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: To elucidate the role of estrogen in the regulation of tumor angiogenesis in the pituitary of female rats , the density of blood vessels was analysed using factor VIII related antigen ( FVIIIRAg ) immunohistochemistry and the expression of vascular endothelial growth factor / vascular permeability factor ( VEGF / VPF ) was examined by Western blot and immunohistochemical analysis .
### Output Text:
To elucidate the role of <span class="chemical">estrogen</span> in the regulation of tumor angiogenesis in the pituitary of female rats , the density of blood vessels was analysed using factor VIII related antigen ( FVIIIRAg ) immunohistochemistry and the expression of vascular endothelial growth factor / vascular permeability factor ( VEGF / VPF ) was examined by Western blot and immunohistochemical analysis . 