Female Reproductive Anatomy

In order to avoid confusion, there are a few other anatomical terms we should review:

• Anterior is used to describe a structure that is towards the front. For example, the heart is largely anterior to the lungs. • Posterior is used to describe a structure that is towards the back. For example, the lungs are largely posterior to the heart. • Superior is used to describe a structure that is towards the head. For example, the eyes are superior to the mouth. • Inferior is used to describe a structure that is towards the feet. For example, the mouth is inferior to the eyes. • Medial is used to describe structures in relation to the midline of the body. • Lateral is used to describe structures further from the midline of the body. In females, the ovaries are the most lateral anatomical structures.

To review – anterior means front, while posterior means back. Superior means top (or closer to the head), while inferior means bottoms (or closer to the feet). The term medial is used to describe structures closer to the midline, while lateral is used to describe structures further from the midline. These terms will be helpful in understanding the relationship of the reproductive organs to one another.

FEMALE REPRODUCTIVE ANATOMY

The female reproductive system consists of the ovaries, fallopian tubes, uterus, cervix, and vagina. The ovaries, fallopian tubes, uterus, cervix and upper two-thirds of the vagina are located in the pelvis, which means that the vast majority of the system is protected by this bony structure. It is useful to keep in mind that these organs share space in the pelvis with the structures of the urinary and gastrointestinal systems. Figure 3 shows these major female reproductive structures:

Figure 3.

To understand where the organs of the female reproductive system are located within the pelvis, let’s first consider the coronal plane. Figure 4 shows these structures in the coronal plane:

Figure 4.

Recall that the coronal plane runs from head to toe and separates the body into anterior and posterior segments – or front and back segments. The coronal planes allow us to consider how the organs of the pelvis relate to the midline. In this view, we can appreciate that the uterus, cervix, and vagina are midline structures. The fallopian tubes extend laterally on both sides of the uterus. The fallopian tubes are hollow structures contiguous with the smooth muscle of the uterus. The fallopian tubes open to the ovaries. The ovaries are connected to the uterus by the ovarian ligament.

In the coronal view, you can see the close association of the female reproductive system to the urinary bladder and female rectum. Most of the time, this does not affect fertility but, as you’ll see, things like endometriosis affect the uterus, bladder, and rectum or even the sigmoid colon, leading to bladder or GI symptoms often seen in this disorder.

In addition, the fact that the bladder is a hollow structure that can be kept full of urine makes it helpful in using ultrasonography to better visualize the female reproductive structures using a transabdominal view of the uterus and ovaries.

The uterus, cervix and vagina form a hollow continuum located immediately behind the bladder, and the rectum is the most posterior structure in this view. This orientation also helps explain why pregnant women often have to urinate frequently. The bladder –the most anterior structure in this view – sits between the pubic bone and the uterus. As the uterus grows during pregnancy, it can put pressure on the bladder.

Let’s next consider the sagittal plane. Recall, that the sagittal plane is a vertical plane that divides the body in half into left and right segments. This view allows you to see how the organs of the pelvis relate to one another. In this view, the bladder is the most medial structure and that the vagina, cervix, and uterus, are also medial. The furthest lateral structures are the ovaries, which are paired structures on either side of the uterus.

Let’s now consider each of the organs of the female reproductive tract individually.

The ovaries are the gonadal structures of the female reproductive system. Females have two ovaries – a right ovary and a left ovary. The ovaries are oval shaped, and each ovary is about the size of an almond. The ovaries are held in place by the broad ligament. The ovaries are attached to the uterus by the ovarian ligament. The blood supply to the ovary comes from the ovarian artery. The right and left ovarian arteries branch directly off of the abdominal aorta right below the level of the renal artery. Figure 5 shows the major ligaments of the female reproductive system:

Figure 5.

The ovaries contain two general populations of cells – germ cells and follicle cells. Germ cells migrate into the gonadal tissue very early in embryonic development. Follicular cells (which are also referred to as granulosa cells) act as support cells for the germ cells. We will discuss the function of these cell populations in a few minutes.

