Fig. 1

Proliferation of cardiomyocytes during cardiac regeneration post cryo-injury. a A commercial liquid nitrogen cooled cryo-gun was used to generate a cryo-lesion at the apex of the heart. b Different gold tip attachments allowed matching to animal size. c A necrotic lesion was produced with edema still present at 6-days post injury (Dpi). d The lesion was identified by a robust inflammatory infiltrate with visible peripheral nuclei (DAPI) at 6 dpi. The ventricle was fully regenerated by 60–90 dpi and appeared normal, both superficially e, and histologically f, relative to uninjured controls. g Histological staining was performed with Acid Fuchsin orange G (AFOG) whrere collagen stains blue, muscle of the myocardium in orange/red and the nucleus black. h Time course of Bromodeoxyuridine (BrdU) labelling of animals daily (i.p. injection). i Identification of cardiomyocyte (CM) specific proliferation used nuclear localized Nk2.5 antibody staining and dual positive BrdU+ (Green)+ Nkx2.5+ cells (Red). j CM specific and non-CM proliferation was measured during the first 5 weeks of regeneration within 200 μM of the borderzone or 100μM of the valve region (at the junction between the atrium and the ventricle). At least three sections per animal and three animals per time-point were used. k Cartoon of the general spatial localization of CM and non-CM proliferation in the axolotl heart over the first 3 weeks of regeneration. Non-cardiomyocytes shown in green and Nkx2.5+ CM shown in pink. Cryo lesion in ventricle shown in red. Scale bar = 100 μM