Timing of cardiomyocyte growth, maturation, and attrition in perinatal sheep.

Studies in altricial rodents attribute dramatic changes in perinatal cardiomyocyte growth, maturation, and attrition to stimuli associated with birth. Our purpose was to determine whether birth is a critical trigger controlling perinatal cardiomyocyte growth, maturation and attrition in a precocial large mammal, sheep (Ovis aries). Hearts from 0-61 d postnatal lambs were dissected or enzymatically dissociated. Cardiomyocytes were measured by micromorphometry, cell cycle activity assessed by immunohistochemistry, and nuclear number counted after DNA staining. Integration of this new data with published fetal data from our laboratory demonstrate that a newly appreciated >30% decrease in myocyte number occurred in the last 10 d of gestation (P < 0.0005) concomitant with an increase in cleaved poly (ADP-ribose) polymerase 1 (P < 0.05), indicative of apoptosis. Bisegmental linear regressions show that most changes in myocyte growth kinetics occur before birth (median = 15.2 d; P < 0.05). Right ventricular but not left ventricular cell number increases in the neonate, by 68% between birth and 60 d postnatal (P = 0.028). We conclude that in sheep few developmental changes in cardiomyocytes result from birth, excepting the different postnatal degrees of free wall hypertrophy between the ventricles. Furthermore, myocyte number is reduced in both ventricles immediately before term, but proliferation increases myocyte number in the neonatal right ventricle.

Cardiomyocyte enlargement, proliferation and maturation during chronic fetal anaemia in sheep.

Chronic anaemia increases the workload of the growing fetal heart, leading to cardiac enlargement. To determine which cellular process increases cardiac mass, we measured cardiomyocyte sizes, binucleation as an index of terminal differentiation, and tissue volume fractions in hearts from control and anaemic fetal sheep. Fourteen chronically catheterized fetal sheep at 129 days gestation had blood withdrawn for 9 days to cause severe anaemia; 14 control fetuses were of similar age. At postmortem examination, hearts were either enzymatically dissociated or fixed for morphometric analysis. Daily isovolumetric haemorrhage reduced fetal haematocrit from a baseline value of 35% to 15% on the final day (P < 0.001). At the study conclusion, anaemic fetuses had lower arterial pressures than control fetuses (P < 0.05). Heart weights were increased by 39% in anaemic fetuses compared with control hearts (P < 0.0001), although the groups had similar body weights; the heart weight difference was not due to increased ventricular wall water content or disproportionate non-myocyte tissue expansion. Cardiomyocytes from anaemic fetuses tended to be larger than those of control fetuses. There were no statistically significant differences between groups in the cardiomyocyte cell cycle activity. The degree of terminal differentiation was greater in the right ventricle of anaemic compared with control fetuses by 8% (P < 0.05). Anaemia substantially increased heart weight in fetal sheep. The volume proportions of connective and vascular tissue were unchanged. Cardiomyocyte mass expanded by a balanced combination of cellular enlargement, increased terminal differentiation and accelerated proliferation.

The placenta in the integrated physiology of fetal volume control.

Almost all water that enters the conceptus of the sheep enters via the placenta. The forces that drive water are hydrostatic and osmotic. The placental channels that allow water to cross into the fetus have not been identified by microanatomic means. Although an "equivalent pore" system can account for the diffusional entry of small hydrophilic solutes, it can be calculated that the filtration coefficient of this system is too small to account for the demonstrated trans-placental water flows. It is possible that a second much less numerous system of large pores permits the flow of water, but that is by no means certain. The placenta does not control the amount of water that enters the conceptus; nor does any other single fetal structure. And water entry is not dependent on the volume of water already present. However, the combined physiological properties of the fetal heart, kidneys, somatic tissues and placenta constitute a consistent explanation of fetal water volume control.

Intramembranous solute and water fluxes during high intramembranous absorption rates in fetal sheep with and without lung liquid diversion.

OBJECTIVE: To examine mechanisms that mediate increased intramembranous solute and water absorption. STUDY DESIGN: Intramembranous solute and water fluxes were measured in fetal sheep under basal conditions and after intraamniotic infusion of lactated Ringer's solution of 4 L/d for 3 days with and without lung liquid diversion. RESULTS: Intramembranous sodium, potassium, chloride, calcium, glucose, and lactate fluxes increased 2.5- to 7.9-fold, were linearly related to volume fluxes (r = 0.83-0.99), and were unaffected by lung liquid. All clearance rates, except that of lactate, increased to equal the intramembranous volume absorption rate during infusion. CONCLUSION: Under basal conditions, passive diffusion makes a minor and bulk flow a major contribution to intramembranous solute absorption. During high absorption rates, the increase in solute absorption above basal levels appears to be due entirely to bulk flow and is unaffected by lung liquid. The increased bulk flow is consistent with vesicular transcytosis.