The fallopian tubes – also called the uterine tubes – are tubular structures extending from the uterine fundus, bilaterally. The fallopian tubes transport eggs from the ovary to

the uterus. The fallopian tubes are contiguous with the uterus and consist of an inner mucosal layer and an outer smooth muscle layer. The mucosal layer contains ciliated cells that help propel the egg towards the uterus. Similarly, the smooth muscle layer contracts to facilitate egg transport.

The fallopian tubes can be divided into four sections – the isthmus, the ampulla, the infundibulum, and the fimbriae. The isthmus extends off the uterus and is the most medial part of the tube. The ampulla of the fallopian tube is lateral to the isthmus. The ampulla is a curved section of the tube, and it is the location of fertilization.

The infundibulum is the funnel-shaped portion of the tube that opens to the ovary. Fimbriae are attached to the infundibulum. Fimbriae function by sweeping released eggs from the ovary into the infundibulum. From the infundibulum, the egg can then travel through to the ampulla, isthmus, and into the uterine cavity.

The fallopian tube is the major site of fertilization of the female egg, after which it is the zygote that gets transported to the uterus. While fertilization can occur anywhere along the fallopian tube, the most common site for this process is the ampulla. You need to realize that much of the fallopian tube interiorly is very narrow. The total length of each fallopian tube is 7 to 12 centimeters, while the external diameter is less than 1 centimeter. The narrowest part of the fallopian tube is the part where it opens into the uterus. At this point, it is just 1 millimeter in internal diameter. In the ampulla, the internal diameter is about six millimeters in diameter.

The fimbriae represent the lateral ends of the fallopian tube. These form fingers that essentially wrap around the ovaries in order to give the ruptured egg the best opportunity to flow into the fallopian tube. While the relationship between the fimbriae and ovary is a close one, this structure actually is not enclosed but opens directly into peritoneal cavity itself. It provides a portal through which menstrual blood and tissue, ascending infectious organisms, or pus can pass out of the female reproductive tract in order to enter the pelvis. In addition, the narrowness of the fallopian tube in parts makes it easy for this structure to impact fertility. If an infection occurs that scars the fallopian tube, this greatly impairs fertility by blocking the passage of the egg or zygote, leading to an increased chance of an ectopic pregnancy.

The fallopian tube has multiple layers. The inner epithelial layer has four types of cells within it. There are secretory cells that do not have cilia and ciliated cells that contain the cilia, which beat in the direction of the uterus. There are intercalary cells sometimes called peg cells spaced between other cells. Small cells called indifferent cell make up the fourth type of epithelial cell in the fallopian tube. The numbers and appearance of these cells depend greatly on the estrogen and progesterone levels of the female, which means they function differently depending on the menstrual phases, pregnancy status, and menopausal status of the woman. This alone may play a significant role in fertility.

The uterus is a hollow organ that functions as the womb in human reproduction. The uterus is comprised of three layers – the endometrium, the myometrium, and the perimetrium. The perimetrium is the outermost layer. The perimetrium is a serous membrane covering similar to the peritoneum and pericardium. The myometrium is the middle layer. The myometrium is comprised of smooth muscle and functions to expel the neonate during labor and delivery. The innermost layer is the endometrium, which is the layer involved in the female menstrual cycle.

Before we discuss the endometrium and myometrium separately, let’s first discuss the uterus as a whole. The uterus is located within the pelvic cavity and is contiguous with the fallopian tubes superiorly and the cervix inferiorly. Within the pelvis, the uterus is supported by a variety of muscles and ligaments. The pelvic floor muscles support the uterus nearly as much as the supporting ligaments. Women with pelvic floor dysfunction will not have the musculature to support the uterus and uterine prolapse through the vagina may occur. Laxity of the supporting ligaments also affects uterine prolapse.

Among the different supporting ligaments of the uterus are the round ligaments that connect the anterior and superior surface of the uterus to the labia majora, passing through the internal inguinal rings and the inguinal canal. These stretch as the uterus grows in pregnancy. The second set of ligaments is the uterosacral ligaments that fan out from the lateral uterus to the sacrum. These contain muscle fibers, blood vessels, parasympathetic nerve fibers, and lymphatics. There are also the paired transverse or cardinal cervical ligaments that extend from the cervix and insert into the fascia near the

levator ani muscles and obturator muscles. The broad ligament comes from peritoneal folds that cover the fallopian tubes, housing the major uterine structures.

