Female Reproductive System
The female reproductive system (or female genital system) serves multiple functions: it is responsible for producing the eggs necessary for reproduction, facilitating the occurrence of reproduction, and also for producing the female sex hormones that maintain the reproductive cycle and that play a direct or indirect role elsewhere in the body.
The female internal reproductive organs include the vagina, uterus, Fallopian tubes and ovaries. The external components of the reproductive system include the mons pubis, pudendal cleft, labia majora, labia minora, Bartholin's glands and clitoris.
Functionally speaking, the internal part of the female reproductive system consists of two main parts: the ovaries and the uterus.
The ovaries of a woman are both gonads and endocrine glands—analogous to the testes in a man, and this structure in the woman is found in the pelvic cavity on both sides of the uterus. The ovary has two extremities: the tubal extremity where the Fallopian tube attaches to the ovary by the infundibulopelvic ligament (also known as the suspensory ligament of the ovary), and the uterine extremity where the uterus is attached to the ovary via the ovarian ligament.
The uterus, or womb, functions to host the developing fetus during gestation, produces vaginal and uterine secretions, and allows sperm to pass through the Fallopian tubes in order to fertilize an egg. At one end of the uterus, the cervix opens into the vagina, while the other end is connected to both Fallopian tubes.
Externally, the vagina meets the “outside” organs at the vulva, which includes the labia, clitoris and urethra. Specifically, the vagina is attached to the uterus through the cervix.
The ovaries are the site of oogenesis, or gamete production. They periodically release egg cells (ova; female gametes) every 3 to 4 weeks in the average adult female that grow within the ovarian follicular environment. Follicles consist of different types and densities of cells, where the size of a follicle is indicative of the stage of oocyte development. Once an oocyte has finished its maturation process in the ovary, it is ovulated, or released by the follicle to begin its descent through the Fallopian tube.
After the follicular phase, ovulation occurs roughly halfway through the menstrual cycle, which is then followed by the luteal phase. In other words, the follicular phase is the portion of the menstrual cycle during which the ovarian follicles mature, and this phase ends upon ovulation. The days bordering ovulation around days 10 to 18 of the 28-day menstrual cycle are the most fertile phase.
The actual process of ovulation is controlled by the hypothalamus and by the hormones released by the anterior lobe of the pituitary gland (structures both found in the brain). Changes in the levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), both released by the pituitary gland, vary throughout the menstrual cycle; levels of LH and FSH are closely correlated to levels of estrogen and progesterone in this cycle as well. Specifically, ovulation is characterized by a sharp spike in the levels of LH and FSH, which corresponds with and results from a peaking of estrogen levels during the follicular phase.
To summarize, the menstrual cycle begins with the follicular phase (days 1 to 14), and ends with the luteal phase (days 14 to 28). The basal temperature of the body will be on average lower (as low as 36.4 degrees C) during the follicular phase, and higher (up to 37 degrees C) during the luteal phase. The average time from the beginning of the last menstrual period (LMP) until ovulation is around 14.6 days, with substantial variation between females based on multifactorial causes that include hormonal differences, chemical and hormonal imbalances, stress, environment, and so forth. During the follicular phase, estrogen levels will continually increase and reach its peak around day 14 to facilitate growth and maturation of the ovarian follicle, in addition to preparing the uterus for conception and fertilization. Estrogen will induce the endometrial glands to produce proliferative endometrium (the inside lining of the uterus), which is the site of implantation if fertilization occurs. Peaking estrogen levels will then trigger a spike in LH and FSH levels that result in ovulation of the primary oocyte due to rupture of the follicle. However, the follicle will remain functional and reorganize into the corpus luteum, which is responsible for secreting progesterone that changes the way the endometrium proliferates. Transitioning into the luteal (post-ovulatory) phase, levels of estrogen, LH, and FSH will now decrease as progesterone levels increase and reach a peak around day 21. In this phase, the corpus luteum continues to exert a paracrine action to maintain the endometrium, and the proliferative pattern of the endometrium changes to a secretory lining due to increased progesterone that provides a hospitable environment for one or more fertilized eggs (blastocysts) to implant. Importantly, if a blastocyst does not implant during this phase and provide feedback to the body by triggering the production of human chorionic gonadotropin (hCG) by the placenta, and provide continual feedback through pregnancy with the proper levels of estrogen and progesterone, the endometrial lining is either reabsorbed (estrous cycle) or shed (menstrual cycle). The reason for this is that in the absence of progesterone, the arteries that supply blood to the functional endometrial layer will constrict such that the cells undergo ischemia and apoptosis, which leads to menstruation and loss of the un-fertilized egg. (The basal endometrial layer that is adjacent to the myometrium and below the functional layer will not be shed at any time during the menstrual cycle).
If after ovulation sperm intercepts the egg released from the ovary as it passes through the Fallopian tube into the uterus, a single sperm can enter the egg and fertilize it. Fertilization usually occurs in the Fallopian tubes (also known as oviducts), and once the fertilized egg implants itself on the uterine wall, the process of embryogenesis and morphogenesis will begin, thus beginning the pregnancy process. Once the embryo develops into a fetus that is able to survive outside of the womb, the cervix will dilate and contractions of the uterus will evacuate the fetus through the vagina, in a process known as birth.
The ova, or eggs of a female, are formed by the time a female is born. That is to say, at birth, females possess the total number of eggs they will have for the remainder of their lives. A female is born with about 2 million eggs in her ovaries, and the older she gets, fewer follicles will remain. This is different from males who will continually produce sperm beginning from after puberty until well into their old age.
Menopause will occur later in a woman’s life when she is usually around 45 to 55 years of age. This is due to the fact that a decreased number of follicles will mean less estrogen and progesterone are produced since they are less responsive to gonadotropins (sex hormones; LH, FSH, etc.). In effect, blood vessels will constrict and dilate in response to shifting hormone levels, and the sudden dilation of cutaneous arteries can result in the classically known hot flashes—a spreading sense of heat that radiates from the abdomen to the thorax, neck and face. On average, menopause will occur at 52 years of age, and menopause is generally considered to have occurred once menstruation has ceased for one or more years.
The breasts and mammary glands are also involved in reproduction, where the glands will develop during pregnancy and remain active in the lactating breast—only atrophying when a woman ceases to nurse.