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 hilar neurons immunoreactive for Prox - 1 , a granule - cell - specific marker , was estimated using the optical fractionator method .
### Output Text:
The number of hilar neurons immunoreactive for Prox - 1 , a granule - cell - specific marker , was estimated using the optical fractionator method . 