MATURATION OF CARDIAC MYOCYTES IN FETAL AND NEWBORN SHEEP: A STUDY EXAMINING CHANGES IN MYOCYTE NUCLEAR NUMBER AND VOLUME

Judith H. Burrell, Adrian M. Boyn, Vasumathy Kumarasamy, Albert Hsieh, Stewart I. Head, Eugenie R. Lumbers, Department of Physiology & Pharmacology, School of Medical Science, University of New South Wales, Sydney, 2052, Australia/.

During fetal life the heart enlarges by hyperplasia and hypertrophy of cardiac myocytes. Terminal differentiation of myocytes in the late gestation fetal sheep is characterised by transition from uninucleated to binucleated cells, which can no longer divide. Growth and maturation of the fetal myocytes and the effects of surgery and anaesthesia were studied in both non operated and chronically catheterised fetal sheep.

Myocytes were isolated from the right (RVFW) and left ventricular free walls (LVFW) of hearts from 25 fetal sheep aged 77-146 days, (term = 150 days), from 6 saline infused cannulated fetal sheep aged 129 days, from 3 newborn lambs and 2 older lambs. Myocytes were digested with collagenase, stained with ethidium bromide and examined using confocal microscopy. The proportions of uninucleated and binucleated cells were determined, myocyte volume measured and total number of myocytes calculated.

At 77 days <2.5 %, at 135 days 50%, and at 6 weeks after birth 90% of myocytes were binucleated. In the fetus, the size of both uni- and binucleated myocytes in the RVFW increased with gestation age, but in the LVFW only the binucleated myocytes showed a gestation dependent increase in cell volume (P < 0.05). LVFW myocytes were smaller and there was a larger number than in the RVFW (P < 0.001). Although myocyte numbers in both LVFW and RVFW increased throughout gestation, 4 days after birth RVFW myocyte number (P = 0.002) and mass (P = 0.006) had decreased. Anaesthesia and surgery had no effect on myocyte maturation in 129 day fetuses.

This study describes myocyte maturation in fetal sheep and young lambs and shows that the capacity of the developing heart to grow through hyperplasia is reduced by the time of birth, and that there is a reduction in RVFW myocyte number after birth.