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Figure 1. Luminal atrial smooth muscle was not detected in most vertebrates. Red represents cTnI and green represents SMA, as detected by fluorescent immunohistochemistry. (A) American alligator, (B) pink‐tongued skink, (C) African clawed frog, (D) longnose gar, (E) mouse, and (F) lesser redpoll bird (all detected SMA in B, E, F was within arterial walls). Scale bars are 1 mm. a, atrium; as, atrial septum; pv, pulmonary vein; sv, sinus venosus; v, ventricle; oft, outflow tract.
Figure 2. Luminal smooth muscle was not detected in atrium from spectacled caiman (A), cane toad (B), or caecilian (C). Red represents cTnI and green represents SMA, as detected by fluorescent immunohistochemistry. Scale bars are 1 mm. (r/l)a, (right/left) atrium; as, atrial septum; sv, sinus venosus; c, conus.
Figure 3. Smooth muscle in human atrial wall. Red represents cardiac cTnI and green represents SMA, as detected by fluorescent immunohistochemistry. (A) leftanterior atrium and (B) left atrial appendage. Scale bars are 1 mm.
Figure 4. The phylogenetic distribution of smooth muscle in different regions of the heart in eight turtle species. Red represents cTnI and green represents SMA, as detected by fluorescent immunohistochemistry. Scale bars are 100 μm for all species, except for C. senegalensis and T. scripta (500 μm), and C. carbonaria (1 mm). Phylogeny based on Crawford et al. (2015).
Figure 5. Assessment across turtles of the proportion of signal from SMA (green) out the total signal from SMA and cTnI (red), measured by the ImageJ Plugin function “RGB measure.” The yellow lines indicate the region of interest within which the measurements were made. Two specimens of the Pond slider with intermediate (A) and high (B) proportions (numbers in green) of SMA in the sinus venosus and atria. (C) Across turtles, the sinus venosus was proportionally richer in SMA than the atria, which in turn were richer in SMA than the ventricle (P‐values of paired two‐tailed T‐tests, numbers in columns are the number of assessed sections). (D) The proportion of SMA in the sinus venosus was significantly positively related to the proportion of SMA in the atria (Pearson correlation, P < 0.001).
Figure 6. The interspecific variation in atrial smooth muscle in turtles. (A) Mean area of smooth muscle as a percentage of total muscle (smooth + cardiac muscle) area in eight species. Values are means ± SD. (B–D) Red represents cardiac Troponin I and green represents smooth muscle actin, as detected by fluorescent immunohistochemistry. (B) A prominent case of atria smooth muscle in T. scripta. (C) Intermediate levels of smooth muscle seen in C. senegalensis. (D) Near absence of atrial smooth muscle in C. carbonaria. For B–D, scale bars are 1 mm. a, atrium; as, atrial septum; pv, pulmonary vein; sv, sinus venosus; v, ventricle; oft, outflow tract.
Figure 7. There was no significant relationship between atrial smooth muscle (% area of total muscle) and body mass in Trachemys scripta (linear regression).
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