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: A marked corticomedullary gradient of prostaglandin synthesis was observed in all groups .
### Output Text:
A marked corticomedullary gradient of <span class="chemical">prostaglandin</span> synthesis was observed in all groups . 