Anatomy, Abdomen and Pelvis: Female Internal Genitals

Structure and Function

The uterus is the central anatomical landmark of the female internal genitalia and pelvic anatomy. The highly muscular, childbearing organ measures approximately 3 x 2 x 1 inches in size in a nulliparous woman. Even though the uterus is primarily a pelvic organ, during later stages of pregnancy, it can reach up to the epigastric region due to hypertrophy and hyperplasia of the myometrium.

The most common anatomical position of the uterus is anteverted-anteflexed. Anteversion is the angulation between the long axis of the cervix and the vagina, which nears a right angle.[1][2] Anteflexion is the angulation between the body's long axis and the cervix, an obtuse angle of around 120° to 125°. In an anteverted-anteflexed position, the uterine body sits within the vesicouterine space. In rare cases, the uterus may be retroverted-retroflexed, causing the uterine body to be within the pouch of Douglas. 

The shape of the uterus has been described as "pyriform" on examination in a sagittal plane. The uterus has 4 parts: the fundus, body, isthmus, and cervix.[3][4] The uterine body is the largest portion, while the uterine isthmus is where the uterine body meets the cervix.[4] The fundus is the part of the uterus above the opening of the fallopian tubes into the uterine cavity. The uterine cavity has a lining membrane called the endometrium. The endometrium will shed during menstruation; inflammation of this membrane is endometritis, which, when chronic, can lead to infertility.[5] 

The cervix is the lower, short, broad cylindrical part of the uterus extending from the internal to the external os and has a narrow central lumen, which opens into the uterine cavity as the internal os and into the vagina as the external os. The external os is seen as a circular opening in nulliparous females and as a horizontal slit in multiparous females. This horizontal slit divides the cervix into an anterior and posterior lip. The narrow upper third of the cervix is the isthmus, which defines the lower uterine segment and is the site for the lower segment cesarean sections.[6]

The ovaries are the primary organs of the female reproductive system. They are oval-shaped gonadal structures, approximately 3 x 1.5 x 1 cm in size, and homologous with the testis in males. They are present in pairs with the long axis oriented downward and forward. The ovaries are within the pelvis, just lateral to the uterus. They lie within the ovarian fossa, in front of the ureters, and behind the external iliac vessels.[3] In nulliparous females, they are pinkish with a smooth outer surface, but in multiparous women, they are more greyish with a puckered surface due to repeated ovulations.

The ovaries are entirely covered with peritoneum except for the mesovarian (anterior) border through which all the blood vessels, nerves, and lymphatics pass. They contain thousands of follicles. Each month, 1 follicle will develop into the dominant (Graafian) follicle, which releases an ovum during ovulation.[4] On ultrasound, the ovary appears to have an echoic central area and a peripheral region, with the former corresponding to the stroma and the latter corresponding to the cortex.[3] The ovaries secrete 2 steroid hormones, estrogen and progesterone, under the direct control of follicle-stimulating hormone and luteinizing hormone secreted by the pituitary. These hormones play a significant role in many aspects of the female life.[7][8][9] 

The ovaries link to the uterus through fallopian tubes, which carry the zygote to the uterine cavity for implantation. The fallopian tubes (oviducts or uterine tubes) divide into the fimbriated infundibulum, ampulla, isthmus, and intramural parts.[10] The ampullary part is the usual site for fertilization.[11] The fallopian tube also helps bring the sperm and ovum to this site of fertilization. The infundibulum is the later part, funnel-shaped, with finger-like projections hanging from the sides, called fimbriae. This part opens into the peritoneal cavity and is in contact with the ovaries through some of its fimbriae. The intramural section is the narrow and medial part of the fallopian tube, which is present within the uterine wall. Each region represents characteristic physiologic features.

Salpingitis is a bacterial infection of the fallopian tube. The inflammation may be acute or chronic, most commonly resulting from Neisseria gonorrhoeae or Chlamydia trachomatis. These conditions can lead to fallopian tube scarring and predispose one to infertility and ectopic tubal pregnancy. A tubal factor caused by disruption of the integrity of the fallopian tubes by infusion, endometriosis, or surgical complication is an important consideration when evaluating women with unexplained infertility.[10] Ectopic tubal pregnancy most commonly occurs in the ampulla of the fallopian tube. Common predisposing factors include salpingitis, pelvic surgery, or pelvic inflammatory disease. This diagnosis may be confused with appendicitis in a young woman, and early differentiation between the 2 diagnoses is crucial.

