely packed item burrows [128] whereas the Devonian Treptichnus shows even more than regular blueprint of burrows branching. This is suggestive of a very tiresome directional evolution of behaviour.

The early Palaeozoic graptolites offer an opportunity, unique in the whole living globe, to written report the evolution of a circuitous behaviour in a long geological time bridge. These extinct organisms were shown to be pterobranch hemichordates by Kozłowski [129,130] who pointed out that the fusiform units (fuselli) of their colony skeleton imply the same mode of secretion every bit in Recent Rhabdopleura, that is with the glandular preoral lobe. For a long time the nature of the additional layered tissue (proven to exist collagenous by Towe and Urbanek [131]), which gave graptolite colonies (rhabdosomes) their strength attributable to secondary thickening, remained enigmatic. Eventually, Crowther [132,133 ] documented irregularly meandering 'bandages' which comprehend the rhabdosome surface, apparently corresponding in width to the graptolite animal preoral lobe. Finally, fossilised zooid bodies [ 134] confirmed their hemichordate amalgamation. The pterobranch model seems thus fully consistent with the bachelor evidence [135], which means that all the complexity of the rhabdosome organisation depends solely on the behavioural activity of the animals forming the colony (clone).

Carbonised collagenous skeletons of the graptolite colonies are mutual fossils in Palaeozoic rocks. Of special interest are pelagic graptolites, being widely distributed geographically and thus suitable for evolutionary studies. Their phylogeny is well known [136] and they are traditionally believed to be the fastest evolving fossils organisms of the Ordovician and Silurian and are used every bit guide fossils for the standard stratigraphic subdivision of those periods. Unfortunately, evolutionary studies are rarely performed at the species level and bachelor quantitative data on the morphologic development are very sparse.

Evolution of collagenous pterobranch 'nests'

The evolution from the oldest Middle Cambrian Rhabdopleura-like benthic pterobranchs [137 ] to pelagic graptoloids was the near dramatic at the early postlarval stages of the colony development (astogeny). Originally, the larva produced a dome-shaped skeleton (prosicula) immediately afterward settling over the substrate. Later the metamorphosis was completed, the first zooid perforated its wall and started to secrete a collagenous tube (metasicula) with its pre-oral disc. Zooids of subsequent generations perforated the creeping base of operations of the tubes forming the network of the colony ( Fig. x).

Fig. 10. Diagrammatic presentations of the transformation of astogeny in the evolution from the rhabdopleurid pterobranchs, through benthic graptolites with dimorphic thecae to pelagic graptoloids; three subsequent stages in the colony development are shown.

In the grade of the evolution leading from the rhabdopleurids to the benthic graptolites, a potent size dimorphism developed amid zooids, expressed in the diameter of their tubes (thecae). The dimorphism was apparently sexual and the smaller tubes (bithecae) are assumed to belong to males. Advanced benthic graptoloids developed erect coniform colonies [138] past gradually reducing the creeping phase in their astogeny, until the first perforation developed at the top of cylindrical prosicula. At this stage in the latest Cambrian lineage of Rhabdinopora the development of protosicula ceased to be preceded by settlement of the larva and the whole astogeny proceeded in the water. How this was executed, remains a mystery [139]. With the transition to the pelagic mode of life the male tubes (bithecae) became restricted to the few first generations (frequently they opened into the female tubes) and finally disappeared. This is suggestive of a far-reaching reduction in size of male zooids, unable to produce their own thecae, or their complete disappearance. In any case this would mean the lack of gene period between graptolite colonies, thus the biological species concept does not apply to them! The lack of a mutual cistron pool apparently did not accept whatsoever influence on the rate of evolution of the graptolites. The alternative, that formerly female zooids became hermaphroditic, seems less likely and cannot exist supported by the irregular pattern of variability in graptolite fossil assemblages [140] which is to be expected when interbreeding is lacking.

However, what is the most important for the topics of this review, in the grade of the evolution the astogeny became progressively more and more ordered. The class of the colony, which was apparently of much hydrodynamic importance, was determined already at the beginning of the astogeny, by the direction of budding of the first few generations of zooids. Despite numerous reversals of the evolution, the main tendency in the evolution of the pelagic graptoloids, as nicely shown past Mitchell [141] and Melchin [142], was a reversal in the orientation of the thecae in respect to the probably stabilising fin (nematularium) and progressing reduction in number of thecal rows (stipes). During this evolution, the zooids which initially built their tubes parallel to the sicula, did it in the opposite management, starting from the distal office of the stipes and reaching the stage when already the single tube of the first generation was opposite to the sicula and produced only a unmarried stipe. In a lineage of the graptoloids with extremely thin colony skeleton with two rows of tubes connected together along the mid-line, the number of zooids was reduced and became finite in the astogeny (Holoretiolites). In farthermost cases in that location was actually merely a single fully developed zooid in the colony (Corynites), which ways that coloniality was secondarily lost in a very sophisticated way [143].

