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

Abstract

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.

Cardiac corticosteroid receptors mediate the enlargement of the ovine fetal heart induced by chronic increases in maternal cortisol

“…In early gestation, cardiac growth is mostly a result of the production of new myocytes originating through cell division and proliferation (Smolich 1995). After w115 days of gestation in sheep, however, cardiac growth results primarily from increases in myocyte size (Jonker et al 2007). Myocytes lose their ability to divide and proliferate shortly after birth in an event in which there is nuclear division without subsequent cell division (Oparil et al 1984).…”

“…Myocytes lose their ability to divide and proliferate shortly after birth in an event in which there is nuclear division without subsequent cell division (Oparil et al 1984). In fetal sheep, the number of terminally differentiated or binucleate myocytes increases from w115 days of gestation through term, and heart growth during this period is due to increases in both myocyte size and myocyte proliferation (Jonker et al 2007). Theoretically, cortisol could be stimulating growth through either hypertrophy or hyperplasia, or possibly even both.…”

ANG II modulation of cardiac growth and remodeling in immature fetal sheep

“…In a subset of fetuses (n ϭ 5 for each group), the hearts were dissected into anatomical components, and each component was weighed and immediately frozen in liquid nitrogen. For the remaining fetuses (n ϭ 4 for each group), fetal hearts were enzymatically dissociated on a Langendorff apparatus, and the cardiomyocytes were fixed for morphometric analysis, as previously described (19). Ewes were euthanized by an intravenous administration of pentobarbital sodium and phenytoin sodium (120 mg/kg barbiturate, euthasol solution; Virbac, Fort Worth, TX).…”

“…In addition to developmental changes in vascularity and myocyte myofibrillar and mitochondrial content, there is a marked increase in the number of cardiomyocytes within the heart, as well as a transition from mononucleated to binucleated (terminally differentiated) myocytes (19,38). In the sheep heart, the transition from mononucleated to binucleated cells begins around 100 days of gestation (term ϳ145 days), such that, at term, ϳ70% of cardiomyocytes are binucleated, or terminally differentiated (19).…”

“…Therefore, we sought to determine whether ANG II stimulates heart growth and cardiomyocyte hyperplasia and/or hypertrophy in the immature fetal heart independent of the effects of cardiac load. Fetal sheep were directly infused with ANG II at 100 days of gestation to target the transition from mononuclear to binucleated (terminally differentiated) cardiomyocytes in the fetal heart (19). Coinfusion of the vasodilator sodium nitroprusside (NTP) was utilized to normal blood pressure during ANG II administration.…”