Ontomimetic environment resulted in 3D-dhECT CMs exhibiting mature ultrastructural features. (A) Day 24 3D-dhECT CMs developed first characteristics of a muscle cell including mitochondria (m) and Z-lines (Z). (B) Ultrastructural images of 124 day old 3D-dhECT CMs showed sarcomere structures, including Z-lines (Z), I-bands (I), H-zones (H), intercalated discs (ID), gap junctions (GJ), and mitochondria (m). (C) Additionally, CMs contained caveolae (C) adjacent to basement membrane (bm), mitochondria with well-defined cristae, and T-tubules (T) adjacent to Z-lines, a key component of functional mature CMs. Insert: magnified view processed to highlight T-tubules.