An interesting question thus emerges, how precise was the behavioural control of the early astogeny and how it adult in the development. Unfortunately, data on the population variability of the early astogeny in benthic cock graptolites (dendroids) are missing. This is understandable, as their primeval astogenetic stages existence covered with the collagenous cortical tissue, require sophisticated and time consuming techniques to be revealed [144]. It is much easier to study early stages of astogeny in pelagic graptolites, as thousands of juvenile colonies can easily be obtained from a single sample. In the higly ordered early astogeny of the Middle Ordovician graptolites with biserial colonies, the kickoff post-sicular generation zooid perforated the wall of the sicula in location which was presumably adamant by the mode of budding, thus developmentally controlled. In contrast, the sinuous course of the theca was a result of the graptolite creature peculiar behaviour, equally the structure was built-up past the organism with fusellar collagenous units secreted by its preoral lobe glands. The following succession of alternate left and right directions of budding of new zooids and construction of their thecae was apparently a mixture of developmental and behavioural controls. Very few specimens departed from the precisely determined order of budding direction ([145]: p. 266). The final morphology of particular thecae and the whole rhabdosome depended solely on the behavioural mechanisms, although some morphologic gradients are typical for the primeval parts of the colony, suggestive of sicula-born physiologic control [146,147] or accumulation of morphogenetic factors during the colony growth [148,149]. The available bear witness is thus suggestive of an extremely precise control of the early astogeny of the graptolites, including the left–correct side recognition by the zooids amalgam the collageneous colony skeleton with the secretion of their preoral discs.

Behavioural asymmetry had its expression not merely in the style of establishing ('budding') new colony tubes (thecae). In several cases asymmetry of apertural structures developed in the development of graptolites, the best documented being the Belatedly Silurian lineages of Lobograptus, Cucullograptus, and Neocucullograptus. In a series of samples from the borehole Mielnik in eastern Poland, with a very good stratigraphic control, Urbanek [150,151] documented a gradual development of asymmetry in hood-like apertural modifications of those graptolites (Fig. eleven). In each of the three cases asymmetry developed independently from a bilaterally symmetric bequeathed status. Moreover, the lineage of Neocullograptus developed asymmetry in apertural structures which were built in basically different way, as indicated by a unlike microstructure of the collagenous tissue (microfusellar fabric). The control of asymmetry was precise from the beginning and practically all specimens in samples of Cucullograptus and Neocucullograptus have their left apertural lappet enlarged, whereas in the lineage of Lobograptus this was the right lappet which was larger (Fig. 11). Urbanek ([150]: p. 337) interpreted the asymmetry of apertural structures every bit an expression of an anatomical asymmetry of the lophophore. However, as the fuselli were manifestly deposited by an animal crawling over the colony skeleton information technology was a effect of its behaviour that the hood was asymmetric, fifty-fifty if truly an disproportionate anatomy was fit into it afterwards construction.

Fig. eleven. Recurrent development of disproportionate apertural structures in thecae of Late Silurian monograptids (based on [150,151]).

Read full chapter

URL:

https://www.sciencedirect.com/scientific discipline/article/pii/B9780080434049500127

Marine Invertebrates: Genetics of Colony Recognition☆

Rick Grosberg , David Plachetzki , in Reference Module in Life Sciences, 2017

Genetics of Allorecognition in Botryllus schlosseri

The formal and molecular genetics of allorecognition are better understood in the colonial ascidian B. schlosseri than in whatever other invertebrate system. Ascidians are soft-bodied invertebrate chordates (Phylum Chordata) that vest to the Subphylum Tunicata, a clade that diverged from its sister taxon, the Craniata (including the vertebrates), over 600 Ma. The life cycles of botryllid ascidians such as B. schlosseri offering many opportunities for allorecognition behavior to be expressed. First, the fertilized egg develops into a motile tadpole-like larval stage. This tadpole stage possesses all the diagnostic chordate features, including a notochord, a dorsal hollow nervus tube, and pharyngeal gill slits. The larvae swim for a few minutes to hours, and then attach to hard substrates, oft dispersing so little that they settle in the vicinity of their kin, apparently using shared allorecognition alleles to detect their relatives. Once attached, the tadpole metamorphoses into a minute, founding oozooid. During the metamorphic transition to an attached phase, the juvenile Botryllus loses all of its chordate features, except for its gill slits, which it retains for respiration and feeding. The oozooid then asexually buds off additional zooids ( Fig. ane(a)), which in turn bud however more zooids. Repeated cycles of asexual budding ultimately requite ascent to a modular colony of genetically identical zooids, each with its own set up of ovaries and testes. The zooids lie embedded in a gelatinous matrix (the tunic), interconnected by a ramifying and anastomosing blood vascular arrangement (Fig. 1(b)). Colony size has no intrinsic physiological or structural limit; consequently, B. schlosseri colonies can continually grow, often encountering themselves (self-recognition) or conspecifics (allorecognition) equally they expand (Fig. i(c)). When colony edges meet, they interact via ampullae, finger-similar projections of their vascular network (Fig. 2(a)). The ampullae may either fuse, establishing claret flow between the colonies, or turn down, a response accompanied by cytotoxic reactions and the formation of a barrier between incompatible colonies.

Fig. one. Colony construction and ecology of the colonial ascidian Botryllus schlosseri. (a) The adult zooids (the modular, cloned units that etch a colony) of B. schlosseri form star-shaped systems embedded in a gelatinous tunic. Numerous tiny saccular ampullae of the tunic'south interzooidal blood-circulatory complex fringe the colony'south circuitous blood vascular system. (b) A group of adult zooids, asexual buds (which sequentially and synchronously develop into new zooids), and peripheral finger-similar projections of the claret-vascular system called "ampullae" (sites of allorecognition). (c) Competition for spatial resources is violent. Private colonies of B. schlosseri (outlined in white dashed lines) surrounding a single colony of Botrylloides leachi, a species closely related to Botryllus (cherry dashed lines).

