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Endothelial cell self-fusion during vascular pruning.

Lenard A, Daetwyler S, Betz C, Ellertsdottir E, Belting HG, Huisken J, Affolter M - PLoS Biol. (2015)

Bottom Line: Because of the lack of an in vivo system suitable for high-resolution live imaging, the dynamics of the pruning process have not been described in detail.In pruning segments, endothelial cells first migrate toward opposing sides where they join the parental vascular branches, thus remodeling the multicellular segment into a unicellular connection.Often, the lumen is maintained throughout this process, and transient unicellular tubes form through cell self-fusion.

View Article: PubMed Central - PubMed

Affiliation: Biozentrum der Universität Basel, Basel, Switzerland.

ABSTRACT
During embryonic development, vascular networks remodel to meet the increasing demand of growing tissues for oxygen and nutrients. This is achieved by the pruning of redundant blood vessel segments, which then allows more efficient blood flow patterns. Because of the lack of an in vivo system suitable for high-resolution live imaging, the dynamics of the pruning process have not been described in detail. Here, we present the subintestinal vein (SIV) plexus of the zebrafish embryo as a novel model to study pruning at the cellular level. We show that blood vessel regression is a coordinated process of cell rearrangements involving lumen collapse and cell-cell contact resolution. Interestingly, the cellular rearrangements during pruning resemble endothelial cell behavior during vessel fusion in a reversed order. In pruning segments, endothelial cells first migrate toward opposing sides where they join the parental vascular branches, thus remodeling the multicellular segment into a unicellular connection. Often, the lumen is maintained throughout this process, and transient unicellular tubes form through cell self-fusion. In a second step, the unicellular connection is resolved unilaterally, and the pruning cell rejoins the opposing branch. Thus, we show for the first time that various cellular activities are coordinated to achieve blood vessel pruning and define two different morphogenetic pathways, which are selected by the flow environment.

No MeSH data available.


Related in: MedlinePlus

Cell rearrangements during pruning of type I and II.Stills from time-lapse movies illustrating cell rearrangements in type I pruning with lumen collapse before cell rearrangements (A) and type II pruning with cell rearrangements before lumen collapse (B) in transgenic embryos Tg(fliep:GFF)ubs3,(UAS:mRFP),(5xUAS:cdh5-EGFP)ubs12. Cell–cell junctions are green (VE-cad-EGFP), and cell cytoplasm is red. Black-and-white pictures show respective channels alone. Key steps of pruning are shown. Green arrows mark multicellular contacts (cell–cell junction length), white arrows mark transcellular lumen, and grey dotted lines mark unicellular fragments without lumen. Asterisks mark nuclei of cells contributing to the branch. (A) Pruning type I. A small, lumenized branch is made of two cells connected by two parallel lines of junctions (1). Lumen collapses when the branch is still multicellular (2–3); after lumen collapse, cells move away from each other, and cell–cell contact surface shrinks, generating a nonlumenized, ajunctional segment (4). Eventually, only the last bridging cell remains (5, grey arrow) prior to final detachment (not shown). See also S8 and S9 Movies and S3 Fig. (B) Pruning type II. Cellular architecture of a multicellular branch (1) is simplified to a branch made mainly by two cells connected by parallel lines of junctions (2). Further cell rearrangements lead to the formation of a partially unicellular tube (3). The lumen eventually collapses (4), and the cell body migrates towards the left-side major branch (5) until only a last, narrow cell extension connects two major branches (6). See also S10 Movie. (C) A graph representing percentage of pruning type I (dark blue) and II (light blue) in all pruning events analyzed in transgenic embryos of Tg(fliep:GFF)ubs3,(UAS:mRFP),(5xUAS:cdh5-EGFP)ubs12 and TgBAC(kdrl:mKate-CAAX)ubs16, respectively. Scale bar: 10 μm.
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pbio.1002126.g003: Cell rearrangements during pruning of type I and II.Stills from time-lapse movies illustrating cell rearrangements in type I pruning with lumen collapse before cell rearrangements (A) and type II pruning with cell rearrangements before lumen collapse (B) in transgenic embryos Tg(fliep:GFF)ubs3,(UAS:mRFP),(5xUAS:cdh5-EGFP)ubs12. Cell–cell junctions are green (VE-cad-EGFP), and cell cytoplasm is red. Black-and-white pictures show respective channels alone. Key steps of pruning are shown. Green arrows mark multicellular contacts (cell–cell junction length), white arrows mark transcellular lumen, and grey dotted lines mark unicellular fragments without lumen. Asterisks mark nuclei of cells contributing to the branch. (A) Pruning type I. A small, lumenized branch is made of two cells connected by two parallel lines of junctions (1). Lumen collapses when the branch is still multicellular (2–3); after lumen collapse, cells move away from each other, and cell–cell contact surface shrinks, generating a nonlumenized, ajunctional segment (4). Eventually, only the last bridging cell remains (5, grey arrow) prior to final detachment (not shown). See also S8 and S9 Movies and S3 Fig. (B) Pruning type II. Cellular architecture of a multicellular branch (1) is simplified to a branch made mainly by two cells connected by parallel lines of junctions (2). Further cell rearrangements lead to the formation of a partially unicellular tube (3). The lumen eventually collapses (4), and the cell body migrates towards the left-side major branch (5) until only a last, narrow cell extension connects two major branches (6). See also S10 Movie. (C) A graph representing percentage of pruning type I (dark blue) and II (light blue) in all pruning events analyzed in transgenic embryos of Tg(fliep:GFF)ubs3,(UAS:mRFP),(5xUAS:cdh5-EGFP)ubs12 and TgBAC(kdrl:mKate-CAAX)ubs16, respectively. Scale bar: 10 μm.

