Reepitheliazation is the process of regrowing epithelium (skin) over damaged, external parts of the body. Endotheliazation is the same thing, only internally: growing a type of skin cells back on the parts of the aorta damaged by surgery.
For a "simple" valve job, the skin takes about three months to fully overgrow any areas that need it.
If there is work also done on the aortic root, or replacement of any part of the aorta, the process takes about six months. This is especially true when various types of cloths (like Dacron velour) are used to form the new aortic tube. The skin actually grows into and throughout the fabric and eventually coats it as well.
This skin, of course, doesn't look like your external skin, but it performs a very similar protective function.
Throughout the time that your endothelial layer is healing, you have a greater risk of developing endocarditis, and a significantly higher risk of stroke. Obviously, there is more of an opening for infection, and things tend to stick to the unfinished healing layers that can sometimes lead to a clot.
This is part of the reason that even tissue valve recipients have to take warfarin (Coumadin) or clopidogrel (Plavix) for a few months after surgery for stroke avoidance. During this time, dental work is also greatly discouraged, due to the higher risk of infection.
The body doesn't need to grow cell layers over a replaced valve. In fact, it can in rare instances cause issues, particularly with mechanicals. There are a very few, experimental valves that try to act as scaffolds to grow your own cells over the valve (Cryolife has one approved, I believe). So far, they've not done as well as hoped for length of useful valve life.
Best wishes,