Fig. 2. Allorecognition reactions in Botryllus schlosseri. (a) Initial contact between the ampullae of 2 colonies. (b) Vascular fusion between 2 colonies that share one or both alleles at their FuHC locus. (c) Cytotoxic rejection betwixt ii incompatible colonies that lack a shared FuHC allele. Arrows denote initial points of contact.

All-encompassing breeding studies, dating back to the early 1960s, and more contempo genetic mapping experiments, show that the effect of these interactions – fusion or rejection – depends on a single highly polymorphic allorecognition locus, at present called fuhc. Alleles at fuhc are expressed codominantly in B. schlosseri. Upon contact, individuals that share identical sequences in fuhc fuse (Fig. 2(b)), whereas pairs of colonies that do not share an allele pass up (Fig. 2(c)). The DNA sequence of fuhc shows no obvious homology to any known vertebrate locus. Nevertheless, like other genetic systems mediating allorecognition, the fuhc locus displays an extreme level of allelic variation, with several studies yielding estimates of polymorphism in backlog of 100 alleles, and heterozygosities that approach 1.0. These extraordinary levels of genetic polymorphism mean that individuals are just likely to share alleles with themselves (self) and close relatives; thus, this polymorphism permits individuals to discriminate kin relationships with far greater resolution than if fewer alleles were nowadays in the population. As well, considering only a unmarried shared fuhc allele is required for fusion, individuals that are simply related as kin, not clones, can fuse. Thus, genetic chimeras (single colonies composed of multiple genotypes) arise with observable frequency in Botryllus, but are only probable to form between close kin.

Contempo work, together with improvements on the typhoon genome sequence of B. schlosseri, have revealed a number of nuances to our understanding of the mechanism of fuhc factor product action in mediating the allorecognition response. First, fuhc appears to encode a number of expressed proteins that play different roles in mediating allorecognition. In addition, the beginning described genomic element, cfuhc, has recently been shown to encode two proteins, cfuhc sec and cfuhc tm , one secreted and the other membrane bound. cfuhc sec and cfuhc tm are the near polymorphic genes in the fuhc locus and their sequences strongly segregate with fusion/rejection phenotypes, thus explaining much of the upshot of tissue compatibility in B. schlosseri. However the precise molecular details of how cfuhc sec and cfuhc tm proteins mediate histocompatibility outcomes is still unclear (Fig. 3).

Fig. 3. Schematic of the fuhc locus. Tiptop is a genetic map, with the genomic clones making up the minimal tiling path of the physical map shown below. Specific contigs are represented by assuming lines, and the blue line shows the contig in which all polymorphic genes take thus far ben discovered. Predicted genes located in the locus are shown below. Red arrows are putative allorecognition genes, bluish arrows are genes with homologs in the NCBI database (with a threshold of east-ten), and grey arrows are predicted genes unique to Botryllus. The dotted vertical lines marker the boundaries and genetic markers inside a 1.two cM distance. Blackness solid boxes stand for genetic markers. (For estimation of the references to color in this figure fable, the reader is referred to the web version of this article.).

 Reprinted from Taketa, D.A., De Tomaso, A.W., 2015. Botryllus schlosseri allorecognition: Tackling the enigma. Developmental & Comparative Immunology 48 (ane), 254–265, Copyright (2015), with permission from Elsevier.

In add-on, refinements in genomics, including RNAseq and functional gene knock-down approaches, have helped place other genes involved in allorecognition. 2 such proteins are fester and uncle fester. Both fester loci share extensive sequence similarity, likely due to a mutual origin past gene duplication, and both appear to encode membrane bound proteins. fester is highly polymorphic, though to a lesser extent than either cfuhc sec of cfuhc tm . In add-on to sequence polymorphisms, fester expresses a large number of unique mRNA splice products that yield a higher diversity of fester proteins in the population than would be expected from allelic multifariousness lone. Functional studies of fester implicate it in the initiation of the rejection response, only other roles are possible. The other fester locus, uncle fester, shows far less polymorphism than fester and produces far fewer splice-product isoforms. uncle fester besides appears to play a part in rejection, and one current hypothesis is that both fester and uncle fester serve as membrane bound receptors that form heterodimers. Both fester proteins may also collaborate with other genes in the fuhc locus. One of import impediment to understanding the biochemical role of the fester genes in allorecognition is the vast array of splice products they are capable of forming. As such, it is possible that both membrane bound and secreted determinants of allorecognition could derive from the same genes, thus complicating matters immensely.

Other genes in the fuhc locus take also been proposed to function in the allorecognition response based on sequence assay. These include hsp40-fifty and bhf (Botryllus histocompatibility factor). Both genes are highly polymorphic and of unknown function, and sequences of both genes segregate with fusion and rejection phenotypes. Functional gene knock downwardly studies have non been washed for hsp40-Fifty, but such knock-downs led to the abolishment of rejection amongst incompatible pairs in experiments targeting bhf. Interestingly, both hsp40-L and bhf are expressed in the cytoplasm, whereas all other allorecognition proteins discussed to a higher place are either secreted or membrane spring. At that place is clearly much more than to be learned about the complexity and dynamics of fuhc gene function in the allorecgonition response of Botryllus.

