PHYSIOLOGY OF THE MENSTRUAL CYCLE
Anatomy and Embryology of the Ovary
The main functions of mature ovaries are to generate a fertilizable ovum each month and to prepare the endometrium for implantation through the sequential release of estrogen and progesterone. The sexually mature ovary is approximately 3 cm3 in volume. It consists of three structurally distinct regions: 1) the outer cortex containing the surface germinal epithelium and follicles, 2) a central medulla consisting of stroma, and 3) a hilum around the area of attachment of the ovary to the mesovarium. The ovarian follicle, composed of the egg and surrounding granulosa and theca cells, is the fundamental functional unit of the ovary.
By week 5 of gestation, premeiotic germ cells, which originate outside of the embryo proper, arrive at the genital ridge and are termed oogonia. Through three separate processes of mitosis, meiosis, and oogonial atresia, these oogonia gradually evolve to form primary oocytes. The number of primary oocytes reaches a peak of 5 to 7 million at approximately 5 months of gestation. Through the process of atresia, this number decreases to 1 to 2 million by birth and to approximately 500,000 by the first menses. The ovary is perhaps most unique for its continuous process of atresia. This process begins in utero and continues, without regeneration, until a woman reaches the point of menopause, around the fifth decade of life, when approximately 1000 oocytes remain. Throughout life, only 400 to 500 follicles, less than 1% of the total number, will ovulate.
Once formed, the primary oocyte will persist in prophase stage of the first meiotic division until the time of ovulation when meiosis is resumed. The primary oocyte then evolves into a secondary oocyte and the first polar body is formed and extruded. This progression occurs just prior to ovulation but after the luteinizing hormone (LH) surge. At ovulation, the secondary oocyte and the surrounding granulosa cells (cumulus oophorus) are extruded from the ovary and the remaining follicular cells form the corpus luteum. If penetrated by a sperm, the secondary oocyte will undergo a second meiotic division after which the second polar body is extruded.
Abnormal uterine bleeding (AUB)—noncyclic, nonovulatory bleeding unrelated to anatomic lesions or systemic disease
Amenorrhea—absence of menarche by age 16 or no menses for >3 cycles or >6 months in an individual who previously had cyclic menses
Atresia—the degeneration and resorption of one or more ovarian follicles
Corpus luteum—formed by luteinization of the remaining theca and granulosa cells of the dominant follicle after ovulation; produces progesterone and estrogen during the luteal phase
Chronic anovulation—repetitive ovulation failure, which differs from ovarian failure in that viable oocytes remain in the ovary
Dysmenorrhea—a condition marked by painful menstruation
Hematocolpos—sequestration of blood in the vagina with imperforate hymen
Luteinization—the transformation of the mature ovarian follicle into a corpus luteum
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