Lymphatic lacunae of the mucosal folds of human uterine tubes - A rediscovery of forgotten structures and their possible role in reproduction.

The mucosa of uterine tube forms multiple and branched longitudinal mucosal folds and takes part in many reproduction events, such as oocyte pick-up, gamete transport, sperm capacitation, fertilization, and early embryonic development. In the habilitation thesis of German physician Paul Kroemer (1904) was the first to describe the lymphatic lacunae inside the tubal folds (by injection of Indian ink), which the author named the öLymphbahnen" (ölymphatic channels"). Despite the fact that this first description has existed for 110 years, there is no mention of these lacunae in most of the current literature. In this article we present a rediscovery of completely overlooked morphological structures of uterine tubes - the lymphatic lacunae in their mucosal folds. The specimens from the uterine tubes were taken from 72 women (mean age 46.25 years) who underwent transabdominal or laparoscopic salpingectomy. The tissue samples from anatomically different parts of the uterine tubes were used for hematoxylin and eosin staining and for immunohistochemistry. Primary antibodies were used to label and detect podoplanin D2-40, a selective marker of lymphatic endothelia, CD34 antigen, and von Willebrand factor (Factor VIII). In the histological slides of the uterine tubes, there were noticeable slits or gaps within the loose connective tissue of the lamina propria of the mucosal folds. They were lined with one layer of squamous endothelial cells. These öempty spaces" were most prominent in the fimbriae, but were still well recognizable in mucosal folds of the ampulla. They always run through the central part of the fold. As a results of immunohistochemistry, we confirmed that in the centre of every mucosal fold, as well as in the fimbriae of the uterine tubes, dilated lymphatic spaces were situated and were lined with a simple layer of lymphatic endothelial cells (positive for podoplanin and CD34, and negative for Factor VIII). As there is no mention on them in the current Terminologia Histologica, we proposed the term ölymphatic lacunae of tubal mucosal folds and fimbriae" in English and ölacunae lymphaticae plicae mucosae et fimbriae" in Latin. According to our hypothesis, these lymphatic lacunae may be responsible for the thickening of the fimbriae during the oocyte pick-up and the maintenance of the tubal fluid.