Epicardium is required for the full rate of myocyte proliferation and levels of expression of myocyte mitogenic factors FGF2 and its receptor, FGFR-1, but not for transmural myocardial patterning in the embryonic chick heart.

Proper heart development requires patterning across the myocardial wall. Early myocardial patterning is characterized by a transmural subdivision of the myocardium into an outer, highly mitotic, compact zone and an inner, trabecular zone with lower mitotic activity. We have shown previously that fibroblast growth factor receptor (FGFR) -mediated signaling is central to myocyte proliferation in the developing heart. Consistent with this, FGFR-1 and FGF2 are more highly expressed in myocytes of the compact zone. However, the mechanism that regulates the transmural pattern of myocyte proliferation and expression of these mitogenic factors is unknown. The present study examined whether this transmural patterning occurs in a myocardium-autonomous manner or by signals from the epicardium. Microsurgical inhibition of epicardium formation in the embryonic chick gives rise to a decrease in myocyte proliferation, accounting for a thinner compact myocardium. We show that the transmural pattern of myocyte mitotic activity is maintained in these hearts. Consistent with this, the expression patterns of FGF1, FGF2, and FGFR-1 across the myocardium persist in the absence of the epicardium. However, FGF2 and FGFR-1 mRNA levels are reduced in proportion to the depletion of epicardium. The results suggest that epicardium-derived signals are essential for maintenance of the correct amount of myocyte proliferation in the compact myocardium, by means of levels of mitogen expression in the myocardium. However, initiation and maintenance of transmural patterning of the myocardium occurs largely independently of the epicardium.