Read full affiliate

URL:

https://www.sciencedirect.com/scientific discipline/article/pii/B9780128096338011638

Volume II

Jonathan M.W. Slack , in Eye Evolution and Regeneration, 2010

Two.A Distribution of Regenerative Ability

It is oftentimes believed that regenerative power is extensive in "lower" animals, and becomes lost in "higher" ones, but this is only partly true. More of import than the grade of the organism is the nature of its cell renewal programme. For case, nematodes generally do not regenerate at all, while some of the similarly complex annelids can regenerate all parts of the body from a pocket-sized fragment. Nematodes do their growing during the embryonic and larval periods, and in the adult the cell number is stock-still and determinate. By contrast, those annelids that tin can regenerate extensively likewise practice asexual reproduction by fission, so no new machinery needs to be created to enable regeneration.

The principal types of regeneration are bidirectional, monodirectional and hyperplastic (Slack, 1980) (Fig. 2). Bidirectional means that a severed organism tin can regenerate in both directions, usually a head from an inductive-facing cut surface and a tail from a posterior-facing cutting surface. Monodirectional ways that an identical or at to the lowest degree similar distal structure will regenerate from either side of a cutting surface. Monodirectional regeneration relates mostly to appendages such as arthropod and vertebrate limbs, which always regenerate distally, fifty-fifty if a proximal facing cut surface is engineered by experimenters (Butler, 1955). Hyperplastic regeneration refers to tissues such every bit the mammalian liver (Fausto, 2000), where the microstructure and overall size is regenerated, but the gross anatomical course is not. A distinction is also made between and then-called epimorphic and morphallactic regeneration. Epimorphic regeneration refers to the type of regeneration in which a bud of dividing cells is formed and grows to produce the regenerated construction. This is the example for near instances of bidirectional and monodirectional regeneration. Morphallactic regeneration refers to the type of regeneration which is accomplished by rearrangement of existing cells and is finer bars to hydroids (Holstein et al., 2003).

Figure 2. Types of regeneration: (A) bidirectional; (B,C) monodirectional, the lower part shows the celebrated experiment of (Butler, 1955); (D) hyperplastic. Regenerated parts are shown in orange.

Bidirectional regeneration is truly dramatic, as it involves the ability to regrow the main body axis post-obit a severing of the whole trunk (Berrill, 1951; Holstein et al., 2003). Recently, this process has been extensively studied in the planarian worms (Platyhelminthes) (Reddien and Alvarado, 2004), merely information technology is actually shown near profoundly by many nemertean worms, which tin abound a whole new trunk from a modest fragment (Coe, 1934 ). Of more interest in the present context are the annelid worms, because of their possession of a circulatory system with contractile vessels, and they are discussed further below. Also showing bidirectional regeneration are about of the colonial type of animals, where the organism as a whole consists of a ramifying or branching colony spread out over a substratum, such as a submerged rock. Typically the colonies consist of a number of bodies called zooids, which are bounding main anemone-similar in appearance, joined together past a stolon, which is a tube running across the substratum. The individual zooids have a defined and reproducible beefcake, characteristic of the species in question, merely the colony as a whole may vary considerably in shape and size. Colonial animals include many hydrozoa, bryozoa and some ascidians. These are of phylogenetically very unlike grades. The hydrozoa are a course of the Cnidarians, conventionally regarded as diploblastic. The bryozoa are coelomates. The colonial ascidians are chordates, and and then share some characteristics with the vertebrates. Well-nigh of these colonial animals will regenerate zooids from stolons and stolons from zooids, thus meeting the definition of bidirectionality. Among them, only the ascidians accept hearts.

In bidirectional regeneration the same cutting surface can regenerate either a caput or a tail, depending on its orientation. This ways that there is an essential "polarity command" that makes a conclusion about whether the regenerate is to be a head or a tail. This must operate before the process of new part formation itself begins. Whole trunk bidirectional regeneration is normally associated with the type of asexual reproduction in which the body can spontaneously fragment into two or more than parts, each of which develops to reconstitute a new individual. The requirements of asexual reproduction by fission and of bidirectional regeneration are obviously very like. Bidirectional regeneration is also associated with a continuous turnover of cells in which in that location is a continuous flux from undifferentiated "stem cells" to the principal body parts. This is found in both planarians and hydroids, and probably also in some annelids. In planaria in that location is a considerable literature on the pocket-size undifferentiated cells called neoblasts that are responsible for continuous renewal of the whole torso (Salo and Baguna, 2002; Reddien et al., 2005b). Neoblasts are the just cells that commonly divide, and they are considered to comprise, or include, the stem cells that produce the 12–fifteen histologically-distinguishable cell types making upwardly the tissues of the worm. In the steady state, a worm contains about 20% neoblasts and fourscore% differentiated cells. Information technology is not known whether the neoblast population contains subpopulations committed to particular fates, or whether there is a small stem cell pool feeding a large transit amplifying puddle, as is generally the case in college animals. Following transection of a planarian in that location is an aggregating of undifferentiated cells under the wound epithelium which makes upward the regeneration blastema. Blastema is a general term for a regeneration bud containing undifferentiated proliferating cells, although in planarians the term confusingly refers to an unpigmented distal region in which there is no cell division. The cells populating the planarian blastema come up from the dividing neoblasts in the proximal region. The evidence that neoblasts requite rise to the regenerate comes from ii sources. Outset, BrdU-labeling of neoblasts before amputation leads to a blastema containing many BrdU-labeled cells. Second, the ability of worms to regenerate is destroyed past X-irradiation, which is followed by a rapid disappearance of neoblasts. Afterwards such a radiation dose, there is no further production of new cells and the worms volition die a few weeks later.

