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: Although hepatocyte TJs are impaired in cholestasis , attempts to localize the precise site of hepatocyte TJ damage by freeze - fracture electron microscopy have produced limited information .
### Output Text:
Although hepatocyte TJs are impaired in cholestasis , attempts to localize the precise site of hepatocyte TJ damage by freeze - fracture electron microscopy have produced limited information . 