A histological section of the epididymis shows tall columnar cells bearing long, non-motile surface projections that function in absorption and secretory fluid modification. What are these projections?

• A Stereocilia, which are unusually long branched microvilli ✓
• B Kinocilia with a 9 plus 2 microtubule arrangement
• C Primary cilia anchored by basal bodies
• D Microfold processes supported by actin and villin

Explanation

Stereocilia of the epididymal duct are extremely long, sometimes branching microvilli with an actin filament core, not true cilia. They increase surface area for absorption of fluid as sperm mature. Kinocilia have a motile 9 plus 2 axoneme and are found in the respiratory tract and oviduct, which eliminates option B. The core of a cilium is a microtubule axoneme arising from a basal body, whereas the core of stereocilia is actin, the discriminating ultrastructural fact.

Reference: Ross and Pawlina Histology: A Text and Atlas, 7th ed.

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