Monodirectional regeneration ordinarily involves the replacement of amputated appendages such equally legs, antennae or tails. Information technology is displayed past many species of starfish and breakable star (Echinodermata) that can regenerate missing rays (Thorndyke et al., 2001) which, because of the radial symmetry of these animals, might be considered to be the main trunk. A few types of echinoderm, such as the starfish Linckia, can actually regenerate in a bidirectional manner, forming a whole torso from an isolated ray (Kellogg, 1904). Regeneration of external appendages in insects is bars to the larval stages of Hemimetabola, those families which show a gradual progression to adulthood through a series of larval forms. If a leg or antenna is removed from a locust or cockroach, it will grow again and will become visible at the next moult when the old cuticle is shed (French, 1982; Campbell and Tomlinson, 1995). It is non found in the Holometabolous insects which are those, like Drosophila, showing an abrupt metamorphosis during a pupal stage. Appropriately, Drosophila does not testify whatsoever regeneration of adult structures, although the imaginal discs of the larva can regenerate if they are damaged prior to metamorphosis (Bryant, 1975). Regeneration in vertebrates is displayed mainly past the amphibians. Urodeles (newts and salamanders) are often able to regenerate limbs, tails and jaws, and many other structures, both before and later on metamorphosis (Tsonis, 2000). Anurans (frogs and toads) can usually practise so in the polliwog stage, but lose the ability at metamorphosis (Slack et al., 2004). Lizards are well-known for their power to regenerate tails, but the new tail does not include all of the tissues and structures of the original (Simpson, 1970).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123813329000396

Cephalostatins and Ritterazines

Martín A. Iglesias-Arteaga , Jacek Due west. Morzycki , in The Alkaloids: Chemistry and Biology, 2013

2 Occurrence

The title alkaloids have been found, so far, only in two marine organisms: Cephalodiscus gilchristi (cephalostatins) and Ritterella tokioka (ritterazines). The sometime species belongs to phylum Hemichordata that includes around 90 described species of enteropneusts (acorn worms) and around xxx described pterobranchs (among them C. gilchristi). Similar chordates, hemichordates are deuterostomes with pharyngeal gill slits, and most have a dorsal (and sometimes hollow) nerve cord. However, they lack a notochord. As adults, all or nearly all hemichordates are benthic marine animals. Near pterobranchs live in small colonies of individual zooids within a secreted tube on the ocean flooring at depths of v–5000 g. C. gilchristi is a tiny marine worm (∼five mm long) predominately found in shallow, temperate waters. This unusual and relatively rare tube worm is divided into iii body regions – cephalic shield, collar, and trunk – and displays distinctive tentacled arms emanating from the collar dorsal side. 3 Interestingly, C. gilchristi is not confined to the tube only is contained and can move out of the tube onto the tube surface past emitting a secretion from the sucker-similar proboscis of the buds. Tubes containing colonies of these tiny marine animals are unremarkably found attached to bryozoans or sponges (Fig. 2.ii).

Figure 2.2. Colonies of Cephalodiscus gilchristi

(left; reproduced with consent of Professor Christopher B. Cameron, Canada University of Montreal) and Ritterella tokioka (right; reprinted with permission from the American Chemic Society, Ref. 2i). (For color version of this figure, the reader is referred to the online version of this book.)

Ritterella tokioka is a specific species of tunicate. Tunicates, also known as urochordates, are members of the subphylum Tunicata (Urochordata), a grouping of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata, which also includes the vertebrates. 4 The adult form of most tunicates shows no resemblance to vertebrate animals, merely such a resemblance is axiomatic in the larva. The most familiar tunicates are the sea squirts, or ascidians. Adult body of water squirts are sedentary, filter-feeding, cylindrical or globular animals, unremarkably found attached to rocks, shells, pilings, or boat bottoms. The soft body is surrounded by a thick test, or tunic, ofttimes transparent or translucent and varying in consistency from gelled to leathery. The tunic (for which the tunicates are named) is secreted past the body wall of the developed animal. It is composed of cellulose, an about unique occurrence of that textile in the fauna kingdom. The sedentary and colonial marine tunicate R. tokioka is extremely small-scale (adult ∼i mm) and possesses a elementary body structure, being essentially an unsegmented sac with ii siphons.

C. C. Lambert and G. Lambert reviewed the history of research on hemichordates, cephalochordates, and tunicates likewise as diverse aspects of their biology. 5

Read total chapter

URL:

https://world wide web.sciencedirect.com/science/article/pii/B9780124077744000029

Evolutionary Developmental Biology

Christopher J. Lowe , in Current Topics in Developmental Biology, 2021

2 The animals

The Hemichordata consists of ii major classes, the pterobranchs and the enteropneusts. Enteropneusts are lone marine worms that burrow in sandy or muddy sediment and feed by a combination of filter and deposit feeding ranging in size from a few millimeters to a meter in length (Cannon, Rychel, Eccleston, Halanych, & Swalla, 2009; Kaul-Strehlow & Röttinger, 2015 ). Pterobranchs are often pocket-size colonial animals connected to each other by a common stalk living as a series of inter-connected zooids ( Sato, Bishop, & Holland, 2008).

