Identification of Misplaced Endometrial Glands and Stroma in the Myometrium of Foetal Uteri Evocative of Developmental Adenomyosis

Adenomyosis is a benign gynaecological disorder in which endometrial glands and stroma enter the uterine myometrium with varying degrees of spreading. To analyse the presence of developmentally displaced endometrial glands and stroma in the foetal myometrium, a retrospective cohort of 420 foetal uteri, including one monozygotic twin pair, was histopathologically evaluated. The gestational age ranged between 18 and 37 weeks; the clinical characteristics included various foetal malformations with a predominantly normal karyotype, except in one case with trisomy 18. Ectopic endometrial tissue enclosed in the myometrium was discovered in twelve individual foetuses from the cohort (12/420). The investigation of the histogenetic attributes of the misplaced endometrial tissue in both monozygotic twins' (MZ) foetal uteri revealed isolated glands and thin channels containing cords of endometrial-type glands penetrating the myometrium. Through immunohistochemistry, low levels of oestrogen receptors (ERs) were detected, whereas a moderate level of progesterone receptor (PR) expression was observed in the ectopic glandular and stromal cell nuclei in all cases. Additionally, the surrounding periglandular component consistently expressed the vimentin and CD10 stromal cell markers, while the myometrial smooth muscle cells revealed the strong expression of both alpha-Smooth Muscle Actin (α-SMA) and desmin marker proteins.