A worksheet cell may contain a rich formatted text that represents a cell text divided into separate regions - text runs, each with its own set of font characteristics. Each run is defined by the RichTextRun object and is stored in the Runs collection. To view a run's text and font settings, use the RichTextRun.Text and RichTextRun.Font properties, respectively.
To apply rich formatting to the cell text, do one of the following:
Call the CellRange.SetRichText method to assign the rich formatted text to a cell.
For details on how to apply different fonts to specific regions of the cell text, refer to the How to: Apply Rich Formatting to Cell Text document.