I'd say "rarefied" (in the sense of being lost), but ob/gyn is not my specialty...

German term (edited): verdämmernde

"Morphological methods, in particular histochemical technique of secondary fluorescence according to Bertalanffy were used for the assessment of regressive changes of placenta. . . These changes are followed from one side, by an atrophy of the blood vessels and connective tissue cells, and from the other, there increases the number of decidua giant cells, as well as of syncytium and cytotrophoblast. The areas of impaired blood supply contain giant cells and intervening foam cells. These cells according to the authors can be the forms of regressive changes exhibiting relative biological activity influencing trophic status of the placenta. Possible importance of the fibrinoid masses for the immunology of pregnancy which is developing under maternal morbid conditions has been discussed."

But "degenerating" is fine, too, I think.