Atherosclerosis white space?

[Jenn5]

Hiya

(sorry if this is the wrong forum to post in)

I have to design a poster and would really appreciate your help in labelling a picture I found. Basically I'm confused by the three white spaces in the center - if the large one at the top is the artery lumen, what are the two white streaks below it?

Thousand thanks in advance

[canalon]

I am no specialist, but could that be an artifact due to the longitudinal cut of the sample?