Cytonemes with complex geometries and composition extend into invaginations of target cells

Abstract

Cytonemes are specialized filopodia that mediate paracrine signaling in Drosophila and other animals. Studies using fluorescence confocal microscopy (CM) established their general paths, cell targets, and essential roles in signaling. To investigate details unresolvable by CM, we used high pressure freezing and electron microscopy to visualize cytoneme structures, paths, contents, and contacts. We observed cytonemes previously seen by CM in the Drosophila wing imaginal disc system, including in disc, tracheal air sac primordium (ASP), and myoblast cytonemes, and identified cytonemes extending into invaginations of target cells and cytonemes connecting ASP cells and connecting myoblasts. Diameters of cytoneme shafts vary between wide (206 {+/-} 51.8 nm) and thin (55.9 {+/-}16.2 nm) segments at regular intervals. Actin, ribosomes, and membranous compartments are present throughout; rough endoplasmic reticulum and mitochondria are in wider proximal sections. These results reveal novel structural features of filopodia and provide a basis for understanding cytoneme cell biology and function. SummaryCytoneme signaling filopodia of Drosophila cells have regions defined by oscillating diameters, and contents that include ER, mitochondria and ribosomes, and form contacts along invaginations of target cell membranes.