The primary clock for the menstrual cycle is the ovary, as evident from the sequence of events. Two main pituitary-based events, the transient rise in FSH and the LH surge, are timed based on two ovarian events. The regular life span of the corpus luteum and its demise after 14 days determine the timing of FSH rise, while the growth of the dominant follicle and its ability to produce high levels of estrogen induce a switch to positive feedback at the pituitary, leading to the LH surge. Essentially, the dominant follicle communicates to the pituitary that it is ready for ovulation and luteinization.

The oviducts, also known as the uterine tubes or fallopian tubes, are muscular tubes positioned close to the surface of each ovary at the distal ends and passing through the wall of the uterus at the proximal ends. The oviducts are divided into four sections: the infundibulum, which is the open end with fingerlike projections called fimbriae that sweep over the ovary's surface; the ampulla, which has a wide lumen and extensive mucosal folding; the isthmus, which has a narrow lumen and less mucosal folding; and the intramural or uterine segment, which extends through the uterine wall at the superior corners of the uterus.

The main functions of the oviducts include capturing the cumulus-oocyte complex at ovulation and transferring it to the ampullary-isthmus junction for fertilization. Oviductal secretions coat and infuse the cumulus-oocyte complex, ensuring viability and fertilizability. Additionally, the oviducts serve as a site for sperm storage. Sperm can remain viable for up to 5 days after sexual intercourse by adhering to the epithelial cells lining the isthmus. Oviductal secretions also induce capacitation and hyperactivity of sperm. Furthermore, the oviducts secrete fluids that provide nutritional support to the preimplantation embryo.

The timing of embryo movement into the uterus is crucial, as the uterus has a limited implantation window of about 3 days. The oviduct needs to retain the early embryo until it reaches the blastocyst stage (5 days after fertilization) and then allow its passage into the uterine cavity.

The wall of the oviduct consists of the endosalpinx (mucosa), myosalpinx (two-layered muscularis), and perisalpinx (outer-lying connective tissue). The endosalpinx is highly folded, almost obliterating the lumen, and is lined by a simple epithelium comprising ciliated and secretory cells. Cilia, most abundant at the infundibular end, propel the cumulus-oocyte complex towards the uterus. Transport of the ovulated cumulus-oocyte complex solely relies on the cilia present on the fimbriae. Once the complex passes through the oviduct's ostium and enters the ampulla, it is propelled by both cilia and peristaltic contractions of the muscularis.
From this sequence of events it is evident that the ovary is the primary clock for the menstrual cycle. The timing of the two main pituitary-based events—the transient rise in FSH that recruits large antral follicles and the LH surge that induces ovulation—is determined by two ovarian events. These are respectively the highly regular life span of the corpus luteum and its demise after 14 days, and growth of the dominant follicle to the point at which it can maintain a sustained high production of estrogen that induces a switch to positive feedback at the pituitary. In essence the dominant follicle tells the pituitary it is ready to undergo ovulation and luteinization.
The Oviduct
The oviducts (also called the uterine tubes and the fallopian tubes) are muscular tubes with the distal ends close to the surface of each ovary and the proximal ends traversing the wall of the uterus. The oviducts are divided into four sections (going from distal to proximal): the infundibulum, or open end of the oviduct, which has fingerlike projections called fimbriae that sweep over the surface of the ovary; the ampulla, which has a relatively wide lumen and extensive folding of the mucosa; the isthmus, which has a relatively narrow lumen and less mucosal folding; and the intramural or uterine segment, which extends through the uterine wall at the superior corners of the uterus (
Fig. 44.25
The main functions of the oviducts are to: 1.
Capture the cumulus-oocyte complex at ovulation and transfer the complex to a midway point (the ampullary-isthmus junction), where fertilization takes place. Oviductal secretions coat and infuse the cumulusoocyte complex and are likely required for viability and fertilizability.
2.
Provide a site for sperm storage. Women who ovulate up to 5 days after sexual intercourse can get pregnant. Sperm remain viable by adhering to the epithelial cells lining the isthmus. The secretions of the oviduct also induce capacitation and hyperactivity of sperm.
3.
Secrete fluids that provide nutritional support to the preimplantation embryo.
The timing of movement of the embryo into the uterus is critical because the uterus has an implantation window of about 3 days. The oviduct needs to hold the early embryo until it reaches the blastocyst stage (5 days after fertilization) and then allow it to pass into the uterine cavity.
810 SECTION8Berne & Levy Physiology •Fig. 44.25 Schematicofthefemalereproductivesystem.(ModifiedfromWhiteBA,PorterfieldSP.Endocrine Physiology. 4thed.Philadelphia:Mosby;2013.)Fundus of uterusIsthmusAmpulla Infundibulum with fimbriae Uterine (fallopian) tube Ovary Body (corpus) of uterus Internal genitalia External genitalia Cervix of uterus Vagina Clitoris Vaginal introitus Opening of female urethra Mons pubis Labia majora Labia minora
The wall of the oviduct is composed of a mucosa (called the endosalpinx), a two-layered muscularis (called the myosalpinx), and an outer-lying connective tissue (the perisalpinx). The endosalpinx is thrown into many folds, almost to the extent that the lumen is obliterated, and is lined by a simple epithelium made up of two cell types: ciliated cells and secretory cells. The cilia are most numerous at the infundibular end and propel the cumulus-oocyte complex toward the uterus. The cilia on the fimbriae are the sole mechanism for transport of the ovulated cumulus-oocyte complex. Once the complex passes through the ostium of the oviduct and enters the ampulla, it is moved by both cilia and peristaltic contractions of the muscularis.