Single-Cell Sequencing Maps Pregnancy's Early Stages

Recent research has produced the first high-resolution molecular image of the equine endometrium, the inner lining of the uterus, before and after embryo implantation. This image, or cell atlas, reveals significant similarities in immune cells between early human and horse pregnancies, which is unexpected given the substantial differences in their placentas.

In contrast to most mammals, the implantation of the equine embryo is delayed, occurring approximately 40 days post-fertilization.

The factors that regulate this late implantation in the mare, and whether they are unique to horses, remain poorly understood.”

Mandi de Mestre, Dorothy Havemeyer McConville Professor, Equine Medicine, College of Veterinary Medicine, Cornell University

Mandi de Mestre is also the Director of the Baker Institute for Animal Health.

The journal Biology of Reproduction published the study. Joanna Jaworska, an Assistant Professor at the Polish Academy of Sciences in Olsztyn, is the First Author, with Shebl Salem, a Postdoctoral Researcher in de Mestre’s lab, among the Co-authors.

To gain insights into the molecular pathways activated during equine pregnancy, the research team employed single-cell RNA sequencing to analyze the transcriptome (the collection of RNA molecules expressed) of nearly 97,000 individual endometrial cells before and after implantation.

This advanced technique enabled them to identify 40 distinct cell groups, creating an atlas that details which genes are expressed or not expressed in each cell.

This analysis uncovered several new immune subpopulations active during implantation and facilitated a direct comparison of each cell type with those found in early human pregnancy.

Implantation comes with remodeling and complex cellular interactions involving epithelial, stromal, and immune cells, as well as glands and blood vessels. The molecules regulating this process have been undefined, and we had a unique opportunity to understand the critical events in early equine pregnancy.”

Mandi de Mestre, Dorothy Havemeyer McConville Professor, Equine Medicine, College of Veterinary Medicine, Cornell University

The study's data has been compiled into a comprehensive atlas of equine pregnancy, paving the way for future research to investigate which genes may malfunction during miscarriage or other fertility challenges.

The atlas also revealed some surprising findings, such as the prevalence of cells known as endometrial natural killers (eNK) during implantation.

This was surprising to us, as eNK cells are traditionally associated with highly invasive placentae and had only been described in small numbers in the mare. The application of these highly sensitive techniques allowed us to demonstrate that they actually constitute nearly half of all the immune cells in the uterus.”

Mandi de Mestre, Dorothy Havemeyer McConville Professor, Equine Medicine, College of Veterinary Medicine, Cornell University

Based on the genes they expressed, the researchers hypothesize that these cells may play a role in essential adaptations necessary for a successful pregnancy, including regulating maternal immune responses to the fetus, facilitating endometrial remodeling, and providing protection against viral pathogens.

de Mestre remarked, “We know that these cells are the predominant immune cell type present in many other species, which indicates that these cells have been evolutionarily conserved and adapted for the pregnant state across all mammals.”

With these newly recognized similarities between horses and humans, the atlas is expected to provide valuable insights into fertility for both species.

“The data offers a comprehensive transcriptomic resource for researchers interested in exploring early pregnancy in horses, as well as conducting comparative studies in other mammals, including humans,” de Mestre stated.

High-resolution techniques have transformed scientists' understanding of species-specific differences in maternal immunity to the implanting embryo, de Mestre added, and such methods hold promise for identifying novel targets that could influence fertility in both horses and humans.