Theca cells
Cells in the ovary that produce androgens. In PMOS, insulin drives them to make more, which is the start of the insulin-androgen loop.
Theca cells are a layer of cells surrounding each developing follicle in the ovary. One of their normal jobs is to produce androgens, which are then partly converted to oestrogen by neighbouring cells as a follicle matures.
In PMOS, the theca cells are central to the androgen story. Insulin acts directly on them and increases their androgen output. This is the first step of the insulin-androgen loop: high circulating insulin tells theca cells to make more androgen, the androgen contributes to insulin resistance, and the loop reinforces itself.
This is also why the ovary is described as "still listening" while muscle and fat become insulin resistant. The theca cells keep responding to insulin, which is what links the metabolic and androgenic sides of the condition at the cellular level.