Final answer:
During the proliferative phase of the uterine cycle, the stratum functionalis layer of the endometrium thickens in response to estrogen, preparing for potential implantation and pregnancy.
Step-by-step explanation:
The proliferative phase of the uterine cycle involves the thickening of the innermost layer of the uterus, known as the endometrium. This growth is stimulated by the hormone estrogen, which is secreted by the ovaries during the menstrual cycle. The endometrium is composed of two layers: the deeper stratum basalis, which remains relatively constant, and the superficial stratum functionalis, which thickens during the proliferative phase in preparation for a potential pregnancy. If fertilization does not occur, the stratum functionalis is subsequently shed during menstruation.
During the proliferative phase, rising estrogen levels from the maturing ovarian follicles cause the stratum functionalis of the endometrium to grow and thicken. This layer also becomes highly vascular, as blood vessels proliferate to prepare a suitable environment for the implantation of a fertilized egg. The endometrial glands in this layer elongate and the tissue itself prepares to support a potential embryo, marking the uterus's readiness for possible pregnancy.