During pregnancy the maternal organism undergoes a variety of physiological changes to ensure its optimal adaptation to the needs of the developing fetus. The cardiovascular system of the mother is strongly affected as a remarkable increase in blood plasma volume and an eccentric cardiac hypertrophy occur. These morphological and functional alterations are rapidly and completely reversed upon partition. However, until now the molecular mechanisms underlying this surprising physiological adaptation of the maternal cardiovascular system are poorly understood. Because of the recently developed concept of resident organotypic stem cells in the cardiovascular system, our aim is to explore the potential involvement of these cells in the physiological adaptation process.

We have analyzed cell proliferation using immunostaining against pHH3, Ki67 and BrdU incorporation assays in mouse hearts at different stages during pregnancy. The different cardiac cell types, in particular cardiomyocytes, fibroblasts and endothelial cells were identified by co-staining for vimentin, a-actinin, a-smooth muscle actin and PECAM-1, respectively. We found at all stages of gestation proliferation of mostly endothelial cells and fibroblasts, few smooth muscle cells but not cardiomyocytes. The average percentage of Ki67 positive nuclei per total nuclei of ventricular cells increased from virgin controls (0.890.25, n=8) to gestational day 3 (G3, 50222, n=3) and peaked at the end of gestation at G18 (2.120.36, n=6). Moreover, this increased rate of proliferation came to an abrupt halt within hours after delivery (0.440.12, n=2). Immunofluorescence stainings revealed that 2/3 of the proliferating cells were fibroblasts and 1/3 endothelial cells. The lack of cardiomyocyte proliferation during late stage pregnancy was corroborated when using BrdU pulse chase experiments and analysis at the single cell level after enzymatic digestion using Langendorff perfusion. In contrast to cardiomyocytes, we could confirm also at the single cell level proliferation of fibroblasts and endothelial cells. This appears to reflect angiogenesis and extracellular matrix remodeling known to occur en course of the pregnancy induced hypertrophy. In accordance with these findings we could detect an increase of hyperthrophy and of capillary density during pregnancy; both these parameters peaked at the day of delivery and decreased in the following days to the level of non pregnant mice. We are currently investigating the hormones underlying these physiological adaptive processes in cardiac endothelial cells and fibroblasts.