English: These are spiral arteries (in endometrial stroma, endometrial glands can also be seen) temporarily supplying blood to endometrium in the luteal phase of the menstrual cycle.They have thick muscular walls and small lumen ,that is they are high resistance low flow vessels.If pregnancy occurs these spiral arteries undergo remodelling and gets dilated several fold and loses some of its thick musculature, that is they become low resistance high flow vessels.This is essential to supply blood to the developing fetus.

In preeclampsia there is defective/ineffective remodelling of spiral arteries.

Note: They are called spiral arteries since they are coiled and spiral in appearance but what is shown here is its cross section,which will show what appears to be numerous closely packed arteries of similar calibre. Actually this probably is one single artery and due to its coiled nature we are seeing multiple lumens in the same cross section.