The upper part of the uterus is more muscular, while the lower part of the uterus, including the cervix, is more fibrous. The portion above the fallopian tubes is called the fundus. The next lowest portion is the corpus or body. The lower part is the cervix or isthmus. The majority of the endometrial cavity is contained within the corpus.

The myometrium is layered in the uterus. The outer layer is longitudinal along the vertical axis of the uterus. The vascular layer has interlacing spiral groups of smooth muscle fibers and is very vascular. The inner layer is both oblique and longitudinal. These layers together allow for maximum uterine contractions in all directions.

The uterus itself can be in several different positions. Most of the time, the woman who has never been pregnant has a uterus slightly bent in the anterior direction and its axis is also tilted forward. A lesser number of women will have a retroflexed uterus, which is backward curvature, or retroversion, which is backward tilted with respect to the axis. Others are mid-position, with a vertical axis. There is no evidence that any of these positions affects fertility, bleeding patterns, or pain patterns of the woman, even though this was once a common misperception.

The outer layer of the uterus is called the perimetrium, which is an extension of the peritoneum. This is a thin layer of fascia that lies over the uterus on all sides. It has no muscular or secretory activity but largely separates the external border of the uterus.

The endometrium is very important and changes with the different phases of the menstrual cycle. There are three layers, including the pars basalis, the zona spongiosa, and the superficial zona compacta. There are straight branches of the uterine radial arteries that end in the pars basalis. There are spiral branches that penetrate further to make lakes of dilations of the end capillaries.

The endometrium proliferates because of the influence of estrogen, while maturation of this tissue is under the influence of progesterone. In the follicular phase or proliferative phase, the lining thickens up until the time of ovulation. After ovulation is the secretory or luteal phase. The menstrual phase is when the most superficial layer sheds because estrogen and progesterone levels are low. The spiral arteries proliferate in the luteal

phase but clot off in the menstrual phase in order to shed the superficial zona compacta. Menstrual cramps come from ischemia of the uterine lining as the spiral arteries fail.

The blood supply of the uterus is derived from the uterine artery. The uterine artery is a branch off of the internal iliac artery. The uterus also receives blood from the ovarian artery. The ovarian artery arises from the abdominal aorta. The uterine and ovarian arteries often have small connections – or anastomoses - between them. The uterus receives both sympathetic and parasympathetic innervation. The sympathetic innervation to the uterus is derived from the uterovaginal plexus. The uterovaginal plexus arises from the inferior hypogastric plexus. The parasympathetic innervation to the uterus comes from the pelvic splanchnic nerves. The pelvic splanchnic nerves arise from S2 through S4.

There is the inferior aspect of the uterus called the cervix. It is about 2 to 3 centimeters long. There is an intravaginal part called the portio vaginalis, which has nonkeratinizing squamous epithelium on its surface with mucus glands. Below that is the endocervical canal, which is 2 centimeters long. The external os is the opening of the cervix into the vagina. The endocervical canal is lined with columnar epithelium and contains endocervical glands. At the junction of the ectocervix and the endocervix is the squamocolumnar junction. Its features depend on the hormonal environment. This is where cervical cancer originates.

The cervix is contiguous with the body of the uterus. The cervix connects the uterus to the vagina. The cervix is comprised of two distinct regions – the ectocervix and the endocervix. The ectocervix is the inferior region of the cervix that projects into the vagina. The opening of the ectocervix is called the external os. The endocervical canal is the more superior portion of the cervix. The internal os is the opening between the endocervical canal and the body of the uterus. The uterine artery supplies blood to the cervix. The cervix is innervated by the pelvic splanchnic nerves arising from S2 through S4.

The vagina is a muscular tube that extends from the cervix. The vagina consists of four layers – an outermost fibrous layer, a fibromuscular layer, an elastic layer, and an innermost epithelial layer. The blood supply to the vagina comes from the uterine