Anatomy and physiology of the female reproductive system (Gynecologists' Lecture)

The female reproductive system includes all of the internal and external organs that help with reproduction. The internal sex organs are the ovaries, which are the female gonads, the fallopian tubes, two muscular tubes that connect the ovaries to the uterus, and the uterus, which is the strong muscular sack that a fetus can develop in. The neck of the uterus is called the cervix, and it protrudes into the vagina. At the opening of the vagina are the external sex organs, and these are usually just called the genitals and they’re in the vulva region. They include the labia, the clitoris, and the mons pubis.
The ovaries are a pair of white-ish organs about the size of walnuts. They’re held in place, slightly above and on either side of the uterus and fallopian tubes by ligaments. Specifically, there’s the broad ligament, the ovarian ligament, and the suspensory ligament. And the suspensory ligament is particularly important because the ovarian artery, ovarian vein, and ovarian nerve plexus pass through it to reach the ovary. If you slice the ovary open and look at it (don’t try this at home) there’s an outer layer called the cortex, which has ovarian follicles scattered throughout it, and an inner layer called the medulla, which contains most of the blood vessels and nerves.
At birth, the ovarian cortex has around two million follicles - that’s roughly the population of Paris - and they’re called primordial follicles. Each primordial follicle has a single immature sex cell called the primary oocyte at the core, and a layer of follicular cells surrounds this. The primary oocyte has 46 chromosomes, but eventually it has to turn into a gamete with only 23 chromosomes. To do this, the primary oocytes have to complete meiosis 1, and in a person’s lifetime only about 400 successfully do that. This process of oocyte development follows that of follicular development, which can be broken into three stages.
The first stage lasts from infancy to puberty, and during this stage the primary oocyte remains stuck in the prophase step of meiosis 1. So, in other words, the cell is living, but not dividing. Meanwhile, the primordial follicle turns into a primary follicle, meaning that the follicular cells that surround the primary oocyte develop into granulosa cells.
The second stage of follicular development begins for a few lucky primary follicles with the first menstrual cycle in puberty, and a few more primary follicles go into the second stage with each subsequent menstrual cycle. In the second stage, the primary follicles develop into secondary and eventually tertiary, or graafian follicles. In a secondary follicle, the primary oocyte is still in the prophase step of meiosis 1, but now the follicle has additional layers of granulosa cells, as well as theca cells. Theca cells make androstenedione, a sex hormone precursor, and granulosa cells use the enzyme aromatase to convert it into estradiol - a member of the estrogen family, as well as progesterone.
In a graafian follicle, a central cavity called the antrum forms within the follicle, and the granulosa cells secrete a nourishing fluid for the primary oocyte directly into that antrum. The second stage takes roughly 70 to 85 days and results in a few fast-growing graafian follicles. The third stage of follicular development starts when the graafian follicles are ready and occurs during the follicular phase of the menstrual cycle.
So let’s briefly switch gears and go over the highlights of the menstrual cycle to put that follicular phase into context. The menstrual cycle starts on the first day of menstrual bleeding, lasts 28 days on average from then. Assuming a 28-day cycle, the follicular phase makes up the first two weeks of the menstrual cycle, and the luteal phase, the last two weeks.
The menstrual cycle is ultimately controlled by the hypothalamus, which is at the base of the brain. Before puberty, the hypothalamus constantly secretes small amounts of a hormone called gonadotropin-releasing hormone, or GnRH. That GnRH travels to the nearby pituitary, which secretes two hormones of its own - follicle stimulating hormone, or FSH, and luteinizing hormone, or LH. Once puberty hits, the hypothalamus starts to secrete GnRH in pulses, sometimes more and sometimes less, and pituitary FSH and LH make the ovarian follicles develop.
All right, so as a quick recap: the female reproductive system comprises of internal sex organs, such as the ovaries, the fallopian tubes, the uterus and the vagina, as well as external sex organs such as the labia, the mons pubis and the clitoris. The ovaries are the female gonads, and they produce the ova, as well as the female sex hormones. Both the ovaries and the uterus are subject to the pulsatile hormonal activity of the hypothalamus and pituitary glands.The uterus is where pregnancy develops, and the baby comes into the world through the birth canal, or the vagina, that connects the internal and external sex organs.