Rediscovering the Rete Ovarii: the Development, Role, and Function of a Secreting Auxiliary Structure to the Ovary

An aspect of ovary structure and function that has been given little attention is the rete ovarii (RO). Although the RO appears in drawings of the ovary in early versions of Gray’s Anatomy, it has disappeared from recent textbooks, and often is dismissed as a ‘functionless vestige’ of the adult ovary. The role, function, and development of the RO have largely evaded understanding by scientists for the past 154 years since its first description. In the process of studying ovarian morphological development, we identified a marker, PAX8, that strongly labels the RO and reveals that it is an actively developing tripartite structure consisting of the intraovarian (IOR), connecting (CR), and extraovarian rete (EOR), all three of which persist in adult life. The early RO develops within the mesonephros, alongside the Wolffian and Müllerian duct. Although both the male (Wolffian) and female (Müllerian) ductal primordia are present at the outset of gonadogenesis, only one of the two ductal systems develops. Once the male pathway is initiated, testis cells produce Anti-Mullerian hormone (AMH), leading to Müllerian ductal degeneration, and testosterone, promoting the development of the Wolffian duct and mesonephric tubules into the Rete Testis (RT), the efferent ductules, and the epididymis. In contrast, during female development, AMH is absent, and the Müllerian duct differentiates into the oviduct and uterus. These findings led to the idea that the mesonephric ducts are exclusively “male” primordia, while the Müllerian ducts are exclusively “female” primordia. This interpretation was consistent with the old idea that the RO was a degenerating structure derived from remnants of the mesonephric tubules still detectable in females. Using confocal imaging we found that the RO derives from the mesonephric tubules, but also from the first segment of the main Wolffian duct thought to only persist in XY as the Wolffian duct. We found that the Wolffian duct is severed during establishment of the RO to create the blind ended EOR, indicating the persistence of the “male” ductal system in the female reproductive tract. We also found bridges of cells from the Mullerian rudiment toward the presumptive CR (XX) and efferent ductules (XY), suggesting the intermingling of Müllerian Duct and mesonephric tubule cells in both sexes. This alters the paradigm that the mesonephric duct is exclusively a male-specific structure, while the Müllerian duct is exclusively a female-specific structure.The function and role of the RO previously heavily relied on histology and serial sections – leading to a confusion on the description and function of the RO. Some early histology of the structure, and work presented here, suggest that cells of the EOR are secretory. Using microinjections into the EOR, we found that it is fluid-filled and that luminal contents flow towards the ovary. Using proteomics and transcriptomics we found that the EOR produces proteins that are essential for ovarian function and homeostasis. These proteins, among others, are hormonally regulated. Labelling for cellular component markers revealed that a subset of the epithelial cells of the EOR are ciliated and exhibit cellular trafficking capabilities. Labelling for innervation and vasculature revealed that the cells of the EOR are closely associated with neuronal projections and vasculature. The direct proximity of the RO to the ovary suggest that it is functionally linked to the ovary and may play an important role in ovary development and homeostasis. Based on the cell biology, transcriptome and proteome of the RO,I hypothesize that the RO acts as an antennae for the ovary and plays an important role in ovary homeostasis and fertility.