Placental insufficiency decreases cell cycle activity and terminal maturation in fetal sheep cardiomyocytes

Abstract

Umbilicoplacental embolization (UPE) in sheep has been used to investigate the effects of placental insufficiency on fetal development. However, its specific effects on the heart have been little studied. The aim of this study was to determine the effects of placental insufficiency, induced by UPE, on cardiomyocyte size, maturation and proliferation. Instrumented fetal sheep underwent UPE for either 10 or 20 days. Hearts were collected at 125 +/- 1 days (10 day group) or 136 +/- 1 days (20 day group) of gestation (term approximately 145 days). Cell size, maturational state (as measured by the proportion of binucleated myocytes) and cell cycle activity (as measured by positive staining of cells for Ki-67) were determined in dissociated cardiomyocytes. UPE fetuses were hypoxaemic, but mean arterial pressures were not different from controls. UPE fetuses were lighter than control fetuses (10 days: -21%, P < 0.05; 20 days: -27%, P < 0.01) and had smaller hearts, but heart weight was appropriate for body weight. Neither lengths nor widths were different between control and UPE cardiomyocytes at either age. Ten days of UPE did not significantly alter the proportion of binucleated myocytes or cell cycle activity in either ventricle. However, 20 days of UPE reduced cell cycle activity in both ventricles by approximately 70% (P < 0.05); the proportion of binucleated myocytes was also lower in UPE fetuses at this age (left ventricle: 31.1 +/- 12.0 versus 46.0 +/- 6.6%, P < 0.05; right ventricle: 29.4 +/- 12.3 versus 46.3 +/- 5.3%, P < 0.05). It is concluded that in the absence of fetal arterial hypertension, placental insufficiency is associated with substantially depressed growth of the heart through suppressed proliferation and maturation of cardiomyocytes.

Figure 1. Fetal arterial oxygen, arterial pressure and heart rate during UPE

A, partial pressure of oxygen (P_a,O2), B, arterial oxygen saturation (S_a,O2), C, mean arterial pressure (MAP), and D, heart rate, in control and umbilicoplacental embolization (UPE) fetuses (20 day groups only). Mean data for the control group are shown by the continuous line ± s.d. (shaded area). Data for UPE fetuses (symbols, mean ± s.d.) are shown as baseline (first symbol), when target level of hypoxaemia had been reached (second symbol) and 2 h after UPE had ceased for the day (third symbol). For days when UPE was not required, data are only included in the baseline point. Data shown are for experimental days 1, 4, 7, 10, 13, 16, 19 and 21.

Figure 2. Fetal plasma cortisol concentration and renin activity

A, plasma cortisol concentration, and B, plasma renin activity, in control (•) and UPE (○) fetuses in the 20 day study group. Data are means ± s.d., *P < 0.05, **P < 0.01 versus age-matched control; n=5 in all groups, except n=4 for UPE cortisol values.

Figure 3. Cell cycle activity in cardiomyocytes of control and UPE fetuses

Cell cycle activity (as determined by positive staining for Ki-67) in mononucleated myocytes (expressed as a proportion of total mononucleated myocytes) from hearts of control and UPE fetuses at 125d GA (10d UPE) and 136d GA (20d UPE). Data are means ± s.d., *P < 0.05 versus age-matched control; n=6 controls in both age groups, n=5 UPE fetuses in both age groups.

Figure 4. Binucleation of cardiomyocytes of control and UPE fetuses

Proportion of binucleated myocytes in the left and right ventricles of control and UPE fetuses at 125d GA (10d UPE) and 136d GA (20d UPE). Data are means ± s.d., *P < 0.05 versus age-matched control; ^#P < 0.05, ^##P < 0.01 versus 125 day control; n=6 controls in both age groups, n=5 UPE fetuses in both age groups.