Antral follicle
A small, fluid-filled sac in the ovary that holds an immature egg. The structures that get miscounted as "cysts" in the old PCOS name.
An antral follicle is a small, fluid-filled sac inside the ovary, each holding an immature egg. Every cycle, a cohort of them begins to develop, and normally one becomes dominant and releases its egg at ovulation.
In PMOS, this maturation tends to stall. More follicles develop partway and pause, so at any given moment the ovary holds a higher number of small antral follicles than usual. On an ultrasound this appears as the "string of pearls" pattern.
This is the structure the word "polycystic" got wrong. Antral follicles are not cysts. They are normal ovarian structures present in greater number, which is part of why the 2026 rename moved away from "polycystic ovary syndrome" to PMOS.