Microtubules polymerized in D2O-Split in two

Keep eye on the white circle, microtubules splits into two. Just before splitting the tubule curled in a shape of an upside down question mark. The upper part of tubule follows it nose and the lower curled part splits at the curve and follow that tangent. It looks like to me that tubule are elastic but at tight turns/curls (Over few number of protofilaments), they are pretty rigid like a rod and easily break. I think this is what happened here; it took a tight turn/curl and the bond between one protofilament circle and next, breaks, though the break does not have to be shapely between the circles it can have any arbitrary forms.