In addition to a description of the female cycle in terms of ovarian function, the cycle can also be described in terms of the changes in the endometrium of the uterus. These changes occur because the development of the endometrium is timed by the cyclic changes in the secretion of estradiol and progesterone from the ovarian follicles. Three phases can be identified on the basis of changes in the endometrium: (1) the proliferative phase; (2) the secretory phase; and (3) the menstrual phase (fig. 20.35, bottom).
The proliferative phase of the endometrium occurs while the ovary is in its follicular phase. The increasing amounts of estradiol secreted by the developing follicles stimulate growth (proliferation) of the stratum functionale of the endometrium. In humans and other primates, coiled blood vessels called spiral arteries develop in the endometrium during this phase. Estradiol may also stimulate the production of receptor proteins for progesterone at this time, in preparation for the next phase of the cycle.
The secretory phase of the endometrium occurs when the ovary is in its luteal phase. In this phase, increased progesterone secretion stimulates the development of uterine glands. As a result of the combined actions of estradiol and progesterone, the endometrium becomes thick, vascular, and "spongy" in appearance, and the uterine glands become engorged with glycogen during the phase following ovulation. The endometrium is therefore well prepared to accept and nourish an embryo should fertilization occur.
The menstrual phase occurs as a result of the fall in ovarian hormone secretion during the late luteal phase. Necrosis (cellular death) and sloughing of the stratum functionale of the endometrium may be produced by constriction of the spiral arteries. It would seem that the spiral arteries are responsible for menstrual bleeding, since lower animals that lack these arteries do not bleed when they shed their endometrium. The phases of the menstrual cycle are summarized in figure 20.37 and in table 20.6.
The cyclic changes in ovarian secretion cause other cyclic changes in the female reproductive tract. High levels of estradiol secretion, for example, cause cornification of the vaginal epithelium (the upper cells die and become filled with keratin). High levels of estradiol also cause the production of a thin, watery cervical mucus that can easily be penetrated by spermatozoa. During the luteal phase of the cycle, the high levels of progesterone cause the cervical mucus to thicken and become sticky after ovulation has occurred.
Abnormal menstruations are among the most common disorders of the female reproductive system. The term amenorrhea refers to the absence of menstruation. Dysmenorrhea refers to painful menstruation, which may be marked by severe cramping. In menorrhagia, menstrual flow is excessively profuse or prolonged, and in metrorrhagia uterine bleeding that is not associated with menstruation occurs at irregular intervals.
Clinical Investigation Clue
Remember that Gloria is experiencing amenorrhea. Considering the previous clues, which is most likely to be responsible for her amenorrhea—decreased functioning of her endometrium, ovaries, anterior pituitary, or hypothalamus?
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.