The three-dimensional structure of the central region in a synaptonemal complex: a comparison between rat and two insect species, Drosophila melanogaster and Blaps cribrosa.

The highly ordered central region of the synaptonemal complex (SC) in Blaps cribrosa has recently been studied by electron microscope tomography (EMT), and a simple three-dimensional model presented. Using the same experimental approach we have now compared the central region in Blaps with the central regions in Drosophila melanogaster and rat. In all three species, the SCs exhibit a central element (CE) flanked by two lateral elements (LEs). The central region between the two LEs is crossed by transverse filaments (TFs). The Blaps CE element is the most ordered one with a well-defined ladder-like structure with two longitudinal components bridged by a number of regularly spaced transverse components, the rungs of the ladder. At the junctions between the longitudinal and transverse components there are prominent dense structures. The CE is multi-layered with the ladders of the separate layers in approximate register. In Drosophila the transverse CE components are as distinct and well organized as in Blaps, while in rat they are present but are less frequent and less well ordered. The longitudinal CE components in Drosophila are often fragmented and even more so in rat. The tomographic analysis revealed that in all three species the central region contains the same structural units: a single TF associated with two short pillars (or globules), which correspond to the junction structures. A fibrous lattice connects the two pillars/globules on the same TF forming the transverse CE component and those on adjacent TFs forming the longitudinal CE component; fibers between pillars/globules also link consecutive CE layers together. In the longitudinal component the number of fibrous bridges between the pillars/globules is related to the conspicuousness of the longitudinal component, i.e. Blaps has most, Drosophila almost as many, and rat considerably fewer bridges. We conclude that the central region in rat, Drosophila and Blaps contains the same basic structural unit but the degree of order and concentration of the units differ: a higher density seems to be accompanied by a higher order within the CE.