Initial endometriosis showing direct morphologic evidence of metaplasia in the pathogenesis of ovarian endometriosis.

It is believed that ovarian endometriosis may be generated by a celomic metaplastic process from existing epithelium in the ovary. However, no morphologic evidence of metaplastic process has been described. In this study, we intended to identify the earliest morphologic changes of endometriosis within the ovary to examine if evidence of metaplasia exists. Included in this study were 110 ovarian endometriosis cases and 30 benign ovaries without endometriosis but with ovarian epithelial inclusions (OEIs). Among the 110 well-established ovarian endometriosis cases, 34 cases showed areas of initial endometriosis (IE), which is defined as lesions showing direct transitions from normal-looking ovarian tissue to areas of minimal formation of endometriosis and/or to areas of full-blown endometriosis. We further divided IE into two types: type I IE was present on the ovarian surface, which was associated with ovarian surface epithelia; type II was located within the ovarian cortex, which was associated with OEIs. Sections containing IE, OEIs, and well-formed endometriosis were subject to CD10 and aromatase immunostaining. In IE lesions, the number of CD10-positive cells were significantly higher than the number of that in OEIs, but lower than that of well-formed endometriosis areas (p < 0.05). Aromatase expression was detected in both epithelial and stromal components of the IE lesions, indicating that estrogen local production may be involved in this initial process of endometriosis. Microvessel density was higher in IE lesions than in areas of OEI (p < 0.05). Based on the morphologic characteristics of IE, we believe that IE represent a spectrum of the earliest morphologic changes of endometriosis identifiable by routine microscopy. The morphologic transitions from ovarian surface epithelium or OEI to IE lesions provide direct metaplastic evidence for the pathogenesis of ovarian endometriosis. This metaplastic process may not only involve the ovarian epithelial cells, but also stromal components. Local production of estrogen, probably in high-levels, may be related to the initial process of endometriosis, although detailed mechanisms remain to be clarified.