Left and right ventricular contributions to the formation of the interventricular septum in the mouse heart

Identification of interventricular septum precursor cells in the mouse embryo

Little is known about the formation of the interventricular septum (IVS), a central event during cardiogenesis. Here, we describe a novel population of myocardial progenitor cells in the primitive ventricle of the mouse embryo, which is characterized by expression of lysozyme M (lysM). Using LysM-Cre mice we show that lysozyme expressing cells give rise to the IVS and to a part of the left ventricular free wall, demonstrating that these heart regions are developmentally related. LysM+ precursors are not of hematopoietic origin and develop in the absence of transcription factors that regulate lysozyme expression in macrophages. LysM-deficient mice lack an overt cardiac phenotype, perhaps due to compensation by the related lysozyme P, which we also found to be expressed in the developing heart. Direct visualization of lysM expression, using LysM-EGFP knock-in mice, showed that ventricular septation is initiated at embryonic day 9 by the movement of myocardial trabeculae from the primitive ventricle towards the bulbo-ventricular groove and revealed the dynamics of IVS formation at later stages. Our studies predict that LysM-Cre mice will be useful to inactivate genes in the developing IVS.

Combinatorial signaling in the heart orchestrates cardiac induction, lineage specification and chamber formation

The complexity of mammalian cardiogenesis is compounded, as the heart must function in the embryo whilst it is still being formed. Great advances have been made recently as additional cardiac progenitor cell populations have been identified. The induction and maintenance of these progenitors, and their deployment to the developing heart relies on combinatorial molecular signalling, a feature also of cardiac chamber formation. Many forms of congenital heart disease in humans are likely to arise from defects in the early stages of heart development; therefore it is important to understand the molecular pathways that underlie some of the key events that shape the heart during the early stages of it development.

Transcriptional pathways in second heart field development

The heart is the first organ to form and function during vertebrate development and is absolutely essential for life. The left ventricle is derived from the classical primary or first heart field (FHF), while the right ventricle and outflow tract are derived from a distinct second heart field (SHF). The recent discovery of the SHF has raised several fundamental and important questions about how the two heart fields are integrated into a single organ and whether unique molecular programs control the development of the two heart fields. This review briefly highlights the contributions of the SHF to the developing and mature heart and then focuses primarily on our current understanding of the transcriptional pathways that function in the development of the SHF and its derivatives.