The vagina is a fibromuscular canal ranging from 6 to 8 cm in length and anatomically is located anteriorly to the rectum and posteriorly to the wall of the urinary bladder and the urethra.[12] This canal directs downwards and forwards. Because of the oblique nature of the vagina, the anterior wall is slightly shorter, ie, about 6 cm, compared to the posterior wall, which is about 8 cm. The upper segment of the vagina forms a vaginal vault by being enclosed within the vaginal portion of the cervix.[13] 

The cervix extends into the vagina, forming a channel between the 2 structures. Thus the cervix is divided into a supravaginal and a vaginal part. The narrow space between the cervix and the wall of the vaginal is the fornix. The recess between the anterior lip of the cervix and the anterior wall of the vagina is the anterior fornix. Similarly, the recess between the posterior lip of the cervix and the wall of the vagina is the posterior fornix. The spaces on either side between the cervix and the lateral walls of the vagina are known as the lateral fornices. The posterior fornix is the deepest and is a common site for ruptures, foreign bodies, and malignancies.[14][15][16][17]

Embryology

Before the determination of phenotypic sex, the gonads are classified as "unisex" and have the potential to develop into testes or ovaries.[18] The embryologic development of the female internal genitals has been a topic of research in recent years. The determination of female sexual differentiation is by the absence of the SRY (sex-determining region on Y) gene, a gene on the short arm of the Y chromosome (Yp11). When present, the SRY gene will cause the indifferent embryo to develop into the male phenotype.

The belief is that the ovaries derive from the gonadal ridge while the primordial germ cells develop in the epiblast and migrate to endoderm cells in the yolk sac.[19] They finally appear in the genital ridges in the sixth week of gestation. The ovaries initially develop within the abdomen and migrate to the pelvis later in fetal life. Even though they migrate into the pelvis, they still retain their blood supply, nerve supply, and lymphatic drainage back to their site of origin, the upper abdomen.[20]

The gubernaculum forms the ovarian ligament and the round ligament of the uterus. In the recent decade, there has been ongoing research to identify a female-determining sex gene. So far, the "Z" gene hypothesis, which names FoxL2 as a possible ovary-determining factor, has been postulated.[21] Other genes believed to be involved, including Dax1 and Wnt4, may have similar effects; however, many unanswered questions remain regarding this pathway.[18] 

The paramesonephric (Mullerian) duct is the embryologic derivative of the genital ducts. The cranial portions form the uterine tubes, while the caudal parts form the uterovaginal primordium. The uterovaginal primordium gives rise to the uterus, cervix, and superior third of the vagina. The vaginal plate is a derivative of the sinovaginal bulbs, which canalize to form the lower two-thirds of the vagina.[22] These derivatives are surrounded by a layer of myometrium that develops from the mesenchyme. The outermost covering is the perimetrium, which develops from folds of the peritoneum.

Embryologic defects that are important to consider include unicornuate uterus anomalies, where 1 paramesonephric duct fails to develop; didelphys anomalies, where there is a complete lack of fusion of the paramesonephric ducts; and bicornuate uterine anomalies, where there is a partial fusion of the paramesonephric ducts.[23][24] Bicornuate uterine anomalies are associated with the highest risk of infertility. The mesonephric (Wolffian) duct disappears in the case of females, except for a small cranial part that remains as the epoophoron and paroophoron. The remnant of the mesonephric duct is called the Gartner duct.[25] A small caudal portion of this duct can be visualized and is referred to as a Gartner duct cyst within the wall of the uterus or vagina.[26]

Blood Supply and Lymphatics

The ovarian artery is a branch of the abdominal aorta; it reaches the ovary through the suspensory ligament of the ovary. Although ovaries are pelvic organs, they derive their blood supply from the abdominal aorta, indicating their site of origin. The uterine artery, the main arterial supply of the uterus, originates from the anterior division of the internal iliac artery and the umbilical artery in a common trunk.[27] The uterine artery is highly tortuous and travels across the pelvic floor in the cardinal ligament and runs anteriorly and superiorly to the ureter close to the lateral fornix of the vagina, ending by anastomosing with the ovarian artery within the broad ligament of the uterus (see Image. Female Reproductive System Blood Supply). Ligation of the uterine artery during a hysterectomy is typically at the level of the internal cervical os.[27] The small, inferior branch of the uterine artery becomes the vaginal artery and supplies the cervix and vagina. The vaginal artery may also arise from the internal iliac artery.

