thank you all for your answers. You right, with the example of Michel Fortin, no problem, but with my page, it didn’t work!
I look for the problem and I found it: if in the html part, you have some html characters like “& nbsp;” or “& copy;” (without space after & of course), there no interpretation of the markdown syntax. This problem don’t seem to come from Markdown Extra, I use it wih other CMS like picoCMS and it work with html characters.