2.2. Biomaterial Collection and Processing

Menstrual blood was collected on days 2–3 of the cryo-ET cycle directly from the uterine cavity using an ET catheter Guardia Access Transfer Catheter K-JETS-7019 (Cook Medical Inc., Bloomington, IN, USA) connected to a syringe. According to the feedback of the study participants, this method appeared to be painless. External genitalia as well as the vagina and cervix were disinfected with a Octenisept spray (Schulke & Mayr GmbH, Norderstedt, Germany) prior to the introduction of the ET catheter. Vaginal secretions were cleaned by cotton buds. The amount of aspirated menstrual blood was varying, in most cases it was not possible to sample more than 0.5 mL. Due to the high viscosity of the biomaterial, the catheter was often clogged and had to be rinsed with 0.2 mL of sterile (0.9%, NaCl) saline solution.

The menstrual blood samples were centrifuged at 5500 rpm for 10 min. The menstrual supernatant was transferred to separate tubes. Both menstrual sediment and supernatant were stored at −80 °C until further analysis.