Slightly off topic, but on a clear blue sky it's possible to directly visualize the white blood cells running around on your retina. I love watching them go about business, and I think I heard it can even be used diagnostically to do a manual WBC count in extremis for leukoproliferative disease.

They're pretty tiny though, I'm not sure if you'd actually see a center in the RBCs

The lighter centers could have been diffraction around the object and reconvergence. Or some kind of signal processing effect at the retinal level.

The retinal vessel network is a fun thing to inspect as you say. It works best for me when the sun is high in the sky. The bright, featureless blue brings out the branching network very well.