There is an increasing understanding that some mitral valve pathologies have developmental origins. The time course of valvulogenesis varies by animal model, where in cattle, the branched chordae tendineae architecture becomes fully developed at full term. The mechanism by which chordae tendineae bifurcate during fetal development remains unknown. The current study presents a detailed description of bovine chordae tendineae formation and bifurcation during fetal development. Analysis of Movat Pentachrome-stained histological sections of the developing mitral valve apparatus were accompanied by micro-CT imaging. TEM imaging of chordae branches and common trunks allowed measurement of collagen fibril diameter distributions. We observed a proteoglycan-rich “transition zone” at the junction between the fetal mitral valve anterior leaflet and chordae tendineae with “perforations” lined by MMP1/2 and Ki-67 expressing endothelial cells. This region also contained clusters of proliferating endothelial within the bulk of the tissue. We hypothesize this zone marks a region where chordae tendineae bifurcate during fetal development. In particular, perforations, created by localized MMP activity serve as a site for the initiation of a “split” of a single chordae attachment into two. This is supported by TEM results that suggest a similar population of collagen fibrils runs from the branches into a common trunk. A clear understanding of normal mitral valvulogenesis and it’s signaling mechanisms will be crucial in developing therapeutics and/or tissue-engineered valve replacements.

There is increasing evidence that some adult mitral valve pathologies may have developmental origins involving errors in cell signaling and protein deposition during valvulogenesis. While early and late gestational stages are well-documented in zebrafish and small mammalian models, longitudinal studies in large mammals with a similar gestational period to humans are lacking. Further, the mechanism of chordae tendineae formation and multiplication remains unclear. This current study presents a comprehensive examination of mitral anterior leaflet and chordae tendineae development in a bovine model (with a gestational period of 9 months). Remarkably distinct from small mammals, bovine development displayed early branched chordae, with increasing attachments only until birth, while the anterior leaflet grew both during gestation and postnatally. Chordae also exhibited accelerated collagen deposition, maturation, and crimp development during gestation. These findings suggest that the bovine anterior leaflet and chordae tendineae possess unique compartmentalized processes of development despite being a continuous collagenous structure.