In a section of spinal cord, the neuronal cell body shows intensely basophilic granular clumps in its cytoplasm on Nissl staining. These granules extend into dendrites but are conspicuously absent from one region of the neuron. Where are they absent, and what do they represent?

• A Initial segment; lysosomes awaiting transport down the axon
• B Axon terminals; mitochondria clustered near synapses
• C Axon hillock; stacks of rough endoplasmic reticulum with free polyribosomes ✓
• D Dendritic spines; Golgi-derived vesicles for neurotransmitter packaging

Explanation

Nissl substance consists of rough endoplasmic reticulum and free ribosomes, reflecting high protein synthesis. It extends into dendrites but stops abruptly at the axon hillock, a key feature distinguishing the axon from dendrites on histology. The absence at the hillock is a standard examination point.

Reference: Junqueira's Basic Histology: Text and Atlas, 16th ed.

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