Phagocyte dynamics in the blastogenetic cycle of the colonial ascidian Botryllus schlosseri: Cell senescence, segregation and clearance after efferocytosis.

In the colonial ascidian Botryllus schlosseri, phagocytes are involved in the clearance of apoptotic cells and corpses during the periodical generation changes or takeovers (TOs) that assure the renewal of the colonial zooids. The persistent respiratory burst associated with efferocytosis, leads to the induction of senescence. Indeed, giant, senescent phagocytes are abundant in the colonial circulation at TO, whereas, in the other phases of the colonial blastogenetic cycle, they colonise the ventral islands (VIs), a series of mesenchymal niches located in the lateral lacunae of the mantle, on both sides of the subendostylar sinus.

Sixty years of experimental studies on the blastogenesis of the colonial tunicate Botryllus schlosseri.

In the second half of the eighteenth century, Schlosser and Ellis described the colonial ascidian Botryllus schlosseri garnering the interest of scientists around the world. In the 1950's scientists began to study B. schlosseri and soon recognized it as an important model organism for the study of developmental biology and comparative immunology.

Differentiation and Induced Sensorial Alteration of the Coronal Organ in the Asexual Life of a Tunicate.

Tunicates, the sister group of vertebrates, possess a mechanoreceptor organ, the coronal organ, which is considered the best candidate to address the controversial issue of vertebrate hair cell evolution. The organ, located at the base of the oral siphon, controls the flow of seawater into the organism and can drive the "squirting" reaction, i.e.

Sexual and asexual reproduction in the colonial ascidian Botryllus schlosseri.

The colonial tunicate Botryllus schlosseri is a widespread filter-feeding ascidian that lives in shallow waters and is easily reared in aquaria. Its peculiar blastogenetic cycle, characterized by the presence of three blastogenetic generations (filtering adults, buds, and budlets) and by recurrent generation changes, has resulted in over 60 years of studies aimed at understanding how sexual and asexual reproduction are coordinated and regulated in the colony. The possibility of using different methodological approaches, from classical genetics to cell transplantation, contributed to the development of this species as a valuable model organism for the study of a variety of biological processes.

Cytodifferentiation of hair cells during the development of a basal chordate.

Tunicates are unique animals for studying the origin and evolution of vertebrates because they are considered vertebrates' closest living relatives and share the vertebrate body plan and many specific features. Both possess neural placodes, transient thickenings of the cranial ectoderm that give rise to various types of sensory cells, including axonless secondary mechanoreceptors. In vertebrates, these are represented by the hair cells of the inner ear and the lateral line, which have an apical apparatus typically bearing cilia and stereovilli.

Evolutionary diversification of secondary mechanoreceptor cells in tunicata.

Background: Hair cells are vertebrate secondary sensory cells located in the ear and in the lateral line organ. Until recently, these cells were considered to be mechanoreceptors exclusively found in vertebrates that evolved within this group. Evidence of secondary mechanoreceptors in some tunicates, the proposed sister group of vertebrates, has recently led to the hypothesis that vertebrate and tunicate secondary sensory cells share a common origin.

Evolutionary conservation of the placodal transcriptional network during sexual and asexual development in chordates.

Background: An important question behind vertebrate evolution is whether the cranial placodes originated de novo, or if their precursors were present in the ancestor of chordates. In this respect, tunicates are of particular interest as they are considered the closest relatives to vertebrates. They are also the only chordate group possessing species that reproduce both sexually and asexually, allowing both types of development to be studied to address whether embryonic pathways have been co-opted during budding to build the same structures.

The oral sensory structures of Thaliacea (Tunicata) and consideration of the evolution of hair cells in Chordata.

We analyzed the mouth of three species, representative of the three orders of the class Thaliacea (Tunicata)--Pyrosoma atlanticum (Pyrosomatida), Doliolum nationalis (Doliolida), and Thalia democratica (Salpida)--to verify the presence of mechanoreceptors, particularly hair cells. In vertebrates, hair cells are well-known mechanoreceptors of the inner ear and lateral line, typically exhibiting an apical hair bundle composed of a cilium and stereovilli but lacking an axon. For a long time, hair cells were thought to be exclusive to vertebrates.

Expression of a Musashi-like gene in sexual and asexual development of the colonial chordate Botryllus schlosseri and phylogenetic analysis of the protein group.

Tunicates are the unique chordates to possess species reproducing sexually and asexually. Among them, the colonial ascidian Botryllus schlosseri is a reference model for the study of similarities and differences in these two developmental pathways. We here illustrate the characterization and expression pattern during both pathways of a transcript for a gene orthologous to Dazap1.

Germline cell formation and gonad regeneration in solitary and colonial ascidians.

The morphology of ascidian gonad is very similar among species. The testis consists of variable number of testicular follicles; the ovary consists of ovarian tubes that are thickened forming the germinal epithelium with stem cells for female germ cells with the exception of botryllid ascidians. Peculiar accessory cells that would be germline in origin accompany the oocytes.

Differentiation of papillae and rostral sensory neurons in the larva of the ascidian Botryllus schlosseri (Tunicata).

During the metamorphosis of tunicate ascidians, the swimming larva uses its three anterior papillae to detect the substrate for settlement, reabsorbs its chordate-like tail, and becomes a sessile oozooid. In view of the crucial role played by the anterior structures and their nerve relations, we applied electron microscopy and immunocytochemistry to study the larva of the colonial ascidian Botryllus schlosseri, following differentiation of the anterior epidermis during late embryogenesis, the larval stage, and the onset of metamorphosis. Rudiments of the papillae appear in the early tail-bud stage as ectodermic protrusions, the apexes of which differentiate into central and peripheral bipolar neurons.

