Fol. Biol. 2019, 65, 36-42
Cardiac Enlargement in the Chick Embryo Induced by Hypothermic Incubation Is Due to a Combination of Hyperplasia and Hypertrophy of Cardiomyocytes
Hypothermic incubation of chicken eggs leads to smaller embryos with enlarged hearts, originally described as hypertrophic. Over the years, however, accumulated evidence suggested that hyperplasia, rather than hypertrophy, is the predominant mechanism of cardiac growth during the prenatal period. We have thus set to re-evaluate the hypothermia model to precise the exact cellular mechanism behind cardiac enlargement. Fertilized chicken eggs were incubated at either 37.5 °C (normothermia) or 33.5 °C from embryonic day (ED) 13 onward (hypothermia). Sampling was performed at ED17, at which point wet embryo and heart weight were recorded, and the hearts were submitted to histological examination. In agreement with previous results, the hypothermic embryos were 29% smaller and had hearts 18% larger, translating into a 67% increase in the heart to body weight ratio (P < 0.05 for all parameters). The cell size was essentially the same between control and hypothermic hearts in all regions analysed. Likewise, there was no significant relationship between the cell size and heart weight; however, in the hypothermic hearts, there was a trend showing positive correlation between cell sizes in different cardiac regions and heart weight. Proliferation rate, determined on the basis of anti-phosphohistone H3 immunofluorescence, showed an overall increase in the hypothermic group, reaching statistical significance (P = 0.02, t-test) in the right ventricle. The proliferation rate was similar among different regions of the same heart. However, the correlation between the proliferation rate and heart weight was only small (r2 = 0.007 and r2 = 0.234 for the normothermic and hypothermic group, respectively). We thus conclude that hyperplasia is the predominant response mechanism in this volume-overload model; mechanistically, decreased heart rate at lower temperature increases the end-diastolic and stroke volume, minimizing the drop in cardiac output through the Frank- Starling mechanism.
Keywords
embryonic heart, cell size, cell proliferation, immunofluorescence.
Funding
This study was supported by the Czech Science Foundation grants 16-02972S and 18-03461S, Ministry of Education, Youth and Sports of the Czech Republic (PROGRES-Q38, INTER-COST LTC17023, and LM2015062 Czech-BioImaging), Charles University UNCE, Grant Agency of Charles University 1456217, OP RDE CZ.02.1.01/0.0/0.0/16_013/0001775 Modernization and support of research activities of the national infrastructure for biological and medical imaging Czech-BioImaging, and institutional funding from the Czech Academy of Sciences RVO: 67985823.
References
Copyright
This is an open-access article distributed under the terms of the Creative Commons Attribution License.
