Patterning and morphogenesis of the intricate but stereotyped oikoplastic epidermis of the appendicularian, Oikopleura dioica

Abstract

Mechanisms for morphogenetic processes that generate complex patterns in a reproducible manner remain elusive. Live imaging provides a powerful tool to record cell behaviors. The appendicularian, Oikopleura dioica, is a planktonic tunicate that has a rapid developmental speed, small number of cells (less than 3500 cells in a juvenile), and a transparent body. The trunk epidermis, called the oikoplastic epithelium (OE), has elaborate cellular arrangements showing a complex pattern to secrete so-called "house" made of extracellular components. The OE is characterized by invariant number, size, and shape of the monolayer epithelial cells. Pattern formation is achieved during 5h of larval development without growth of the body, making this a suitable system for live imaging of a two-dimensional (2D) sheet. First, we subdivided the OE and defined several domains by cellular resolution, and systematically gave names to the constituent cells, since there is no variation among individuals. Time-lapse imaging of the epidermal cells revealed region-specific pattern formation processes. Each identified domain served as a compartment into which distribution of descendant cells of founder cells is restricted. Regulation of orientation, timing, and the number of rounds of cell divisions, but not cell death and migration, was a critical mechanism for determination of final cell arrangement and size. In addition, displacement of epithelial sheet plates was observed in the Eisen domain. Stem-cell-like cell divisions, whereby large mother stem cells generate a chain of small daughter cells, were involved in formation of the Nasse region and ventral sensory organ. These are the first examples of this kind of stem-cell-like cell division in deuterostomes. Furthermore, labeling of the left or right blastomere of the two-cell-stage embryo, which roughly gives rise to the left or right side of the body, respectively, revealed that the boundary of the descendant cells does not match with the midline of the trunk epidermis. Left and right descendants largely invade into the opposite side in an invariant way, suggesting the possibility that specification of the OE cell identities may occur later in development, most probably around hatching, and depending on cell position in the OE epithelial sheet. These detailed descriptions of OE patterning processes provide basic and essential information to analyze further cell behaviors in the generation of elaborate and intricate but stereotyped 2D cellular patterns in this advantageous model system for developmental and cell biological studies in chordates.

The filter‐house of the larvaceanOikopleura dioica. A complex extracellular architecture: From fiber production to rudimentary state to inflated house

“…Recently, Kishi et al (2017) These proteins seem to be specific to appendicularians, because in a comparison between the genomes of Oikopleura dioica and Ciona robusta (then still called C. intestinalis, but see Brunetti et al, 2015, Hosp et al (2012) could not identify homologous genes in the C. robusta genome. However, the authors showed that Linking this molecular level knowledge to higher structural levels has been attempted, yet little can be stated in this respect.…”

“…Recently, Kishi et al (2017) have investigated the ontogenetic pattern of cell divisions and movement that leads to the stereotypic arrangement of oikoplastic cells and presented a clear nomenclature for the individually recognizable cells based on the positions of nuclei that we adopt in the present study. In a pioneering study, Thompson et al (2001) characterized specific expression domains of individual combinations of genes in the different oikoplastic fields among them the diverse and numerous oikosins, coding for the respective proteins.…”

“…Detailed descriptions and an elaborate nomenclature of the relative positions of the oikoplastic cells exist (Lohmann, 1933; Körner, 1952; Thompson et al, 2004; Kishi et al, 2017), and detailed observations of the distribution of oikosins, a highly diverse group of proteins are available (Sagane et al, 2010, Sagane et al, 2011, Thompson et al, 2001). However, very little is known about the structural arrangement of the rudimentary house and the ultrastructural morphology of the oikoplastic cells (Flood & Deibel, 1998).…”

A genome database for a Japanese population of the larvacean Oikopleura dioica

“…Here, we determined the genome sequence of Japanese O. dioica. Using the same population, we have developed tools for molecular, cellular, and genetic techniques, including gene knockdown methods (Omotezako et al, 2013(Omotezako et al, , 2015(Omotezako et al, , 2017, fluorescent live imaging of larvae (Kishi et al, 2014(Kishi et al, , 2017, and maternal and zygotic transcriptomes of the eggs and larvae (Wang et al, 2015).…”

“…These have been used to gain insights into cell migration (Kishi et al, 2014), patterning of epidermal cells (Kishi et al, 2017), left-right patterning (Onuma et al, 2020) and meiotic arrest of the unfertilized egg (Matsuo et al, 2020). Moreover, transcriptome-wide comparison has implied a high level of sequence variation between the Japanese and Norwegian O. dioica (Wang et al, 2015): only 91.0% and 94.8% of exon sequences were conserved at nucleotide and amino acid levels on average, respectively.…”