As someone who has worked on these animals now for 20 years, I can attest to the many challenges of using hemichordates as developmental model species, both at the level of finding, collecting and working with the adult animals, but besides challenges around working with their embryos. Research progress on pterobranchs has been slow largely due to their scarcity, with fewer global sites described, but there may be a diversity of these animals still waiting to be discovered (Tassia et al., 2016). Enteropneusts are broadly distributed globally, simply more easily found on reef flats in tropical regions and intertidally in more than temperate zones. The majority of their multifariousness and biomass may be found subtidally, as intertidal populations generally require protected trophy or inlets. Recent studies have found a surprising deep body of water diverseness, again suggesting further exploration volition uncover additional diversity (Osborn et al., 2012).

The phylogenetic position of hemichordates as sis group to echinoderms within the Ambulacraria is well established (Bromham & Degnan, 1999; Cameron, Garey, & Swalla, 2000; Cannon et al., 2014; Furlong & Holland, 2002) forming the Ambulacraria. Too, their close human relationship with chordates generally well supported by phylogenomic analyses (Fig. 1A ). However, their relationship to acoels and Xenoturbellids remains controversial with some studies grouping Xenoturbellids into the deuterostomes, and some group acoels and Xenoturbellids every bit a monophyletic group (Xenacoelomorpha) into deuterostomes (Bourlat et al., 2006; Dunn et al., 2008; Mwinyi et al., 2010; Philippe et al., 2011, 2019) or as basally branching bilaterians (Hejnol et al., 2009; Rouse, Wilson, Carvajal, & Vrijenhoek, 2016; Simakov et al., 2015; Srivastava, Mazza-Curll, van Wolfswinkel, & Reddien, 2014). Nevertheless the cadre grouping of deuterostome taxa composed of chordates, hemichordates and echinoderms has been stable for some time. However, a recent preprint (Kapli et al., 2020) proposes that support for the classical deuterostome clade is equivocal, and an alternative radical difference to this classical grouping is equally probable; namely a possible closer affinity of chordates to protostomes, leaving the Ambulacraria as a clade dissever from chordates and paraphyletic deuterostomes. Such a reorganization would require a reconsideration of how we call back about chordate origins, and claim a thorough investigation, but is exterior of the scope of this review.

Fig. 1

Fig. 1. Hemichordate torso program and phylogenetic relationships. (A) Summary of phylogenetic relationships of deuterostome taxa based on Philippe et al. (2011) and Hejnol et al. (2009). (B) Phylogenetic relationships within hemichordates showing the relationships between pterobranchs and enteropneusts, and the interrelationships betwixt enteropneust families. Based on Cannon et al. (2014). (C) Diagrams of a generalized enteropneust showing the organisation of the developed body program and primal anatomical features including the nervous system showing wide distribution of a neural plexus and ii nerve cords.

 Panel A and C: figure modified from Lowe, C.J., Clarke, D.N., Medeiros, D.M., Rokhsar, D.S., & Gerhart, J. (2015). The deuterostome context of chordate origins. Nature, 520, 456–465.

Our understanding of the relationships inside hemichordates is now greatly improved with better sampling, and while earlier molecular studies suggested that pterobranchs may nest within enteropneusts (Cameron et al., 2000), a more recent study organizes pterobranchs and enteropneusts into two monophyletic groups (Cannon et al., 2014). Enteropneusts are organized into 4 families; the Harrimanidae, Spengelidae, Ptychoderidae, with the deep sea Torquaratoridae nested within the Ptychoderidae (Fig. 1B) (Cannon et al., 2014).

Read full chapter

URL:

https://world wide web.sciencedirect.com/science/article/pii/S0070215320301356

Comparative Reproduction

Susan D. Hill , ... Daniel H. Shain , in Encyclopedia of Reproduction (Second Edition), 2018

Clitellata

Clitellate annelids are distinguished from other worms by their characteristic reproductive structure, the clitellum, whose role is to secrete an egg sheathing, or cocoon. The clitellum appears equally a thickened sleeve or saddle a few segments in length within the anterior tertiary region of the worm. During periods of sexual maturity, it swells with thousands of glands that secrete proteinaceous material for edifice the cocoon (Coleman and Shain 2009; Rossi et al., 2013).

Clitellates are hermaphrodites (i.e., individuals containing both male and female reproductive structures) and both male and female gonopores go out from the clitellum. Like polychaetes, most clitellates reproduce sexually merely some proliferate apace by asexual fragmentation or budding, while others (e.thousand., Naididae) can alternate between sexual and asexual modes of reproduction depending upon environmental atmospheric condition (Parish, 1981).

Sexual Reproduction

Though some clitellates are capable of self-fertilization (e.g., Tubifex tubifex) or parthenogenesis, almost cross fertilize by copulation. This involves physical contact betwixt ii individuals and the mutual exchange of sperm, ordinarily via an intromittent organ or past the transfer of a sperm-filled sac chosen a spermatophore. Spermatophores are cemented to each other in the prostate seedling and transferred to the ventral body surface of the partner, typically in the clitellar region. Sperm exit the spermatophore, and past a mechanism not fully understood find their way via the coelomic sinuses to ovaries, where the eggs are fertilized internally.

