1.1 Introduction
A thorough understanding of pelvic anatomy and normal physiological transitions during the reproductive phase is required to be able to competently care for girls and women throughout their lifespan.
This chapter outlines basic knowledge of gynaecological anatomy and the reproductive cycle that you will require as a foundation for further knowledge.
1.2 Normal (embryological) development of the genital tract
Key events
The development of the female genital tract has three key embryologic events. These are the differentiation of the gonad into an ovary, female differentiation of internal genital organs and female differentiation of external genital organs (Figure 1.1).
Figure 1.1: Female genital tract development. (a) Development of the ovaries; (b) Development of internal genitalia and (c) Development of external genitalia.
3The embryologic gonad forms an ovary in the female fetus and a testicle in the male fetus. The testicle produces Müllerian-inhibiting substance (MIS), which inhibits the development of female internal genitalia in males (see below).
The ovary does not produce MIS. The fetal ovary also forms all primordial ova that the female will have throughout her future lifespan. New oocytes are not created past this point.
The internal genital organs are pluripotent in early fetal development and contain the Müllerian ducts and Wolffian ducts, both of which are closely associated with the embryonic kidney. The Müllerian ducts can develop into female internal genitalia (fallopian tubes, uterus, cervix and upper vagina), while the Wolffian ducts can develop into male internal genitalia.
The external genital organs include the labia majora, labia minora, clitoris and lower vagina. These have precursor structures (genital tubercle and labioscrotal folds) that respond to either fetal oestrogen or testosterone to differentiate into female or male external genital organs, respectively.
Developmental sequence
Both internal and external genital organ development is guided by sex steroid production from the gonad together with the presence or absence of MIS.
Internal genitalia
In a normal female fetus, the absence of testosterone and MIS causes the Wolffian ducts to regress and the Müllerian ducts to fuse in the mid-line, forming the upper vagina, cervix and uterus. The uppermost parts do not normally fuse and remain separate on either side of the uterus as the fallopian tubes (Figure 1.2). The lowermost part fuses with the developing lower vagina from the external genital organs and the vagina is canalised (opened) (Figure 1.3).
This principle of mid-line fusion, followed by vertical fusion with the developing external genitalia, is fundamental to understanding normal internal genital anatomy and how errors in this process can lead to failure of development of one side, failure of mid-line fusion for all or part of the Müllerian duct and failure of vertical fusion with the external genital organs with anatomical obstruction.4
It is also possible for the lower genital tract to erroneously connect with the alimentary tract and form a common exit termed the ‘cloaca’.
External genitalia
The external genitalia develop from the fetal genital tubercle and labioscrotal folds. Oestrogen results in the urethra remaining posterior to the developing clitoris and not running through the genital tubercle, as it differentiates (as occurs in a male fetus). Similarly, the labioscrotal folds do not meet in the mid-line and form labia, which meet posterior to the developing vagina.5
Figure 1.4: (a) Mid-line fusion problems and (b) Vertical fusion problems – transverse vaginal septum.
The lower vagina meets and fuses with the upper part of the vagina formed by the internal genitalia and the dividing tissue breaks down (canalises) to form a patent vaginal canal (Figures 1.1 and 1.3).
1.3 Pelvic anatomy
Bony pelvis
The pelvis comprises four bones: two innominate bones laterally and the sacrum and coccyx posteriorly. They are held together by strong ligaments and covered by muscle and fascia. Each innominate bone has three parts, which fuse together by puberty: (1) the wide ileum, located laterally; (2) the ischium, inferior, and the bone used to sit; and (3) the pubis, which meets the opposite side in the mid-line at the pubic ramus, the most anterior part of the bone (Figure 1.5).
The pelvic cavity is the space bounded by the bones of the pelvis. It is divided into the greater (false) and lesser (true) pelvises.
Lesser bony pelvis
The lesser pelvis, which contains the bladder and reproductive organs, is the part of the pelvic cavity between the pelvic inlet and the pelvic outlet (Figure 1.6).7
Figure 1.5: (a and b) Bony pelvis. The lesser pelvis lies between the pelvic inlet and the pelvic outlet. The greater pelvis is above the pelvic inlet.
