In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The number of stained microglia cells was significantly higher in the PDN treated groups in the prefrontal cortex than in controls ( moderate - dose , 29 . 1 ; high - dose , 28 . 4 ; control , 17 . 7 cells per field ; p < 0 . 01 ) .
### Output Text:
The number of stained microglia cells was significantly higher in the <span class="chemical">PDN</span> treated groups in the prefrontal cortex than in controls ( moderate - dose , 29 . 1 ; high - dose , 28 . 4 ; control , 17 . 7 cells per field ; p < 0 . 01 ) . 