This is nothing to be concerned about. The body goes through a lot of changes during pregnancy, and that includes changes in the breast.  The bits you're talking about could be a mixture of normal dead skin cells that have gotten darker than usual  (similar to the the skin around the nipple getting darker). 

Do not go scrubbing your nipples clean because that would just be uncomfortable and sore, and unnecessary. 

Sounds to me like your breasts are doing what they should be doing - getting ready for breastfeeding. (this happens even if you aren't planning on breastfeeding).

Darker pigmentation around the nipple and areola are common during pregnancy.  It is caused by hormones, and doesn't mean there is a problem.

The slits you are talking about are the nipple getting ready for being a milk outlet.  The nipple is a collection of spongy tissue (like coral in appearance), and the slits you have noticed are the parts where the milk will pass from you to baby.  The white skin you are seeing is just skin.  It isn't anything to be concerned about.  It appears white because that skin doesn't get to see the sun, so it stays very white.