Final answer:
Birbeck granules are rod-shaped particles with a central stripe resembling a tennis racket, unique to Langerhans cells in the skin, and are observed best under an electron microscope.
Step-by-step explanation:
Birbeck granules typically possess a unique structure that can be compared to that of a tennis racket or rod-shaped particles with a central stripe. These granules are distinctive to Langerhans cells, which are specialized dendritic cells found in the skin. The granules are named after British electron microscopist Michael Birbeck, who first described them in the early 1960s.
Birbeck granules are characterized by their ultrastructural appearance, which is best observed under an electron microscope. Although the function of Birbeck granules is not completely understood, they are thought to be involved in antigen processing.