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Background: Ascidians significantly change their body structure through metamorphosis, but the spatio‐temporal cell dynamics in the early metamorphosis stage has not been clarified. A natural Ciona embryo is surrounded by maternally derived non‐self‐test cells before metamorphosis. However, after metamorphosis, the juvenile is surrounded by self‐tunic cells derived from mesenchymal cell lineages. Both test cells and tunic cells are thought to be changed their distributions during metamorphosis, but the precise timing is unknown. Results: Using a metamorphosis induction by mechanical stimulation, we investigated the dynamics of mesenchymal cells during metamorphosis in a precise time course. After the stimulation, two‐round Ca2+ transients were observed. Migrating mesenchymal cells came out through the epidermis within 10 min after the second phase. We named this event "cell extravasation." The cell extravasation occurred at the same time as the backward movement of posterior trunk epidermal cells. Timelapse imaging of transgenic‐line larva revealed that non‐self‐test cells and self‐tunic cells temporarily coexist outside the body until the test cells are eliminated. At the juvenile stage, only extravasated self‐tunic cells remained outside the body. Conclusions: We found that mesenchymal cells extravasated following two‐round Ca2+ transients, and distributions of test cells and tunic cells changed in the outer body after tail regression. Key Findings: We found mesenchymal cells were released from multiple points in the epidermis of the trunk (extravasation) after mechanical stimulation.Extravasation occurs simultaneously with backward movement of the epidermis.Before metamorphosis, mesenchymal cells already migrated beneath the epidermis.Three‐dimensional timelapse imaging revealed that extravasated cells were divided into two groups: faster migrating cell groups than before metamorphosis and slower cell groups.After extravasation, non‐self‐external cells are eliminated and self‐extravasated cells surrounded the outer body. Both cells are thought to be responsible for defense. [ABSTRACT FROM AUTHOR]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2006.01.010 Byline: Peter A. Thomason (a)(c), Satoshi Sawai (a), Jeffry B. Stock (a)(b), Edward C. Cox (a) Keywords: Morphogenesis; Dictyostelium; Histidine kinase; dhkK; Slugger; Culmination; Phosphorelay; Cyclic AMP Abstract: A key event in Dictyostelium development is the formation of the Mexican hat. This corresponds to a commitment step in morphogenesis that irreversibly signals progression from the slug stage to the fruiting body. We describe the characterization of the dhkK gene that controls this morphogenetic step. Null mutants of dhkK repeatedly attempt, and fail, to undergo morphogenesis from the slug to the Mexican hat, causing them to exhibit a 'slugger' phenotype, which cannot be corrected by co-development with wild-type cells. The dhkK gene encodes a putative receptor histidine kinase whose expression is enriched in prestalk cells in the slug. Uniquely for a histidine kinase, DhkK is located in the nuclear envelope. Entry into culmination requires the DhkK response regulator domain, which appears to directly regulate cyclic AMP signaling. Author Affiliation: (a) Department of Molecular Biology, Princeton University, Princeton, NJ 08544, USA (b) Department of Chemistry, Princeton University, Princeton, NJ 08544, USA (c) School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Article History: Received 15 June 2005; Revised 31 August 2005; Accepted 9 January 2006

Author(s): Satoshi Sawai (corresponding author) [1]; Peter A. Thomason [1]; Edward C. Cox (corresponding author) [1] Nutrient-deprived Dictyostelium amoebae aggregate to form a multicellular structure by chemotaxis, moving towards propagating [...]

Marine invertebrate larvae are known to begin metamorphosis in response to environmentally derived cues. However, little is known about the relationships between the perception of such cues and internal signalling for metamorphosis. To elucidate the mechanism underlying the initiation of metamorphosis in the ascidian, Ciona intestinalis type A (Ciona robusta), we artificially induced ascidian metamorphosis and investigated Ca2+ dynamics from pre- to post-metamorphosis. Ca2+ transients were observed and consisted of two temporally distinct phases with different durations before tail regression which is the early event of metamorphosis. In the first phase, Phase I, the Ca2+ transient in the papillae (adhesive organ of the anterior trunk) was coupled with the Ca2+ transient in dorsally localized cells and endoderm cells just after mechanical stimulation. The Ca2+ transients in Phase I were also observed when applying only short stimulation. In the second phase, Phase II, the Ca2+ transient in papillae was observed again and lasted for approximately 5–11 min just after the Ca2+ transient in Phase I continued for a few minutes. The impaired papillae by Foxg-knockdown failed to induce the second Ca2+ transient in Phase II and tail regression. In Phase II, a wave-like Ca2+ propagation was also observed across the entire epidermis. Our results indicate that the papillae sense a mechanical cue and two-round Ca2+ transients in papillae transmits the internal metamorphic signals to different tissues, which subsequently induces tail regression. Our study will help elucidate the internal mechanism of metamorphosis in marine invertebrate larvae in response to environmental cues. [ABSTRACT FROM AUTHOR]