On microscopic examination enamel rods have:
**Core Concept**
Enamel rods are the primary structural units of enamel, the hardest substance in the human body. They are composed of highly mineralized, non-living, organic-inorganic composite material that provides a hard, calcified layer covering the crown of teeth.
**Why the Correct Answer is Right**
The correct answer is **B**. Enamel rods are indeed prisms, which are rod-shaped structures composed of highly mineralized, crystalline hydroxyapatite. They are the building blocks of enamel and provide its unique mechanical properties. The orientation of enamel prisms is usually parallel to the surface of the tooth, giving enamel its characteristic hardness and durability.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because enamel prisms are not hexagonal in shape. While enamel is composed of highly ordered, crystalline structures, the individual prisms are actually rod-shaped.
* **Option C:** This option is incorrect because enamel does not have a cellular structure. Enamel is a non-living, acellular tissue that is produced by ameloblasts during tooth development.
**Clinical Pearl / High-Yield Fact**
The structure and composition of enamel prisms are crucial for understanding the etiology and treatment of dental caries. The unique orientation of enamel prisms can affect the progression of caries and the effectiveness of dental restorations.
**Correct Answer:** B. Enamel prisms.