Mentions: In pruning type I, the first recognizable step was the collapse of lumen in the vessel branch, which was transformed this way into a nonlumenized, multicellular cord with continuous junctional connections, often visible as two parallel lines (Fig 3A1–3A3 and S8 Movie). Subsequently, cells moved away from the pruning branch and incorporated into the neighboring major branches, eventually leaving a single bridging cell in between the latter (Fig 3A3 and Fig 3A4). This way, the vessel architecture changed from multicellular to unicellular, as visualized by changes in the junctional pattern. Initially, the junctions were continuous (Fig 3A1–3A3, green arrow), and after cell rearrangements, we observed ajunctional areas indicating unicellular vessel segments (Fig 3A4, grey arrow). During this transformation, the junctions of the last bridging cell changed from continuous along the cell body into two separate junctional contacts on both poles of the cell. With those junctions, the bridging cell was connected to the opposing major branches (Fig 3A4). Eventually, one of these connections shrank to a single cytoplasmic extension when the cell body migrated and incorporated into the opposing major branch (Fig 3A5 and S8 Movie). Finally, this last contact was resolved and regression completed (S3 Fig and S9 Movie).


Endothelial cell self-fusion during vascular pruning.

Lenard A, Daetwyler S, Betz C, Ellertsdottir E, Belting HG, Huisken J, Affolter M - PLoS Biol. (2015)

Cell rearrangements during pruning of type I and II.Stills from time-lapse movies illustrating cell rearrangements in type I pruning with lumen collapse before cell rearrangements (A) and type II pruning with cell rearrangements before lumen collapse (B) in transgenic embryos Tg(fliep:GFF)ubs3,(UAS:mRFP),(5xUAS:cdh5-EGFP)ubs12. Cell–cell junctions are green (VE-cad-EGFP), and cell cytoplasm is red. Black-and-white pictures show respective channels alone. Key steps of pruning are shown. Green arrows mark multicellular contacts (cell–cell junction length), white arrows mark transcellular lumen, and grey dotted lines mark unicellular fragments without lumen. Asterisks mark nuclei of cells contributing to the branch. (A) Pruning type I. A small, lumenized branch is made of two cells connected by two parallel lines of junctions (1). Lumen collapses when the branch is still multicellular (2–3); after lumen collapse, cells move away from each other, and cell–cell contact surface shrinks, generating a nonlumenized, ajunctional segment (4). Eventually, only the last bridging cell remains (5, grey arrow) prior to final detachment (not shown). See also S8 and S9 Movies and S3 Fig. (B) Pruning type II. Cellular architecture of a multicellular branch (1) is simplified to a branch made mainly by two cells connected by parallel lines of junctions (2). Further cell rearrangements lead to the formation of a partially unicellular tube (3). The lumen eventually collapses (4), and the cell body migrates towards the left-side major branch (5) until only a last, narrow cell extension connects two major branches (6). See also S10 Movie. (C) A graph representing percentage of pruning type I (dark blue) and II (light blue) in all pruning events analyzed in transgenic embryos of Tg(fliep:GFF)ubs3,(UAS:mRFP),(5xUAS:cdh5-EGFP)ubs12 and TgBAC(kdrl:mKate-CAAX)ubs16, respectively. Scale bar: 10 μm.
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Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4401649&req=5

