Possible mechanisms for early and intermediate stages of sperm chromatin condensation patterning involving phase separation dynamics

During spermiogenesis in some internally fertilizing molluscs and insects, the post-meiotic spermatid nucleus develops via a sequence of complex patterns of the nuclear contents (chromatin and nucleoplasm) on the way to final chromatin condensation. We have examined the TEM data on these sequences for three species: Philaenus spumarius(a homopteran insect), Murex brandaris (a gastropod mollusc), and Eledone cirrhosa(a cephalopod mollusc). For each of these, spatially quantitative study reveals a constant spacing between pattern repeats through changes from granular to fibrillar to lamellar pattern, followed finally by a shrinkage of the spacing. Therefore we distinguish a "patterning" stage followed by a "condensation" stage. The former appears to demand a dynamic explanation, because there is no sign of structural connections to establish the part of the spacing that crosses the nucleoplasm. We consider types of dynamic mechanism, and show that for "nanostructural" dimensions (tens of nanometers as pattern spacing) reaction-diffusion dynamics are quite inappropriate, but that separation of two fluid phases by a mechanism similar to what is known as "spinodal decomposition" is a very attractive possibility.