Responses of amniotic fluid volume and its four major flows to lung liquid diversion and amniotic infusion in the ovine fetus.

We designed experiments to allow direct measurement of amniotic fluid volume and continuous measurement of lung liquid production, swallowing, and urine production in fetal sheep. From these values, the rate of intramembranous absorption was calculated. Using this experimental design, the contribution of lung liquid to the control of amniotic fluid volume was examined. Fetuses were assigned to 1 of 4 protocols, each protocol lasting 3 days: control, isovolemic replacement of lung liquid, supplementation of amniotic fluid inflow by 4 L/day, and supplementation of amniotic inflow during isovolemic replacement of lung liquid. We found no effect of lung liquid replacement on any of the known flows into and out of the amniotic fluid. Although intramembranous absorption increased greatly during supplementation, the amniochorionic function curves were not altered by isovolemic lung liquid replacement. We conclude that lung liquid does not appear to contain a significant regulatory substance for amniotic fluid volume control.

Sequential growth of fetal sheep cardiac myocytes in response to simultaneous arterial and venous hypertension.

While the fetal heart grows by myocyte enlargement and proliferation, myocytes lose their capacity for proliferation in the perinatal period after terminal differentiation. The relationship between myocyte enlargement, proliferation, and terminal differentiation has not been studied under conditions of combined arterial and venous hypertension, as occurs in some clinical conditions. We hypothesize that fetal arterial and venous hypertension initially leads to cardiomyocyte proliferation, followed by myocyte enlargement. Two groups of fetal sheep received intravascular plasma infusions for 4 or 8 days (from 130 days gestation) to increase vascular pressures. Fetal hearts were arrested in diastole and dissociated. Myocyte size, terminal differentiation (%binucleation), and cell cycle activity (Ki-67[+] cells as a % of mononucleated myocytes) were measured. We found that chronic plasma infusion greatly increased venous and arterial pressures. Heart (but not body) weights were approximately 30% greater in hypertensive fetuses than controls. The incidence of cell cycle activity doubled in hypertensive fetuses compared with controls. After 4 days of hypertension, myocytes were (approximately 11%) longer, but only after 8 days were they wider (approximately 12%). After 8 days, %binucleation was approximately 50% greater in hypertensive fetuses. We observed two phases of cardiomyocyte growth and maturation in response to fetal arterial and venous hypertension. In the early phase, the incidence of cell cycle activity increased and myocytes elongated. In the later phase, the incidence of cell cycle activity remained elevated, %binucleation increased, and cross sections were greater. This study highlights unique fetal adaptations of the myocardium and the importance of experimental duration when interpreting fetal cardiac growth data.

Myocyte enlargement, differentiation, and proliferation kinetics in the fetal sheep heart.

The generation of new myocytes is an essential process of in utero heart growth. Most, or all, cardiac myocytes lose their capacity for proliferation during the perinatal period through the process of terminal differentiation. An increasing number of studies focus on how experimental interventions affect cardiac myocyte growth in the fetal sheep. Nevertheless, fundamental questions about normal growth of the fetal heart remain unanswered. In this study, we determined that during the last third of gestation the hearts of fetal sheep grew primarily by four processes. 1) Myocyte proliferation contributed substantially to daily cardiac mass gain, and the number of cardiac myocytes continued to increase to term. 2) The (hitherto unrecognized) contribution to cardiac growth by the increase in myocyte size associated with the transition from mononucleation to binucleation (terminal differentiation) became considerable from approximately 115 days of gestational age (dGA) until term (145dGA). Because binucleation became the more frequent outcome of myocyte cell cycle activity after approximately 115dGA, the number of binucleated myocytes increased at the expense of the number of mononucleated myocytes. Both the interval between nuclear divisions and the duration of cell cycle activity in myocytes decreased substantially during this same period. Finally, cardiac growth was in part due to enlargement of 3) mononucleated and 4) binucleated myocytes, which grew in cross-sectional diameter but not length during the last third of gestation. These data on normal cardiac growth may enable a more detailed understanding of the consequences of experimental and pathological interventions in prenatal life.