The pelvic inlet is the aperture bordered by the superior margin of pubic symphysis (anteriorly), the arcuate line of each ileum (laterally) and the sacral promontory (posteriorly).
The pelvic outlet is the aperture bordered by the inferior margin of pubic symphysis (anteriorly), the inferior rami of pubis and ischial tuberosities (anterolaterally), the sacrotuberous ligaments (posterolaterally) and the tip of the coccyx (posteriorly).8
Figure 1.6: Contents of lesser pelvis. The internal genital organs lie within the pelvic inlet and outlet. The external genitalia lie below the outlet.
The lesser pelvis is lined laterally by fascia over the pelvic bones, levator muscles of the pelvic floor and the muscles of the pelvic floor (below). Due to upright positioning and the challenges of expanding to accommodate birth of a fetus, this is the largest potential hernial portal in the body and disorders are common.9
Greater bony pelvis
The bones of the greater pelvis, situated superior to the pelvic inlet, include the ilium and ala of sacrum. Mobile contents of the abdominal cavity including the small bowel and some large bowel sit within the greater pelvis. The greater pelvis is bounded by the abdominal wall anteriorly, the L5 or S1 vertebrae posteriorly and the iliac fossae posterolaterally.
Perineum and pelvic floor
The perineum lies inferior to the pelvis (and pelvic floor) and refers to the surface area of the body which sits on a bicycle seat. It is bounded by the symphysis pubis anteriorly; the inferior pubic rami, inferior ischial rami and sacrotuberous ligaments laterally; and the coccyx posteriorly. The anterior half contains the external genitalia and the posterior half contains the anus.
Separating the perineum and the pelvis is a muscular and ligamentous diaphragm known as the pelvic floor, which is traversed by the urethra, vagina and rectum (Figure 1.7). The principal muscle forming the pelvic floor is the levator ani. This thin, yet strong, muscle helps to support the pelvic viscera and is innervated by the pudendal nerve.
10The perineum is divided by an imaginary line passing through the ischial tuberosities into a urogenital triangle anteriorly and an anal triangle posteriorly (Figure 1.8).
Pelvic muscles, nerves and vasculature
Muscles
The pelvic bones provide attachment for major muscle groups involved in movement of the lower limb (e.g. psoas, iliacus, rectus femoris, sartorius, adductors, gluteals, piriformis), spine and trunk (e.g. rectus abdominis, erector spinae, quadratus lumborum, external and internal oblique muscles) and pelvic floor (above).
The muscles of the pelvic floor are more relevant to gynaecology, as they are damaged in parturition and relevant in gynaecological repair.
Nerves
The nerves of the pelvis can be divided into somatic (under voluntary control) and autonomic (involuntary – sympathetic and parasympathetic function) as well as those which supply the organs and tissue of the pelvis and those that pass through to innervate the lower limb.
11Somatic nerves include the pudendal nerve. Autonomic nerves carry sympathetic supply via the hypogastric and sacral nerves and parasympathetic supply via the pelvic nerve.
These nerves are important to gynaecologists, as they may be injured by gynaecologic procedures and parturition (childbirth).
Nerve injury This is graded by severity into neuropraxia, axonotmesis and neurotmesis, meaning compression with conduction disruption, division of the axons and division of the entire nerve in order of increasing severity. Neuropraxia recovers spontaneously with remyelination as does axonotmesis although this takes longer. Neurotmesis requires surgical repair to achieve any recovery of function and this is usually incomplete. Most gynaecologic injuries are neuropraxias and axonotmesis.
Nerve plexuses to lower limb Many major plexuses and lower limb peripheral nerves also pass to the lower limb via the pelvis. These include the lumbosacral plexus formed from the dorsal rami of nerve roots L1-S3 with a small contribution from T12 and its main branches, the sciatic nerve, femoral nerve and obturator nerve. This plexus lies within the psoas muscle and its branches arise from this muscle and arc through the pelvic sidewalls and out through sacral foramina and under the inguinal ligament into the anterior, medial and posterior lower limb (Figure 1.9).
