At this stage, the coiled arteries have reached their maximum development. HA29 contains several cross sections through spiral arterioles (arrows). Due to the coiling, the arteries come in and go out of the plane of section. Look for the smooth muscle wrapped around the lumens. By contrast, the glands (g) have a very different lining. When progesterone levels are high, the stroma of the stratum functionalis becomes edematous in its deeper levels, and forms a deeper spongiosa (s) and a more superficial compacta (c) just beneath the surface epithelium. The stromal cells of the compacta become more compact and rounded in anticipation of their transformation during pregnancy into decidual cells.