||Heparan sulfate proteoglycans (HSPGs) are biologically relevant molecules composed of core proteins and glycosaminoglycan (GAG) chains. The location of HSPGs, on the cell surface or in the extracellular matrix, and their structural heterogeneity place them at a unique advantage to influence signaling pathways and cellcell or cell-matrix interactions. Most of the genes that code for the core proteins and the enzymes that build and modify the GAG chains have been identified. One type of modification, 3-O-sulfation, is catalyzed by a family of 3-O-sulfotransferases (3-OSTs) in zebrafish. Gene expression studies suggest they could modulate different steps of zebrafish development, but the specific roles of each 3-OST have not been elucidated. My dissertation focused on the functions a particular 3-OST, 3-OST-7, perform in zebrafish heart development. To elucidate the functions of 3-OST-7 in zebrafish heart development, I knocked down 3-OST-7 using morpholinos and found that 3-OST-7 controls ventricular contraction. Analysis of the noncontracting ventricle phenotype in 3-OST-7 morphants demonstrated that tropomyosin4 is required to mediate 3-OST-7 regulation of ventricular contraction, placing 3-OST-7 upstream of a novel pathway that controls coordinated sarcomere assembly. Further analysis of the 3-OST-7 knockdown model illustrated that 3-OST-7 functions in two distinct pathways that regulate ventricular maturation. First, 3-OST-7 is necessary for transforming the isometric ventricular cardiomyocytes into their elongated form. Second, 3-OST-7 shares a feedback loop with bmp4 signaling to regulate contraction. These findings demonstrated the specificity of 3-OST-7 action towards influencing cardiac development. Moreover, these studies highlighted the strength of the 3-OST-7 knockdown model in teasing apart the relationship between structure and function. I anticipate that further investigation of this model could advance our understanding of the interrelationships between HSPGs and the signaling pathways that orchestrate cardiac formation.