The veins of the female internal genitals typically correspond to the arteries. The uterine vein follows the course of the uterine artery and is also contained within the cardinal ligament; it drains into the internal iliac vein. The ovarian veins receive blood from the ovaries; the right ovarian vein drains directly into the inferior vena cava, while the left ovarian vein drains into the left renal vein. Malignancies in the pelvis may metastasize to the vertebral column or spinal cord via connections of the pelvic veins. The lymphatic flow from the uterus, cervix, and vagina drains into the internal and external iliac nodes.[28] Lymphatic vessels from the ovary and fallopian tube drain directly into the paraaortic nodes. 

Nerves

The nerve fibers that feed the pelvis are both parasympathetic and sympathetic. The preganglionic fibers are in the sacral parasympathetic (S2-S4) region of the spinal cord. Their processes run into pelvic splanchnic nerves and travel inside the pelvic organs within the intramural plexus (the postganglionic neuron). The sensory fibers from S2-S4 dorsal root ganglia travel with parasympathetic fibers and carry pain sensations from the organs. Sympathetic fibers of preganglionic neurons (T12-L2) form the sacral splanchnic nerves. They contribute to the inferior hypogastric plexus, which is the location of the postganglionic neurons. 

The pudendal nerve provides sensory innervation to the lower vagina. A pudendal nerve block is an option during early labor to relieve maternal pain and may be achieved by piercing the vaginal wall posterolaterally, using the ischial spine as a landmark.[29] The block may also be performed percutaneously along the medial side of the ischial tuberosity. However, this method does not affect pain from uterine contractions and, thus, is no longer commonly used.

Muscles

Several pelvic diaphragm muscles support the uterus, the urogenital diaphragm, and the round, broad, lateral, and cardinal ligaments. The pelvic diaphragm includes the following muscles: piriformis, coccygeus, iliococcygeus, pubococcygeus, and puborectalis. The pubococcygeus, puborectalis, and iliococcygeus muscles are levator ani muscles.

Vaginal delivery is a major cause of levator ani defects, genital organ prolapse, and urinary incontinence.[30] Results from several studies have shown that during the second stage of labor, the pubococcygeus muscle bears the largest tissue strain compared to other pelvic floor muscles.[30] The upper portion of the posterior vagina is suspended and held in place by several lateral connections. Prolapse of this area can occur if there are defects in the perineal body, which most commonly occur following vaginal delivery. The role of episiotomy in the prevention of perineal body tears is controversial.[31][32]

The major ligaments that support the uterus include the round ligament, transverse cervical ligament (cardinal ligament), uterosacral ligament, and broad ligament. The cardinal ligament reaches from the cervix to the sidewall of the pelvis and appears at the base of the broad ligament. The uterosacral ligament plays a significant role in keeping the uterus in the anteverted position. This ligament extends from the cervix to the sacrum and supports the uterus posteriorly. The pubocervical ligament provides anterior support to the uterus, helping to prevent a cystocele. The broad ligament is a double folding of the peritoneum, divided into 4 regions composed of the mesosalpinx, mesovarium, mesometrium, and the suspensory ligament of the ovary. This ligament overlies the ovaries, uterus, and fallopian tubes and secures the uterus laterally to the sidewalls of the pelvis.[30]

When the supportive structures of the pelvis fail, pelvic organ prolapse results; this is a common condition among women and is usually treated with conservative management, mechanical support using pessaries, or surgical repair. Laparoscopic promontofixation is a surgical option that has excellent long-term results.[33] Additionally, laparoscopic promontofixation is associated with low recurrence and morbidity rates and improved quality of life in women with pelvic organ prolapse.[33]