Branches of the lumbosacral plexus with relevance of gynaecology and their significance are tabulated in Table 1.1.
Intrinsic pelvic somatic nerves The most important intrinsic nerve of the pelvis is the pudendal nerve. This arises from the sacral component of the lumbosacral plexus and has sensory supply of the external genitalia, clitoris, perineum, perianal skin and motor supply to the external urethral (voluntary) sphincter and external anal (voluntary) sphincter.
It is damaged in parturition and this damage can be permanent with stretch and ischaemic injury caused by prolonged compression by a fetal head in second stage leading to loss of perineal sensation, sexual sensation and urinary and faecal incontinence.12
Autonomic pelvic nerves The autonomic nervous supply includes sympathetic and parasympathetic innervation.13
Figure 1.10: Autonomic pelvic nerves.Source: Gest TR. (2000). Learning Modules – Medical Gross Anatomy: Introduction to Autonomics. [online] Available from https://anatomy.elpaso.ttuhsc.edu/modules/intro_autonomics_2_module/autonomics_12.html. [Last accessed from August, 2021].
The sympathetic supply of the body is carried along the sides of the vertebral canal as a sympathetic plexus whereas the parasympathetic supply exits at two sites, cranially as the vagus nerve and sacrally as the pelvic nerve (called craniosacral outflow) (Figure 1.10).
Sympathetic innervation is responsible for inhibition of defecation and urination. Parasympathetic innervation is responsible for facilitation of these processes. Parasympathetic innervation controls genital arousal changes and orgasm.
The autonomic pelvic nerves include the hypogastric nerve, the sacral and the pelvic splanchnic nerves. They form the inferior hypogastric plexus deep to the peritoneum in the pre-sacral space and supply sympathetic and parasympathetic innervation to the distal rectum, bladder and genital organs, notably including the cervix, which feels visceral pain via fibres carried in the pelvic nerve.15
Sympathetic fibres are carried in the sacral splanchnic nerves from the sympathetic trunk and parasympathetic fibres arise from S2-S4 and are carried in the pelvic nerve.
Anterior abdominal wall anatomy
The anterior abdominal wall is composed of muscles, nerves, vessels and fascia. It is the site of common incisions for laparoscopy and open surgery, and connects to the pelvic bones. A knowledge of anatomy of the anterior abdominal wall is required to perform safe pelvic surgery.
Muscles
The major muscles forming the anterior abdominal wall are grouped into mid-line and lateral muscles.
Mid-line muscles These are the rectus abdominis and pyramidalis muscles. Together, they make up over half the anterior abdominal wall.
The rectus abdominis muscle extends from the lower costal cartilages superiorly to the pubic crest inferiorly. It is anchored transversely by attachment to the anterior layer of the rectus fascia at tendinous intersections. These fibrous bands give rise to the so-called ‘six-pack’ appearance of the tensed rectus abdominis. The rectus fascia ends at the anatomical landmark known as the arcuate line, where there is a change in arrangement of the layers forming the anterior abdominal wall; this means that incisions made for a caesarean section do not encounter the posterior rectus sheath because it does not exist below the umbilicus (Figure 1.11).
The pyramidalis muscle, which is absent in 20% of people, is anterior to the inferior part of the rectus abdominis and attaches to the anterior pubis. The pyramidalis muscle ends in and tenses the linea alba, the thick mid-line formed by fusion of the two bilateral aponeuroses of the abdominal muscles. The linea alba is wide superior to the umbilicus and then tapers inferior to it.
Lateral muscles These are the external oblique, internal oblique and transverse abdominis muscles.16
Their fleshy bodies become aponeurotic, as they approach the lateral border of the rectus abdominis muscle. These muscles also contribute to the structure of the inguinal ligament.
Nerves
Innervation of the anterior abdominal wall derives from the T7 down to the L1 nerve roots. T7-L1 spinal nerves travel inferiorly and medially giving rise to lateral and anterior cutaneous nerves that traverse the fibres of the abdominal wall muscles to reach the skin.
