Hearts from adult proNgf-HA/⁺ mice exhibit microvascular damage and hearts from juvenile proNgf-HA/⁺ mice show early signs of endothelial cell activation and microvascular damage. (A–C) Transmission electron microscopic images of hearts from 4-mo-old Ngf^+/+, Ngf^+/−, and proNgf-HA/⁺ mice, respectively. Arrowhead, microvascular endothelial cell with fragmented plasma membrane. (D–F) Normal H and E histology of 1-mo-old Ngf^+/+, Ngf^+/−, and proNgf-HA/⁺ hearts. (G–I) Representative transmission electron microscope images of hearts from 1-mo-old Ngf^+/+, Ngf^+/−, and proNgf-HA/⁺ mice. Microvascular endothelial cells from proNgf-HA/⁺ mice exhibit filamentous membrane projections into the vessel lumen (I, arrow). In the proNgf-HA/⁺ myocardium, ∼50% of microvascular endothelial cells examined showed signs of activation as well as perivascular edema (I, asterisk). Additionally, a loss of apposition in the processes of pericyte with endothelial cells is observed (arrowhead). Bars: (A–C and G–I) 2 µm; (D–F) 100 µm. n = 2/genotype.