Physiologic Variants

A common physiologic variant is the retroverted uterus. The presence of a retroverted uterus is not associated with a higher incidence of infertility, urinary tract infection, or hyperemesis gravidarum. However, study results have demonstrated that the incidence of early pregnancy bleeding and spontaneous abortion is significantly higher in those with a retroverted gravid uterus.[34] These patients require careful observation, and the clinician should not perform bimanual manipulation in gravid women with a retroverted uterus.[34] 

Additionally, the origin of the uterine artery may vary in up to 1 in 5 cases.[27] The uterine artery originates directly from the internal iliac, superior gluteal, obturator, or internal pudendal artery.[27] A particularly challenging variation occurs when the uterine artery adopts a C-shaped configuration, with a secondary branch arising directly from the internal iliac artery. Healthcare professionals must anticipate these variants during surgery for successful ligation.

Surgical Considerations

A surgeon must be aware of anatomical relationships when operating on a female. Postpartum hemorrhage is a serious complication, especially after cesarean section. Typically, postpartum hemorrhage management involves clamping or embolization of the uterine artery. If bleeding persists, the surgeon should clamp the internal iliac artery below the origin of the superior gluteal artery. Some study results have suggested that several arteries, including the uterine artery, are recruited to respond to the demand from the gravid uterus.[35] In these cases, treatment of recurrent or persistent postpartum hemorrhage after bilateral uterine artery embolization may be successfully treated with round ligament artery embolization.[35]

Being aware of the position of the ureter during a hysterectomy is crucial; it is especially prone to being ligated because of its proximity to the uterine artery and cervix. The ureter travels under the cardinal ligament and is a vital structure to be mindful of. Furthermore, during a hysterectomy, the uterine artery is usually isolated to prevent unintentional ligation and massive bleeding. After clamping, the uterine artery is ligated at the level of the internal cervical os during a hysterectomy.[27] Femoral hernias may occur in females, but the herniated tissue is usually omentum or bowel. However, there have been rare, reported cases where the ovary, fallopian tube, and/or uterus were contained within the hernia.[36] This possibility should be carefully considered in the management of such cases.

Clinical Significance

Significant clinical correlations to consider when considering the female internal genitalia are as follows:

• The uterus typically sits in an anteverted, anteflexed position. A retroverted uterus is associated with an increased risk of early bleeding in pregnancy and spontaneous abortion. Cesarean section is correlated with a retroverted uterus.[37]
• Embryologic defects in the shape of the uterus, such as bicornuate uterine anomalies, may also lead to infertility and first-trimester spontaneous miscarriages.
• The uterus receives its vascular supply from the uterine artery, an anterior division of the internal iliac artery, which travels within the cardinal ligament. Physiologic variants of the uterine artery may be present in many cases, and surgeons must anticipate these cases for appropriate ligation of the artery.[38]
• There must be careful consideration of the ureter during surgeries involving the internal female genitals to avoid unintentional ligation.
• The ovaries are the gonadal structures of females; they derive from the gonadal ridge. The “Z” gene hypothesis names FoxL2, Dax1, and Wnt4 as possible ovary-determining factors.[39]
• The ovaries obtain their blood supply from the ovarian arteries, and their lymphatic drainage is directed to the paraaortic nodes.
• The fallopian tubes are the connections from the ovary to the uterus. Salpingitis may lead to scarring of the fallopian tubes and predispose to infertility and ectopic tubal pregnancy. A tubal factory must be considered in young women with infertility of undetermined significance.[40]
• A pudendal nerve block is an option to alleviate the pain of labor. However, these blocks are rarely used since the popularity of epidurals has increased. 
• The pubococcygeus muscle bears the largest tissue strain compared to other pelvic floor muscles and is at significant risk of being compromised during vaginal delivery. 
• Study results have shown that the round ligament's embolization may help control recurrent or persistent postpartum hemorrhage.[41] Interventional radiologists should consider this as an alternative for hard-to-control cases.[42]
• There have been rare cases of femoral herniation containing the ovary, fallopian tube, and uterus.[36][43][44]
• Pelvic organ prolapse is prevalent among women.[45] Many treatment strategies exist, including conservative management, mechanical support, or surgical repair.[46] Laparoscopic promontofixation is a surgical approach that generally delivers improved long-term outcomes.