Vessels
The anterior abdominal wall is supplied with blood from three sources:
- Superior epigastric arteries (terminal branches of the internal thoracic artery)
- Inferior epigastric arteries (terminal branches of the external iliac arteries)
Organs
The pelvis contains key organs in the genitourinary and gastrointestinal systems, as distal extensions for excretion as well as true intrinsic pelvic organs. The main organs of the pelvis are the uterus and fallopian tubes, ovaries and the bladder and rectum.
Uterus
Before pregnancy, the uterus measures 8 cm long. This increases to 38 cm by the time a normal pregnancy reaches term, at which stage the uterus lies just under the sternum. The uterus comprises a fundus, two lateral cornua, a body, an isthmus and a cervix (Figure 1.12).
Relations The relations of the uterus are:
- Anteriorly: The uterovesical pouch, separating it from the bladder and loops of the small intestine
- Posteriorly: The rectouterine pouch (of Douglas), separating it from the rectum
The uterus receives blood predominantly from two large uterine arteries, which arise from the internal iliac arteries. The uterine arteries anastomose with terminal branches of the ovarian arteries (direct branches of the aorta).
Gynaecological surgical incisions
Pfannenstiel incision
This is a slightly curved horizontal incision made at the pubic hairline (Figure 1.13). A Joel–Cohen incision is slightly higher and horizontal. Both are frequently used. At the level of a routine Pfannenstiel incision, below the arcuate line of the rectus fascia, the following structures are encountered, from superficial to deep:
- Skin
- Superficial fatty (Camper's) fascia
- Superficial membranous (Scarpa's) fascia
- Rectus fascia (the rectus sheath) enclosing the rectus abdominis muscle
- Pre-peritoneal fat and parietal peritoneum
- The bladder (if it is not empty)
Mid-line Incision
This incision is made through the linea alba. It is a clean and rapid way to access the abdomen because of the absence of blood vessels and nerves in the linea alba.
Mid-line incisions are usually made below the umbilicus; however, when wide abdominal access is required, they are 19extended above the umbilicus. This is rare in benign gynaecology, but common in cancer surgery.
Laparoscopy incision
This is a keyhole incision made to insert a camera or laparoscopic surgical tool into the abdomen, particularly for gynaecological operations. Common laparoscopic incision sites are umbilical, supra-pubic and lateral abdominal.
Care is needed to avoid damaging peripheral nerves and vessels that cross the area of incision or insertion. In particular, this applies to the inferior epigastric vessels with lateral port sites – these should be visualised using the primary entry laparoscope and the planned lateral port entries made away from their path on the internal surface of the anterior abdominal wall. They are visible as pulsatile structures through the parietal peritoneum.
The urinary bladder is also emptied to avoid injury with supra-pubic port insertion.
1.4 Normal menstrual cycle
Hypothalamic-pituitary-ovarian axis
The hypothalamic-pituitary-ovarian (HPO) axis involves each of these organs, which act together to produce reproductive hormones, initiate puberty and regulate the menstrual cycle.
The arcuate nucleus of the hypothalamus secretes pulsatile gonadotropin-releasing hormone (GnRH), which travels in the portal circulation to the anterior pituitary gland. Here, it stimulates the release of follicle-stimulating hormone (FSH) and luteinising hormone (LH). These then act directly on the ovary to produce oestrogen and progesterone (Figure 1.14).
Throughout most of the menstrual cycle, oestrogen and progesterone provide negative feedback on the pituitary gland and hypothalamus, reducing GnRH secretion and reducing the release of FSH and LH. At low levels of oestrogen, LH secretion from the pituitary gland is suppressed but at higher concentrations of oestrogen, LH secretion is stimulated.20
Therefore, once a dominant follicle develops and starts to produce more oestrogen (see ovulatory cycle below), an LH ‘surge’ is triggered, which ultimately results in ovulation.
