At the end of menstrual flow, the uterine cycle enters the early proliferative phase. The uterine glands are sparse and small during this phase, and the epithelial cells develop microvilli and cilia under the influence of estrogen. The stratum functionalis thickens in the late proliferative phase, and the glands become more coiled and densely packed. At ovulation, the secretory phase of the uterine cycle begins. The glands become even more complexly coiled during this phase, and the endometrial lining reaches its maximum thickness, while the stratum basalis and myometrium remain relatively unchanged. The glands have a saw-toothed appearance. In the lumina of the glands, glycogen and glycoprotein-rich secretions can be seen. If there is no fertilization, the placental tissue does not produce hCG and the corpus luteum degenerates. Because the uterine lining does not receive progesterone, the spiral arteries constrict and endometrial tissue becomes ischemic. This results in cell death and stratum functionalis sloughing.