Morphogenesis and Topography of the Mesorectal Fascia.

Background: First described by Carl Toldt in the late 19th century, the mesorectum has since been a topic of anatomical and surgical debate. Its clinical importance was redefined by Heald's introduction of Total Mesorectal Excision (TME), nowadays the golden standard in oncologic rectal surgery. This study aims to elucidate the embryological development and adult anatomy of the mesorectum and the mesorectal fascia, with a focus on clinically significant relations, particularly the peritoneum, and components of the hypogastric plexuses. Methods: We performed anatomical dissections on four 12-15-week-old human fetuses and eight formalin-fixed adult cadavers. In addition, a transverse pelvic section was examined to assess the spatial organization of mesorectal and fascial structures. Results: Our findings confirm the presence of a dorsal mesentery at the rectal level during fetal development, illustrating its transformation into the adult mesorectum. We identified the mesorectal contents in the fetus and examined the course and relations of the superior rectal vessels, hypogastric nerves, and pelvic splanchnic nerves, in both fetal and adult specimens. Conclusions: The observed fetal and adult configurations provide a continuous morphological description of the mesorectum and its compartmental organization within the pelvis. This study enhances the understanding of the mesorectum's embryology, structure, and vital surgical landmarks. By delineating the so-called 'Holy plane' of Heald (the natural avascular plane between the mesorectal and presacral fasciae used during total mesorectal excision) and the delicate connective fibers known surgically as 'angel's hair', which become visible when this plane is correctly entered, rectal and presacral fasciae, and neurovascular elements, provides a comprehensive anatomical framework that may inform surgical plane identification and support future clinical investigations into nerve-sparing rectal surgery.