Final answer:
The basophil nucleus contains a U- or an S- shape and has dark staining granules. Basophils release histamine and heparin.
Step-by-step explanation:
The type of nucleus that contains a U- or an S- shape and has granules that stain very dark is the basophil nucleus. Basophils are a type of white blood cell that release histamine and heparin, and their nucleus is typically U or S-shaped. The granules in their cytoplasm stain dark blue to purple.