Thyroid hormone does not induce maturation of embryonic chicken cardiomyocytes in vitro

Fetal cardiac growth in mammalian models occurs primarily by cell proliferation (hyperplasia). However, most cardiomyocytes lose the ability to proliferate close to term and heart growth continues by increasing cell size (hypertrophy). In mammals, the thyroid hormone triiodothyronine (T3) is an important driver of this process. Chicken cardiomyocytes, however, keep their proliferating ability long after hatching but little information is available on the mechanisms controlling cell growth and myocyte maturation in the chicken heart. Our aim was to study the role of T3 on proliferation and differentiation of embryonic chicken cardiomyocytes (ECCM), enzymatically isolated from 19-day-old embryos and to compare the effects to those of insulin-like growth factor-1 (IGF-1) and phenylephrine (PE). Hyperplasia was measured using a proliferation assay (MTS) and hypertrophy/multinucleation was analyzed morphologically by phalloidin staining of F-actin and nuclear staining with DAPI. We show that IGF-1 induces a significant increase in ECCM proliferation (30%) which is absent with T3 and PE. PE induced both hypertrophy (61%) and multinucleation (41%) but IGF-1 or T3 did not. In conclusion, we show that T3 does not induce maturation or proliferation of cardiomyocytes, while IGF-1 induces cardiomyocyte proliferation and PE induces maturation of cardiomyocytes.