One question about SP-D fuzzyball multimers is whether or not the trimers, or hexamers or dodecamers only have N termini binding end-to-end making the diameter of the multimer about 120nm or whether it is possible that is there another configuration. SP-D multimers with fewer arms (maybe less than 12 or 14 often show a lucent (NOT MEANING luminous but the opposite, as in darker center… like electron lucent in TEM where there is a more background grayscale than a high level of “lightness” or “brightness”. This is what I have seen in multimers of SP-D of low arm count and I am trying to figure out a configuration of the multimer that satisfies this arrangement. In an image of a hexamer (Arryoy et al) there is one which really speaks to the possibility that N termini can bind side by side. Granted, Arroyo et al manipulated the pH of the preparation, but pH changes would occur in vivo in surfactant and other fluids as well, and might just be a trigger for fuzzyball formation that has arms that do NOT span the center N to end of Ntermini, but create a center hollow area with N to side of N binding.
Here is their picture…. N termini are the brightest portion on the lower part of the image. I did not ask for permission to repost this image but thank them.
and a cut and paste model from the above and a SP-D multimer from the same micrograph.  Aside from the butt and feet look