During pregnancy the maternal organism undergoes a variety of adaptive physiological changes. The cardiovascular system is strongly affected and its response is characterized by cardiac hypertrophy. The molecular mechanisms underlying these adaptations are still poorly understood. The recent postulation of resident organotypic stem cells in the cardiovascular system prompted us to explore the potential involvement of these cells and the related signaling cascades involved in this adaptation process in the heart.
We have analyzed cell proliferation using immunostainings in mouse hearts at different stages during pregnancy. The average percentage of proliferating cells in cardiac sections increased from virgin controls to gestational day 3 (GD3), peaked at GD14, and immediately stopped after delivery. Co-staining revealed that the majority of proliferating cells were fibroblasts and endothelial cells (ECs). The lack of cardiomyocyte proliferation was corroborated using BrdU pulse chase experiments analyzed in single cells upon Langendorff perfusion. The proliferation of fibroblasts and ECs reflects angiogenesis, which was underscored by an increase of capillary density, and extracellular matrix remodeling known to occur during pregnancy induced hypertrophy. To mimic the action of pregnancy hormones we implanted pellets with constant release of hormone for 21 days subcutaneously in ovariectomized mice. Capillary density was significantly increased in mice receiving progesterone-pellets alone or in combination with estrogen-pellets without induction of hypertrophy. Thus, increase of capillary density is directly induced by pregnancy hormones. Assessment of differentially expressed genes after hormone treatment revealed induction of angiogenetic factors and their receptors.
We are currently investigating the mechanisms underlying the pregnancy hormone-induced proliferation in cardiac ECs and fibroblasts.