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Unlike mammals, reptiles maintain a sinuatrial delay of the impulse, allowing the partly atrialized sinus venosus to function as a chamber..For reptiles it is unclear where the pacemaker is located and how the sinus myocardium of reptiles compares to the caval vein myocardium of mammals with regards to gene expression profile, electrical activation, and capacity for contraction.The region of the sinus venosus that expressed Isl1 also expressed Tbx3 and Bmp2 (Fig. Between Anolis stages 7 and 19, the Isl1 domain only grew 70% while the remaining sinus venosus myocardium grew 530% (N = 2).Hcn4 was broadly expressed in the sinus venosus and atria of the lizard, but expression was greatest in the region of the Isl1 expression domain (Fig. We did not detect Gja5 (Cx40) in the Isl1 expression domain and in the sinus myocardium proximal to the right atrium (Fig. We also investigated the sinuatrial junction of an embryonic American alligator and again found rich expression of Tbx3 in the sinus myocardium most proximal to the sinuatrial junction (Fig. Gja5 and Scn5a were absent where Tbx3 was present (Fig. The sinuatrial junction of reptiles has the phenotype of the mammalian sinus node.We undertook an anatomical, molecular, electrophysiological and pharmacological investigation of the sinus myocardium in two commonly investigated reptiles, the Anolis lizard and the python.The heart of the Anolis lizard may be seen as representative of most hearts of non-crocodilian reptiles.In proximity of the systemic veins, the Anolis sinus venosus expressed markers of the atrial phenotype Nkx2-5 and Gja5.In conclusion, the reptile heart is driven by a pacemaker region with an expression signature similar to that of the immature sinus node of mammals.

(A) Histological section of sinuatrial junction of the adult Anolis equestrie, showing the left and right leaflet of the sinuatrial valve (lsav and rsav, respectively).

Electrophysiology was exclusively done in adult Anolis and Python.

The activation front of the sinus venosus always propagated away from the sinuatrial junction (Fig. Electrical propagation was slower in the sinus venosus than in the atria (0.18 ± 0.11 vs 0.76 ± 0.58 m/s, N = 3 in Anolis).

Anolis lizards had an extensive sinus venosus of myocardium expressing Tbx18.

A small sub-population of cells encircling the sinuatrial junction expressed Isl1, Bmp2, Tbx3, and Hcn4, homologues of genes marking the mammalian sinus node.

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