Spermatogenesis occurs in paired testisacs and resulting spermatozoa pass through structures including the vas deferens and atrium before longer term storage in the epididymis (Fig. iv). Paired ovisacs give rise to oogonia and oocytes, which laissez passer through oviducts that converge and lead to the female pore. Eggs may acquire yolk past a process chosen vitellogenesis and tin be quite large (up to several mm in some Glossiphoniids), or yolk may be deposited into cocoon fluid equally a nutritional source for developing embryos (albumenotrophy). Sperm from a partner may be stored in specialized compartments chosen spermathecae, and released during egg laying to fertilize eggs internally or in the cocoon.

Fig. 4

Fig. four. Dorsal view of a clitellate (Helobdella sp.) reproductive arrangement (A) Position of reproductive arrangement in body. (B) Dorsal dissection. (C) Schematic reproductive organs. a, atrium; ac, atrial cornua; bone, ovisac; od, oviduct; sd, sperm duct; t, testisac; vd, deferens.

Asexual Reproduction

Many clitellates, excluding leeches, can undergo asexual reproduction past fragmentation, paratomic division (forming a chain of zooids), or parthenogenesis. For naidids, fragmentation is the dominant form of reproduction and enables them to rapidly populate an environment under favorable conditions ( Parish, 1981). Fragmenting worms are too a model system for studying regeneration (e.yard., Lumbriculus variegatus), since both anterior and posterior ends can fully regenerate from as petty as 2 midbody segments!

Cocoon Production

In coordination with egg laying, clitellates secrete a proteinaceous cocoon that provides a microenvironment for embryonic evolution (Coleman and Shain, 2009). The process initiates by the proliferation of clitellum-specific granular cells that differentiate into two cell types, i of which makes gristly protein that builds the cocoon wall and another that secretes a glue-like substance to seal the cocoon ends. Several one thousand thousand granules of each fabric type (i.eastward., cocoon wall, glue) are necessary to construct a single cocoon. Secreted wall granules initially self-assemble to form a sheath around the clitellum. Upon the release of eggs and cocoon fluid from the female pore into the sheath, the worm pulls its head through while simultaneously sealing both ends of the sheath with granulated structures called opercula (Sawyer, 1986). In some leeches (e.g., Helobdella), the cocoon is secreted from a ring of granular cells surrounding the female pore and forms a sac that is sealed with a single operculum. Typically merely a few cocoons are secreted over the course of several hours, but up to ∼fifty have been reported in some worms (e.grand., Myzobdella lugubris). The number of eggs deposited into each cocoon ranges from one to more than 100, depending on the species.

Clitellate cocoons display a variety of morphologies (Fig. 5) but fall into 3 general categories: hard-shelled, membranous and gelled (Siddall and Burreson, 1996). These are distinguished more often than not by the thickness of the fibrous cocoon wall. In some terrestrial taxa (e.thou., Hirudo, Haemopis), a spongy matrix surrounds the cocoon and appears to forbid dessication past trapping water aerosol. The components of clitellate cocoons display extraordinary physiochemical properties (e.g., thermal, chemical resilience), making them valuable resource as biomaterials and bioadhesives.

Fig. 5

Fig. five. Morphology of representative clitellate cocoons. (A) Erpodbella punctata. (B) Theromyzon tessulatum (C) Tubifex tubifex, with juvenile exiting. (D) Eisenia fetida. Arrows place opercula. Scale bar=100 mm.

Parental Care

Annelids tend to show parental intendance of some form, ranging from brooding embryos in tubes to placing them in cocoons. Glossiphoniid leeches (e.grand., Haementeria, Helobdella, Theromyzon) breed their cocoons until embryos hatch, at which time up to several hundred juveniles may attach to the venter of the parent. Throughout the care period, the parent provides protection from predators and ventilates the eggs/young to ensure that they receive sufficient oxygen. In the sanguivorous duck leech, Theromyzon, the parent brings juveniles to their kickoff blood meal, a duck nose, earlier terminating its life cycle.

Some fish leeches (e.chiliad., Piscicolidae) showroom an adaptation that facilitates an early blood meal. Rather than abandoning their cocoons as almost oligochaetes do, they instead cement them to the surface of crustaceans, which are afterwards eaten past fishes. When the young leeches hatch, they readily attach to the fish host's buccal surfaces and migrate to the gills. In some Syllidae oligochaetes, eggs are not released just rather attach to the posterior cease of the worm where they develop into juveniles before becoming independent.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128096338206481

Origin and Functions of Tunicate Hemocytes

Francesca Cima , ... Loriano Ballarin , in The Development of the Immune Organization, 2016

3 Ascidian hemocytes

Ascidian hemocytes are involved in a diverseness of functions, such as: (1) storage and transport of nutrients and catabolites, (two) asexual reproduction, (3) tunic synthesis, (four) allorecognition, and (5) defense reactions. 15–17