Muscle differentiation in a colonial ascidian: organisation, gene expression and evolutionary considerations.

Background: Ascidians are tunicates, the taxon recently proposed as sister group to the vertebrates. They possess a chordate-like swimming larva, which metamorphoses into a sessile adult. Several ascidian species form colonies of clonal individuals by asexual reproduction.

Vascular regeneration and angiogenic-like sprouting mechanism in a compound ascidian is similar to vertebrates.

Tunicates are useful models for comparing differing developmental processes such as embryogenesis, asexual reproduction, and regeneration, because they are the closest relatives to vertebrates and are the only chordates to reproduce both sexually and asexually. Among them, the ascidian Botryllus schlosseri displays high regenerative potential of the colonial circulatory system (CCS). The CCS runs in the common tunic, forming an anastomized network of vessels defined by simple epithelia and connected to the open circulatory system of the zooids.

Does hair cell differentiation predate the vertebrate appearance?

It is generally accepted that the three main chordate groups (tunicates, cephalochordates and vertebrates) originated from a common ancestor having the basic features of the chordate body plan, i.e. a neural tube and a notochord flanked by striated musculature.

A tale of death and life: natural apoptosis in the colonial ascidian Botryllus schlosseri (Urochordata, Ascidiacea).

The colonial ascidian Botryllus schlosseri forms new zooids by blastogenesis, through the formation of palleal buds which progressively grow and mature until adults are formed. At a temperature of 19 degrees C, adult zooids remain active for about one week; then they contract, close their siphons and are gradually resorbed, being replaced by buds which reach functional maturity, open their siphons and begin their filtering activity as adult zooids. This recurrent generation change, known as take-over, is characterised by the occurrence of diffuse programmed cell death by apoptosis.

Haemocytes and blastogenetic cycle in the colonial ascidian Botryllus schlosseri: a matter of life and death.

A recurrent blastogenetic cycle characterizes colonies of the ascidian Botryllus schlosseri. This cycle starts when a new zooid generation opens its siphons and ends with take-over, when adult zooids cease filtering and are progressively resorbed and replaced by a new generation of buds, reaching functional maturity. During the generation change, massive apoptosis occurs in the colony, mainly in the tissues of old zooids.

Tubular sprouting as a mode of vascular formation in a colonial ascidian (Tunicata).

Although phylogenetically related to vertebrates, invertebrate chordate tunicates possess an open circulatory system, with blood flowing in lacunae among organs. However, the colonial circulatory system (CCS) of the ascidian Botryllus schlosseri runs in the common tunic and forms an anastomized network of vessels, defined by simple epithelium, connected to the open circulatory system of the zooids. The CCS originates from epidermal evagination, grows, and increases its network accompanying colony propagation.

Common and divergent pathways in alternative developmental processes of ascidians.

Colonial ascidians offer opportunities to investigate how developmental events are integrated to generate the animal form, since they can develop similar individuals (oozooids from eggs, blastozooids from pluripotent somatic cells) through very different reproductive processes, i.e. embryogenesis and blastogenesis.

Coronal organ of ascidians and the evolutionary significance of secondary sensory cells in chordates.

A new mechanoreceptor organ, the coronal organ, in the oral siphon of some ascidians belonging to the order Pleurogona has recently been described. In contrast to the known mechanoreceptor organs of ascidian atrium that consist of sensory neurons sending their own axons to the cerebral ganglion, coronal sensory cells are secondary mechanoreceptors, i.e.

RGD-containing molecules induce macropinocytosis in ascidian hyaline amoebocytes.

Phagocytes of the compound ascidian Botryllus schlosseri are capable of constitutive macropinocytosis (MP) at sites of membrane ruffling along the leading edge. This gives rise to the formation of initially irregular vesicles which then move to the inside of the cells and acquire a more regular morphology. Both phagocyte spreading and MP are enhanced by the recognition of molecules containing the sequence Arg-Gly-Asp (RGD): this suggests that, as in mammals, integrin activation is involved in the induction of both cell spreading and endocytosis.

External amebocytes guard the pharynx entry in a tunicate (Ascidiacea).

In the present report, we describe the identification of unusual free amebocytes, completely exposed to seawater, which inhabit the inner surface of the oral and atrial siphons of the compound ascidian Botryllus schlosseri (Urochordata). The origin and biological role of these cells were studied by cytochemical and ultrastructural analysis. These amebocytes are mononucleate cells, with numerous round granules, varying in content, and long filopodia, which contact the cuticle protrusions of the tunic in the siphon.

Stomodeal and neurohypophysial placodes in Ciona intestinalis: insights into the origin of the pituitary gland.

The ascidian larva has a central nervous system which shares basic characteristics with craniates, such as tripartite organisation and many developmental genes. One difference, at metamorphosis, is that this chordate-like nervous system regresses and the adult's neural complex, composed of the cerebral ganglion and associated neural gland, forms. It is known that neural complex differentiation involves two ectodermal structures, the neurohypophysial duct, derived from the embryonic neural tube, and the stomodeum, i.

Embryonic versus blastogenetic development in the compound ascidian Botryllus schlosseri: insights from Pitx expression patterns.

The colonial ascidians reproduce either sexually or asexually, having evolved a rich variety of modes of propagative development. During embryogenesis, the fertilized egg develops into a swimming tadpole larva that subsequently metamorphoses into a sessile oozooid. Clonal individuals (blastozooids), resembling oozooids, are formed from few bud-forming multipotent somatic cells, following a wide range of ways that seem to characterize each family of this class.