As to to their frequency and variety, endometrial meta- and hyperplasias are the most impressive examples of the growth and differentiation potential of the Muellerian system and the inductive power of steroid hormones and related inter- and intracellular factors. Characteristic steroid reactions are the glandular hyperplasias, ciliated and squamous cell metaplasias in the case of hyperestrogenism, and the mucinous and clear cell meta- and hyperplasias in the case of gestagen excess. In 1994, the WHO established a new classification of endometrial hyperplasias. This classification takes into account the profound differences in cancer risk, and accordingly demands a clearcut distinction between simple and complex (steroid sensitive, almost always reversible) hyperplasias without atypia, and simple and complex (partial steroid resistant and potential progressive) hyperplasias with atypia. The conclusion drawn from clinical experience is, that the atypical endometrial hyperplasia--and this alone--is the precancerous lesion of the ordinary endometroid carcinoma. In contrast, it is still not clear which biological significance accounts to all the complex squamous, mucinous, clear cell and ciliated cell meta- and hyperplasias. Yet, it is good pathological and clinical practice, to upgrade any metaplastic lesion with nuclear atypia, and to remove the uterus as it is done in almost all atypical endometrial hyperplasias.
