Abstract Body: Background
Mechanistic insight into pediatric and adult heart disease is limited by the absence of a comprehensive reference for normal postnatal human cardiac development. While early embryonic heart formation is well characterized, the cellular and molecular processes that govern postnatal maturation from birth to adulthood in humans remain poorly understood.

Aims
Definition of human heart maturation by constructing an integrated multi-omics atlas of the healthy human heart across postnatal life.

Methods
We performed integrated single-nucleus sequencing and high-resolution spatial multi-omics (Xenium In Situ) of 195 healthy human heart samples from 53 individuals aged 0-75 years (89 pediatric, 84 adult; 173 snRNA-seq, 26 Xenium; 97 LV, 43 RV, 31 LA, 28 RA; 22 female, 31 male). We complement this data by deep-learning-based analysis of cardiac 3D microarchitecture across age to identify structural correlates of cellular maturation.

Results
We identify conserved cellular and spatial heterogeneity within cardiac cell types that change markedly after birth, partially conserved in pig and mouse. The newborn human heart resembles features of the fetal heart that are lost at approximately 6 months of age, when cell identities resemble that of the adult human heart for most cell types. Gene expression in newborn cell states is enriched in specific functional features, including sarcomere immaturity, ribosome content, metabolism, and matrix maturation. Structurally, the postnatal human heart is spatially organized into distinct regions along the epicardial-to-endocardial axis. Coordinated gene programs across cell types drive structural maturation of the human heart from birth to adulthood, including hypertrophic cardiomyocyte growth, multi-nucleation, polyploidization, and changes in the cardiac vasculature and extracellular matrix, which is guided by intercellular signaling of spatially organized cellular communities. We observe an increase in the relative cell type abundance of mesenchymal cells and a reversal of cell state identities towards selected newborn features in the aged heart, indicating age-associated remodeling of the human heart.

Conclusions
We establish a comprehensive reference of postnatal human cardiac development, defining the cellular, spatial, and structural programs that underlie maturation from birth to adulthood.