Structural analysis of endocardial cytodifferentiation

Abstract

Embryonic hearts 8–40 somites (10.0–13.5 days) where subjected to transmission and scanning ultrastructural and histological examination to monitor pathways of endocardial differentiation. Primitive endocardium (day 10.0 or 8 somites) consisted of closely packed cells with smooth luminal surfaces except for cilia at cellular interfaces. Internally these cells possessed minimal secretory potential as indicated by undilated rough-surfaced endoplasmic reticulum (RER) and undeveloped Golgi complexes. The latter had nondistended lamellae and a low population of small (100-nm) bristle-coated and uncoated vesicles. Cytodifferentiation of the primitive endocardium was biphasic. In the outflow tract and AV canal, areas of future cushion tissue (cardiac mesenchyme) formation, endocardium was transformed by day 11.5 (16–18 somites) into cells with amplified secretory potential as evidenced by dilation of RER cisternae, hypertrophy of Golgi lamellae and augmented formation of Golgi vesicles consisting primarily of larger (150–200 nm) uncoated types. The luminal surfaces in these areas became convoluted and flattened while the extracellular (cardiac jelly) surface developed blebs and was studded with globular and fibrillar strands of matrical material. Surface topography and serial sectioning of the initial sites of cushion tissue formation suggested the latter was actually a derivative of endocardium having augmented secretory potential. Conversely, endocardium approached by invaginating myocardial trabeculae (atrium and ventricle) appeared to lose secretory potential as indicated by its (1) progressive attenuation of the cytoplasm, (2) reduction or complete loss of surface projections and associated globular and fibrillar material, (3) regression of RER and Golgi complexes, and (4) acquisition of cystic foci in otherwise nondilated RER. Results therefore indicated that both endocardial surface and internal features could be related to developmental changes in microenvironment and function.