3.1 Hematopoietic Tissues

Hematopoietic sites are mainly located in peripharyngeal regions, conspicuously axiomatic in solitary species every bit "lymphatic nodules," or clusters of stem cells surrounded by hemocytes at various developmental stages. 12,xviii,19 In colonial species of the genus Botryllus, mitotic figures accept been occasionally observed in circulating hemocytes. 20 In the same species, stem prison cell niches accept been identified close to the adult endostyle. 21–23 From this location, they colonize the cell islands, that is, aggregates of hemocytes located forth the sides of the endostyle, from which they enter the circulation and contribute to various zooid tissues, germline and hemolymph included. 23

iii.2 Hemocyte Morphology

Many investigators studied the morphology of living and fixed hemocytes and their ultrastructure. 16,17 Various authors have also proposed unifying classification frameworks, xi,12,24 but a certain degree of incertitude even so persists on the terminology and differentiation pathways of ascidian hemocytes. In our opinion, they can be grouped in the post-obit categories: undifferentiated cells, immunocytes (i.eastward., cells involved in allowed defense force), and storage cells (Fig. ii.2).

Effigy 2.2. Circulating hemocytes of the colonial ascidian Botryllus schlosseri.

(A–C, Eastward) stock-still hemocytes stained with Giemsa solution; (D, F, G) living hemocytes. (A) hemoblasts; (B) hyaline amoebocytes; (C) macrophage-like cell; (D) living morula cell; (E) stock-still morula cells; (F) nephrocyte; (G) blue-pigment cell. Scale bar: 10 μm.

Undifferentiated cells, known as hemoblasts, feature a loftier nucleus/cytoplasmic ratio, a well-defined nucleolus, and a basophilic cytoplasm (Fig. 2.2A). The at present obsolete term "lymphocyte" has also been used in the literature, both as a synonymous of the term "hemoblast" and to indicate a young hemocyte having entered a differentiation pathway. 18,25,26 In Botrylloides leachii and Botryllus schlosseri, hemoblasts show positivity to antibodies against mammalian CD-34. 16,23 In B. schlosseri, new hemoblasts cyclically enter the circulation to replace the hemocytes that undergo apoptosis at the generation change. 27

Circulating immunocytes include both phagocytes and cytotoxic cells.

Phagocytes tin can assume both a spreading and a circular morphology. Spreading phagocytes are also known as hyaline amoebocytes: they are multifunctional cells that can actively move toward foreign cells or particles and ingest them. Their cytoplasm contains fine cytoplasmic granules, unresolvable under the lite microscope, and shows positivity for lysosomal enzyme activities 15,17,28 (Fig. 2.2B). Upon the ingestion of foreign material, they withdraw their projections, assuming a round morphology (in this stage, they are also called macrophages or macrophage-like cells). Circular phagocytes are large cells, with one or more than phagosomes (Fig. 2.iiC). A detailed written report of the properties of round phagocytes in the colonial ascidians B. leachii and B. schlosseri confirmed the presence of hydrolytic enzymes, too as of lipids and lipofuscins inside their phagosomes. 15–17 In B. schlosseri, the presence of a static and a mobile population of phagocytes have been described, the former homing in ventral islands, on both sides of the endostyle. 29

Cytotoxic cells are vacuolated cells containing the enzyme phenoloxidase (PO). They constitute one of the most abundant (sometimes more than fifty%) circulating hemocyte-types and are represented, in most cases, by morula cells (MCs), so-called for the drupe-like morphology they assume afterwards fixation (Fig. 2.iiD–E). In the solitary ascidian Ciona intestinalis, PO-containing cells are represented past univacuolar refractile granulocytes, whereas in Phallusia mammillata, compartment cells and granular amoebocytes comprise the enzyme. 31–33 MCs have a large diameter (10–15 μm) and cytoplasm filled by many vacuoles, compatible in size (effectually 2 μm in diameter) where the enzyme PO, probably stored as an inactive precursor (proPO), resides. 33 In botryllid ascidians, granular amoebocytes are considered to be the directly precursors of MCs, considering they share with the latter similar cytochemical properties and mutual enzymatic content within their granules. 15–17

Storage cells, that is, circulating hemocytes that practice not exert allowed-related functions, consist of vanadocytes, nephrocytes, paint cells, and trophocytes.

Vanadocytes, present in Enterogona, can accumulate vanadium (V) upward to a concentration of 350 mM (the mean V concentration of seawater is 35 nM) inside their vacuoles, complexed with 5-binding proteins. The office of such loftier V concentration is still unexplained. 34–37

Nephrocytes, 10–15 μm in diameter, accumulate urate crystals in Brownian motility inside large vacuoles (Fig. 2.iiF). They represent a sort of "circulating kidney," as tunicates lack a defined excretory arrangement. 11,12,38,39 Nephrocytes can get out the apportionment and contribute to the ascidian pigmentation. 40

Pigment cellsare large hemocytes (upwards to 40 μm in bore) containing blueish, orangish, and/or reddish pigment granules, either in the form of cytoplasmic globules or as crystals in Brownian motion within a few large vacuoles 11,12,41 (Fig. 2.2Yard). Their frequency tends to increase with the age of the organism/colony.

Trophocytes represent the most abundant circulating cells of many colonial ascidians. They are large (10–xv μm in bore), circular cells with the cytoplasm filled with many granules, which store nutrients. They sustain bud morphogenesis, peculiarly in species such as Polyandrocarpa misakiensis, where buds detach very early from the parental organism. 42–44

Read full chapter

URL:

https://www.sciencedirect.com/science/commodity/pii/B9780128019757000025