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Fig. 1. Tracking wild-type cardiomyocytes reveals two morphologically distinct
phases of cell behavior during cardiac fusion. (A) Schematic of
cardiac fusion in the zebrafish embryo. Dorsal views; locations of
cardiomyocyte populations shown in green; location of endocardium at the
20-somite stage indicated with an E. (B-D) Selected images from a
time-lapse of cardiac fusion in a typical wild-type embryo expressing
Tg(cmlc2:egfp) (see Movie 1 in the supplementary material),
exhibiting cardiac morphology at the (B) 16-somite, (C) 18-somite and (D)
20-somite stages. Dorsal views, anterior to the top. (E-I) Paths
traveled by individual cardiomyocytes during (E,F) the initial 50 minutes,
(G,H) the final 80 minutes and (I) the final 20 minutes of fusion. Tracks
displayed represent a subset of the tracked cells in this time-lapse. Each
arrow extends from a cell's starting position to its ending position. Red
arrows, medial movement; yellow arrows, angular movement; asterisks, cells
exhibiting no net displacement over the elapsed time. Cardiac fusion begins
with a phase of coherent medial movement (E,F), followed by a transition to
angular movement in anterior and posterior regions (G,H). Scale bar: in I, 20
µm for B-I. (J-M) Quantitative analysis of the direction of
cardiomyocyte movement. (J,L) Radial bar graphs depict degree of cardiomyocyte
displacement during the first (J) and second (L) phases. For all tracked
wild-type cardiomyocytes, net displacement along the anterior-posterior axis
was calculated relative to a directly medial path (0° displacement).
Grouping the data in 10° increments, the length of each radial bar
reflects the percentage of cells exhibiting each degree of displacement. Red
bars are classified as medial movement, and yellow bars are classified as
angular movement. Significantly more cells exhibit angular movement during the
second phase than during the first phase (52% versus 17%; t-test,
P<0.01). (K,M) Location of cardiomyocytes moving angularly during
first (K) and second (M) phases. Colors overlaid on egfp expression
reflect anterior (purple), central (green) and posterior (blue) regions. Most
of the cells exhibiting angular movement are located in anterior or posterior
regions. In these regions, the percentage of cells moving angularly during the
second phase is significantly greater than that observed during the first
phase (*, posterior: 86% versus 20%; **, anterior: 77%
versus 17%; t-test, P<0.01 for both regions). See
Materials and methods for details of time-lapse analysis and
Table 1 for additional
data.
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