Anatomical location of cardiac fibroblasts and their spatial relationship with cardiomyocytes and endothelial cells at different stages. (A) RNA staining of Col1a1 revealed the spatial pattern of cardiac fibroblasts at different developmental stages. Scale bar=500µm. (B) The development of cardiac fibroblasts can be grouped into four phases. (C) RNA staining analysis of Col1a1, Actn2, and Cd31 revealed the spatial proximity of FB, CM, and EC in E17.5 hearts. (D) Quantification of the number of CMs and ECs that contact with each FB (n=100). Scale bar=500µm and 100µm in the whole heart sections and enlarged sections, respectively.