pbio.1002126.g003: Cell rearrangements during pruning of type I and II.Stills from time-lapse movies illustrating cell rearrangements in type I pruning with lumen collapse before cell rearrangements (A) and type II pruning with cell rearrangements before lumen collapse (B) in transgenic embryos Tg(fliep:GFF)ubs3,(UAS:mRFP),(5xUAS:cdh5-EGFP)ubs12. Cell–cell junctions are green (VE-cad-EGFP), and cell cytoplasm is red. Black-and-white pictures show respective channels alone. Key steps of pruning are shown. Green arrows mark multicellular contacts (cell–cell junction length), white arrows mark transcellular lumen, and grey dotted lines mark unicellular fragments without lumen. Asterisks mark nuclei of cells contributing to the branch. (A) Pruning type I. A small, lumenized branch is made of two cells connected by two parallel lines of junctions (1). Lumen collapses when the branch is still multicellular (2–3); after lumen collapse, cells move away from each other, and cell–cell contact surface shrinks, generating a nonlumenized, ajunctional segment (4). Eventually, only the last bridging cell remains (5, grey arrow) prior to final detachment (not shown). See also S8 and S9 Movies and S3 Fig. (B) Pruning type II. Cellular architecture of a multicellular branch (1) is simplified to a branch made mainly by two cells connected by parallel lines of junctions (2). Further cell rearrangements lead to the formation of a partially unicellular tube (3). The lumen eventually collapses (4), and the cell body migrates towards the left-side major branch (5) until only a last, narrow cell extension connects two major branches (6). See also S10 Movie. (C) A graph representing percentage of pruning type I (dark blue) and II (light blue) in all pruning events analyzed in transgenic embryos of Tg(fliep:GFF)ubs3,(UAS:mRFP),(5xUAS:cdh5-EGFP)ubs12 and TgBAC(kdrl:mKate-CAAX)ubs16, respectively. Scale bar: 10 μm.
Mentions: In pruning type I, the first recognizable step was the collapse of lumen in the vessel branch, which was transformed this way into a nonlumenized, multicellular cord with continuous junctional connections, often visible as two parallel lines (Fig 3A1–3A3 and S8 Movie). Subsequently, cells moved away from the pruning branch and incorporated into the neighboring major branches, eventually leaving a single bridging cell in between the latter (Fig 3A3 and Fig 3A4). This way, the vessel architecture changed from multicellular to unicellular, as visualized by changes in the junctional pattern. Initially, the junctions were continuous (Fig 3A1–3A3, green arrow), and after cell rearrangements, we observed ajunctional areas indicating unicellular vessel segments (Fig 3A4, grey arrow). During this transformation, the junctions of the last bridging cell changed from continuous along the cell body into two separate junctional contacts on both poles of the cell. With those junctions, the bridging cell was connected to the opposing major branches (Fig 3A4). Eventually, one of these connections shrank to a single cytoplasmic extension when the cell body migrated and incorporated into the opposing major branch (Fig 3A5 and S8 Movie). Finally, this last contact was resolved and regression completed (S3 Fig and S9 Movie).

Bottom Line: Because of the lack of an in vivo system suitable for high-resolution live imaging, the dynamics of the pruning process have not been described in detail.In pruning segments, endothelial cells first migrate toward opposing sides where they join the parental vascular branches, thus remodeling the multicellular segment into a unicellular connection.Often, the lumen is maintained throughout this process, and transient unicellular tubes form through cell self-fusion.

View Article: PubMed Central - PubMed

Affiliation: Biozentrum der Universität Basel, Basel, Switzerland.

ABSTRACT
During embryonic development, vascular networks remodel to meet the increasing demand of growing tissues for oxygen and nutrients. This is achieved by the pruning of redundant blood vessel segments, which then allows more efficient blood flow patterns. Because of the lack of an in vivo system suitable for high-resolution live imaging, the dynamics of the pruning process have not been described in detail. Here, we present the subintestinal vein (SIV) plexus of the zebrafish embryo as a novel model to study pruning at the cellular level. We show that blood vessel regression is a coordinated process of cell rearrangements involving lumen collapse and cell-cell contact resolution. Interestingly, the cellular rearrangements during pruning resemble endothelial cell behavior during vessel fusion in a reversed order. In pruning segments, endothelial cells first migrate toward opposing sides where they join the parental vascular branches, thus remodeling the multicellular segment into a unicellular connection. Often, the lumen is maintained throughout this process, and transient unicellular tubes form through cell self-fusion. In a second step, the unicellular connection is resolved unilaterally, and the pruning cell rejoins the opposing branch. Thus, we show for the first time that various cellular activities are coordinated to achieve blood vessel pruning and define two different morphogenetic pathways, which are selected by the flow environment.

No MeSH data available.


Related in: MedlinePlus