Endocytic activity in embryonic cardiac cushion mesenchyme in vivo and in collagen gel lattices

Abstract

Mesenchymal cells, termed cushion tissue (CT) cells, are the principal cellular elements in atrioventricular (AV) endocardial cushions, the latter constituting AV septal and valvular primordia in the embryonic heart. Atrioventricular canals of 2 1/2 day chick embryo hearts were explanted onto collagen gel lattices, wherein cushion endothelial cells acquired the characteristics of CT cells and invaded the gel. The endocytic activity of in situ CT cells was compared to that of gel-cultured CT cells by exposing appropriate preparations to horseradish peroxidase (HRP), cationized ferritin (CF) or to lysozyme. Stimulation by these exogenous proteins resulted in phagocytosis. In addition, all three markers were associated with coated pits, smooth surfaced vesicles (45-60 nm), C- or cup-shaped structures and other lysosomal elements, but were excluded from Golgi cisternae and their entire vesicle population. In CT cells, HRP does not act solely as a "content" marker that reflects fluid-phase uptake. Instead it appears to follow adsorptive endocytic pathways. The endocytic behavior of in vivo and in vitro cushion tissue cells appears to be the same. Endogenous endocytic activity may reflect in part membrane retrieval, which counterbalances the extensive exocytosis observed in these cells.

The Endocardium and Heart Valves

“…Based on appearance, AV and OFT cushions were described as containing "mesenchymal cells beneath the endothelium that lack a basal lamina," and longitudinal analysis of these structures suggested that they morph into the valvular structures (Markwald et al 1977). Early experiments in chick and quail embryos were the first to provide evidence that mesenchyme cells of the cushion are endothelial-derived, based on positive immunoreactivity of the quail endothelial cell marker QH1, in cushion mesenchyme (Markwald et al 1990), and invasion of cushion cells plated on collagen gel lattices (Hay et al 1983). Additional electron microscopy further captured the transformation of endothelial cells into mesenchyme cells as noted by cell-cell separation and migration into the cushion space rich in hyaluronan matrix (Markwald et al 1990).…”

The Heart of The Anatomical Record: Exploring the World of Cardiac Development, with Special Homage to Roger Markwald

“…Roger also published frequently with his mentees/colleagues in The Anatomical Record in areas of cardiac formation, general topics of developmental processes, and issues relating to the journal itself when he was the Editor‐in‐Chief. For example, studies on aspects of cardiac development included those tracing embryonic cardiac cushion formation (Hay, Markwald, and Sage, ; Bernanke and Markwald, ); on distribution of basement membrane antigen in early embryonic hearts (Kitten, Markwald, and Bolender, ); exploration of regions of the embryonic heart where endothelial cells transform into mesenchyme (Sinning, Krug, and Markwald, ); study of the morphogenesis of the sinus venosus, extracardiac mesenchyme, and atrium (Tasaka, Krug, and Markwald, ); immunohistochemical studies of atrial development with Andy Wessels (Wessels et al, ); conotruncal anomalies in the trisomy 16 mouse (Waller et al, ); on identification and detection of the periostin gene in cardiac development (Norris et al, ); and periostin family of proteins as therapeutic targets for heart disease (Litvin et al, . )…”