SEM reveals filopodia-like protrusions in the early embryonic disc of the chick

Abstract

Embryonic development depends on an enormous amount of signaling events between a continuously growing number of cells. According to the classical view, signal transduction is based on either simple diffusion, trans­cytosis or migrating cells. Very recently the existence of filopodia-like protrusions has been proposed as a novel mechanism of long-range intercellular signaling during chick somite and limb development. Analogous thinner structures called cytonemes play a role in the development of Drosophila melanogaster , as well. The present study gives new insight into this problem by adding important information using scanning electron microscopy (SEM). Early chick embryos between stages 4 and 5 (representing late gastrulation and early neurula­tion) were dissected, fixed and critical-point dried and were then fractured in several steps, each step followed by platinum/palladium coating and analysis by SEM. Fracturing was carried out sequentially in the same specimens at different anterior-posterior levels anterior to the primitive node and the ultrastructure of the embryonic disc was analysed with special emphasis on cellular processes. We found cell protrusions of different shape and size in all three layers (mesoderm, epiblast, and hypoblast) of the early embryonic discs. They were most abundant and prominent between the mesodermal cells but also seen in other layers as well as between layers. These protru­sions can be roughly classified into cytoplasmic bridges with mid-bodies, cilia-like protrusions, as well as filopo­dia-like protrusions. As previously shown, cilia of different length are present on the apical surface of epiblast and hypoblast cells. Similarly a plethora of variable protrusions with or without mid-bodies are seen between neigh­bouring cells within embryonic layers. Unexpectedly, we found that cells of the mesodermal layer form a dense net of filopodia-like contacts with the epithelial cells of the epiblast. Our data prove that filopodia-like protrusions are present at earlier developmental stages than previously shown. Their abundance indicates a major role in the intercellular communication during early chick embryonic devel­opment and may provide a structural basis of cell-to-cell communication during gastrulation and early neurula­tion in the chick.