Ovarian cycle
The number of oocytes that a female possesses rapidly declines throughout development. At 20 weeks of gestation, a female human fetus contains 6–7 million oocytes. At birth, she has 1–2 million oocytes and by puberty, 300,000 oocytes. Of these, only 400–500 oocytes will actually reach the stage of ovulation. Menopause occurs when the effective ovarian oocyte supply is depleted, with menopausal ovaries containing mostly dense stroma with interspersed rare oocytes. Unlike sperm, new oocytes cannot be created.21
The ovarian cycle is divided into two phases (Figure 1.15). The follicular phase (from menstruation to ovulation), which lasts for 10–14 days and the luteal phase (from ovulation to menses), which lasts for 14 days.
Follicular phase
At the beginning of the menstrual cycle, ovarian hormones (oestrogen and progesterone) are low. Following the demise of the corpus luteum, the withdrawal of progesterone negative feedback results in an increase in FSH. This results in the recruitment of ovarian follicles, with each follicle 22producing some oestrogen, which in turn causes endometrial proliferation.
As the growing follicles produce inhibin B and more oestrogen negative feedback is exerted on the pituitary gland. This causes a reduction in FSH by the mid-point of the follicular phase. One follicle becomes dominant and grows further, smaller follicles are not able to survive low FSH and stop growing. As the oestrogen concentration progressively increases towards the end of the follicular phase, positive feedback starts and an LH ‘surge’ occurs, triggering ovulation 24–36 hours later.
Luteal phase
At ovulation, oestrogen decreases. The remaining cells from the ruptured follicle form the corpus luteum, which secretes oestrogen, inhibin A and mostly progesterone. Progesterone increases significantly after ovulation. The increase in progesterone, oestrogen and inhibin A result in a decline in FSH and LH (negative feedback). In the absence of fertilisation, the corpus luteum demises after 14 days, causing a decline in progesterone and oestrogen. This removes the negative feedback on the hypothalamus and pituitary gland, resulting in an increase in FSH and re-commencement of the cycle.
Endometrial cycle
Occurring concurrently with the ovarian cycle, the endometrium is prepared to receive a fertilised oocyte (zygote). In the absence of implantation, menstruation occurs. This is the endometrial cycle.
The endometrium is composed of layers (Figure 1.16). The most superficial layer is the decidua functionalis, which comprises the most superficial two-thirds of the endometrium. This layer is very responsive to reproductive hormones and is the part of the endometrium that sheds with menses. It is comprised of the stratum spongiosum (deep layer) and the stratum compactum (superficial layer).
Deep to the decidua functionalis is the decidua basalis. This is the source of endometrial regeneration after menses. This layer has no significant monthly proliferation and is relatively unresponsive to hormones.
The endometrial cycle is divided into two phases that coincide with the phases of the ovarian cycle (Figure 1.15).
The proliferative phase begins after the onset of menses on day 1 of the cycle. Following this, the endometrium is approximately 1–2 mm thick. The increasing oestrogen released by the developing follicles results in mitotic proliferation of the decidua functionalis. The endometrial glands change from being straight, narrow and short into being long and tortuous glands preparing it for implantation. The cell type changes from low columnar into pseudo-stratified, with dense stroma containing minimal vascular structures. It is now ready for the secretory phase.
The secretory phase begins 48–72 hours following ovulation. The increased progesterone from the corpus luteum results in the secretion of protein-rich eosinophilic products from endometrial glands. There is a progressive decline in endometrial oestrogen receptor number, resulting in less proliferation. There is a shift to secretion from glycogen-containing glands on days 19–20 with maximal secretion occurring 6–7 days after ovulation. The endometrium is now ready for the blastocyst.24
In the absence of implantation, the endometrial stroma remains unchanged until post-ovulatory day 7. At this point, endometrial spiral arteries lengthen and coil. At day 24, eosinophilia is visible in the peri-vascular stroma. Two days before menses, there is infiltration with polymorphs, which signals the onset of menses.
The demise of the corpus luteum results in a fall in oestrogen and progesterone as mentioned earlier. This causes spiral artery spasm and ischaemia of the endometrium. There is secretion of proteolytic enzymes, which further destroys the decidua functionalis. Secretion of prostaglandin F2α (PGF2α) results in vasoconstriction with further artery spasm and ischaemia as well as uterine contractions (cramps). Menstruation ensues.