Transmission electron microscopy showing the
symbiont ultrastructure. A - D: symbionts observed in early-embryo stages. (A) Endosymbiotic bacteria (arrowheads) were observed inside a vacuole (V) and close to the nucleus (N). (B) The symbiont matrix is composed of an electron-dense region rich in ribosomes (small white arrow), and an electron-lucid area that contains the DNA fibers (f – open arrowhead) (Bar 0.5 μm). C – At high magnification, it is possible to observe the symbiont in close proximity to the host cell nucleus (N). An electron-dense region is indicative that the symbiont vacuole touches the nuclear envelope (white arrow). It is also interesting to note the association (small black arrow) between the symbiont and the endoplasmic reticulum (RE) containing ribosomes (R). (Bar 0.5 μm). D – The endosymbiont is enclosed by two membrane units: an outer membrane (OM) that faces the vacuole (V) and an inner membrane (IM) that faces the matrix. Note that the symbiont electron-lucid area in the matrix contains DNA fibers (open arrowhead) and that the bacteria are associated with the endoplasmic reticulum (R) with associated ribosome (Bar 0.5 μm).