Effects of intravascular infusions of plasma on placental and systemic blood flow in fetal sheep.

Six singleton fetal sheep of 118-122 days gestational age were instrumented with flow sensors on the brachiocephalic artery, the postductal aorta, and the common umbilical artery and with arterial and venous intravascular catheters. At 125-131 days of gestation, we started week-long continuous recordings of flows and pressures. After control measures had been obtained, the fetuses were given continuous intravenous infusions of adult sheep plasma at an initial rate of 229 ml/day. After 1 wk of infusion, fetal plasma protein concentrations had increased from 34 to 78 g/l, arterial and venous pressures had increased from 42 to 64 and from 2.7 to 3.7 mmHg, and systemic resistance (exclusive of the coronary bed) had increased from 0.047 to 0.075 mmHg.min(-1).ml(-1), whereas placental resistance had increased from 0.065 to 0.111 mmHg.min(-1).ml(-1). Fetal plasma renin activities fell as early as 1 day after the start of infusion and remained below control (all changes P < 0.05). All flows decreased slightly although these decreases were not statistically significant. Thus the increase in arterial pressure was entirely due to an increase in systemic and placental resistances.

Intravascular infusions of plasma into fetal sheep cause arterial and venous hypertension.

Fetal volume control is driven by an equilibrium between fetal and maternal hydrostatic and oncotic pressures in the placenta. Renal contributions to blood volume regulation are minor because the fetal kidneys cannot excrete fluid from the fetal compartment. We hypothesized that an increase in fetal plasma protein would lead to an increase in plasma oncotic pressure, resulting in an increase in fetal arterial and venous pressures and decreased angiotensin levels. Plasma or lactated Ringer solution was infused into each of five twin fetuses. After 7 days, fetal protein concentration was 71.2 +/- 4.2 g/l in the plasma-infused fetuses compared with 35.7 +/- 6.3 g/l in the lactated Ringer-solution-infused fetuses. Arterial pressure was 68.0 +/- 3.6 compared with 43.4 +/- 1.9 mmHg in the lactated Ringer solution-infused fetuses (P < 0.0003), whereas venous pressure was 4.8 +/- 0.3 mmHg in the plasma-infused fetuses compared with 3.3 +/- 0.4 mmHg in the lactated Ringer solution-infused fetuses (P < 0.036). Six fetuses were studied on days 0, 7, and 14 of plasma protein infusion. Fetal protein concentration increased from 31.1 +/- 1.5 to 84.8 +/- 3.8 g/l after 14 days (P < 0.01), and arterial pressure increased from 43.1 +/- 1.8 to 69.1 +/- 4.1 mmHg (P < 0.01). Venous pressure increased from 3.0 +/- 0.4 to 6.2 +/- 1.3 mmHg (P < 0.05). Fetal heart rate did not change. Angiotensin II concentration decreased, from 24.6 +/- 5.6 to 2.9 +/- 1.3 pg/l, after 14 days (P < 0.01). Fetal plasma infusions resulted in fetal arterial and venous hypertensions that could not be corrected by reductions in angiotensin II levels.

Absorption of amniotic fluid by amniochorion in sheep.

Swallowing of amniotic fluid and lung fluid inflow were eliminated in 10 chronically instrumented fetuses. The urachus was ligated, and fetal was urine drained to the outside. At the beginning and the end of 21 experiments of 66 +/- 5 (SE) h duration, all amniotic fluid was temporarily drained to the outside for volume measurement and sampling. Amniotic fluid osmolalities and oncotic pressures were experimentally controlled. Amniochorionic absorption of amniotic fluid depended strongly on the osmolality difference between amniotic fluid and fetal plasma (P < 0.001), but at zero osmolality difference there still was a mean absorption rate of 23.8 +/- 4.7 (SE) ml/h (P < 0.001). Absorption was unaffected by the protein concentration difference between amniotic fluid and fetal plasma, but infused bovine albumin in the amniotic fluid was absorbed at a rate of 1.8 8 +/- 0.4 g/h (P < 0.001), corresponding to a volume flow of fluid of 33.8 8 +/- 6.1 ml/h (P < 0.001). Fluid absorption in the amniochorion is driven in part by crystalloid osmotic pressure, but about 25 ml/h is absorbed by a path that is permeable to protein. That path has the physiological characteristics of lymphatic drainage, although no anatomic basis is known to exist for a lymphatic system in the amniochorion.