In biology, a whorl might occur at the ends of different structures or occur in the middle of structures. It's often used to describe the structures of organs and used in the aid of identification. It's a cluster of cells or tissue that surround another, that starts at the same plain of axis or starts at one point and wraps around that point in an expanding circular pattern.

The Hassall's corpuscle, formed from type VI epithelial reticular cells in the thymus

Thymus

The thymus is a specialized organ of the immune system. The thymus produces and "educates" T-lymphocytes , which are critical cells of the adaptive immune system....

, is an example of a whorl shaped structure.