The fallopian tubes, also known as oviducts or uterine tubes, are a pair of muscular ducts within the female pelvis connecting the ovaries and the uterus. Each tube guides the egg cell (oocyte) released from the ovary toward the uterine cavity. This structure is fundamental to human reproduction because it is the typical location where fertilization occurs. The fallopian tube is a dynamic organ that coordinates movement, provides a supportive environment for gametes, and ensures the resulting embryo is delivered to the uterus at the appropriate time.
The Structure and Segments
The fallopian tube is a slender structure, typically measuring between 10 and 13 centimeters (about 4 to 5 inches) in length. This length is divided into four distinct anatomical regions, sequentially arranged from the ovary to the uterus. The outermost segment is the infundibulum, a funnel-shaped structure positioned nearest to the ovary.
The infundibulum ends in numerous delicate, finger-like projections called fimbriae. These fimbriae are responsible for capturing the egg once it is released during ovulation. Moving inward, the next segment is the ampulla, which is the widest and longest portion of the tube, accounting for approximately two-thirds of its total length.
Following the ampulla is the isthmus, a narrow, muscular section that connects the ampulla to the uterus. This segment has a smaller diameter and a thicker muscular layer. The final portion is the intramural, or interstitial, part, which is the shortest and narrowest segment that passes through the muscular wall of the uterus itself.
The wall of the fallopian tube is composed of three main layers: the innermost mucosa, the middle muscularis, and the outermost serosa. The mucosa is highly folded and covered by a specialized single layer of columnar cells. These cells include ciliated cells, which possess hair-like projections, and secretory cells that produce fluid to nourish the egg and sperm.
The Process of Gamete Transport and Fertilization
The primary function of the fallopian tube is the coordinated transport of gametes—the egg moving toward the uterus and sperm moving toward the ovary—and the facilitation of fertilization. The process begins during ovulation when the fimbriae become engorged with blood and gently sweep the surface of the ovary. This action, coupled with fluid currents created by the beating of cilia, guides the released egg into the infundibulum.
Once the egg enters the tube, two main mechanisms propel it toward the uterus. The first is the sweeping motion of the cilia lining the mucosal layer, which beat in a coordinated wave to push the egg cell. The second mechanism is the peristaltic action of the muscular layer, where rhythmic contractions create wavelike movements that move the contents along.
Fertilization most commonly occurs within the ampulla, the widest part of the tube, after sperm have traveled from the uterus to reach the egg. The fallopian tube provides a supportive environment by secreting fluids that sustain the gametes and facilitate fertilization. This environment is regulated by hormones to ensure optimal conditions.
The journey of the fertilized egg, now a zygote, takes approximately three to four days to travel the full length of the tube before reaching the uterine cavity. The early stages of cell division occur while the developing embryo is still within the tube. The isthmus, the narrow segment closest to the uterus, may act as a temporary regulator, holding the embryo for about 72 hours before relaxing under hormonal influence to allow passage into the uterus for implantation.
Consequences of Tubal Function and Manipulation
The coordinated function of the fallopian tube is necessary for a healthy pregnancy, and any disruption can lead to complications. One significant malfunction is an ectopic pregnancy, which occurs when the fertilized egg implants outside of the uterine cavity, most often within the fallopian tube itself. This condition arises when the embryo is delayed or completely blocked from reaching the uterus.
Damage to the cilia, often caused by previous infections or inflammation, can impair the tube’s ability to push the fertilized egg along its path. If the tube is partially blocked, sperm may still fertilize the egg, but the resulting embryo is too large to pass through the narrowed segment, leading to implantation in the tube. An ectopic pregnancy is a serious medical event because the tube is not designed to support a growing pregnancy and can rupture, causing internal bleeding.
Intentional manipulation of the fallopian tube’s function is the basis for female surgical sterilization, commonly known as tubal ligation. This procedure involves physically interrupting the tube’s continuity by cutting, tying, banding, or sealing off both ducts. The goal is to create a permanent barrier that prevents the sperm and the egg from meeting, halting the reproductive process.
While highly effective, tubal ligation carries a small risk of failure, and if pregnancy occurs, there is an increased chance it will be ectopic. This increased risk is thought to be due to the formation of a small channel or fistula. This channel is large enough for sperm to pass through, but too small for the fertilized egg to complete its journey to the uterus. The procedure achieves contraception by preventing transport, but rare failures can result in the complication the tube’s transport system is designed to prevent.