Vagina, Vulva, and Pelvic Floor

The vulva is the external genitalia of the female and includes the mons pubis, labia majora, labia minora, clitoris, vestibule, vestibular bulb, and greater vestibular glands. The vagina is the female genital canal, extending from the vulva externally to the cervix uteri internally. The structures have sexual, reproductive, and urinary functions and a rich blood supply, mainly arising from the internal iliac artery.

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Development

Embryology timeline

  • Weeks 1–6:
    • Embryo is sexually indifferent/nonbinary.
    • Embryo possesses the following primordial sexual organs:
      • Mesonephric ducts (wolffian ducts)
      • Paramesonephric ducts (müllerian ducts)
      • Urogenital sinus
      • Genital tubercle
      • Urogenital fold
      • Labioscrotal swelling
  • Week 7:
    • Sexual differentiation begins with the development of the gonads.
    • The genes present at fertilization determine if the developing, bipotent gonads differentiate into testes or ovaries. 
    • The presence and/or absence of specific hormones secreted by the gonads determine how the remaining structures differentiate. 
  • Week 20: Phenotypic differentiation is complete.

Female differentiation of the embryo

Male development is triggered by the presence of certain hormones; female development is triggered primarily by the absence of hormones.

  • Male development is driven by 2 primary hormones produced in the testes:
    • Testosterone: stimulates differentiation of the wolffian ducts into the male structures (e.g., epididymis, vas deferens, seminal vesicles, and ejaculatory ducts)
    • Anti-Müllerian hormone (AMH): causes degeneration of the müllerian ducts
  • Developing ovaries do not secrete testosterone or AMH.
  • In the absence of AMH, the paramesonephric/müllerian ducts persist to form the internal female sexual organs: 
    • Fallopian tube
    • Uterus
    • Upper ⅓ of the vagina 
  • In the absence of testosterone:
    • Mesonephric/wolffian ducts degenerate.
    • The urogenital sinus, genital tubercle, folds, and swelling differentiate into female external genitalia.
    • Note: All infants (male and female) are exposed to the mother’s high estrogen levels in utero; therefore, the role estrogen plays in female sexual development is unclear.
  • Urogenital sinus forms:
    • Lower ⅔ of the vagina 
    • Greater vestibular glands (also known as Bartholin glands)
    • Urethral and paraurethral glands (also known as Skene glands)
  • Genital tubercle forms:
    • Glans clitoris
    • Vestibular bulbs
  • Urogenital fold forms: labia minora
  • Labioscrotal swelling: labia majora

Gross Anatomy

Gross anatomy of the vagina

The vagina is the external entrance to the female reproductive tract.

  • A fibromuscular tube connecting the vaginal vestibule (between the labia minora) distally and the uterine cervix proximally
  • Length: approximately 8–10 cm 
  • Functions:
    • Discharge of menstrual fluid
    • Receipt of the penis and semen
    • Delivery of an infant at the end of pregnancy
  • Fornix: 
    • The uppermost portion of the vagina (the annular recess between the cervix and the vagina)
    • Can be divided into anterior, posterior, and lateral portions
  • Hymen: 
    • Folds of vaginal mucosa stretching across the distal portion of the vaginal orifice
    • Embryologically, the hymen separates the urogenital sinus and the vaginal lumen (usually ruptures before birth).
  • Vaginal borders:
    •  Anterior: the urinary bladder and urethra:
      • The urethra is embedded in the anterior wall.
      • The urethra drains the urinary bladder (coursing parallel to the vagina prior to termination at the urethral orifice in the vaginal vestibule).
    • Posterior: the anal canal and rectum
    • Lateral: the muscles of the pelvic floor and ischial bones
    • Superior: the uterus
    • Inferior: the vestibule

Gross anatomy of the vulva

The vulva refers to the external female genitalia and occupies most of the perineum.

  • Borders of the vulva: 
    • Anterior: pubic symphysis
    • Posterior: perineum and anal orifice
    • Lateral: upper medial thighs
    • Superior: muscles of the pelvic diaphragm
  • Mons pubis: hair-bearing region of skin and adipose tissue over the pubic symphysis
  • Labia majora: 
    • Paired prominent longitudinal cutaneous folds containing adipose tissue, which extend from the mon pubis to the perineum
    • Homologous to the skin of the male scrotum
  • Labia minora: 
    • Paired folds of hairless skin between the labia majora 
    • Homologous to the skin of the male penis
  • Clitoris: 
    • The erectile structure partially enclosed by the labia minora (also known as the clitoral hood or prepuce), which is similar to the corpora cavernosa
    • The majority of the structure is internal.
    • Structure:
      • Glans: external portion protruding slightly from the clitoral hood 
      • Body: passes under the pubic symphysis
      • Crura: the body splits like a “Y” and form paired crura, which run along the inferior edges of the pubic bones
    • Homologous to the male glans penis
  • Vestibular bulbs:
    • Elongated masses of corpus cavernosum (erectile tissue) flanking the vaginal orifice 
    • Located bilaterally just deep to the labia majora
    • Become engorged with blood during sexual arousal and contribute to sexual stimulation 
  • Vestibule: 
    • Cavity between the labia minora 
    • Contains:
      • Vaginal and urethral orifices 
      • Greater and lesser vestibular glands
  • Greater vestibular glands (also known as Bartholin glands): 
    • Paired glands located in the vestibule just distal to the vaginal orifice (at approximately 5 o’clock and 7 o’clock if the vaginal orifice was a clock face)
    • Secrete mucus for lubrication during sexual arousal
    • Homologous to bulbourethral glands in men
  • Lesser vestibular glands (also known as Skene glands): 
    • Paired glands located on the distal anterior wall of the vagina adjacent to the external urethral orifice 
    • Homologous to the prostate gland in men
    • Exact function is unknown (believed to secrete lubrication at the urethral opening, which may act as an antimicrobial).
External female genitalia

External female genitalia

Image: “External female genitalia” by Phil Schatz. License: CC BY 4.0

Gross anatomy of the perineum and superficial perineal space

The perineum refers to the external surface, including the vulva, and the shallow “space” beneath.

  • Perineum boundaries:
    • Anterior: pubic symphysis
    • Anterolateral: inferior pubic rami
    • Lateral: ischial tuberosities
    • Posterolateral: sacrotuberous ligaments
    • Posterior: coccyx
  • Triangles: an imaginary transverse line between the ischial tuberosities divides the perineum into 2 “triangles”:
    • Anterior: urogenital triangle (includes the vulva)
    • Posterior: anal triangle (includes the anus)
  • The superficial perineal space (collectively known as the urogenital diaphragm) contains 3 muscles:
    • Ischiocavernosus: 
      • Extends anteriorly from the ischial tuberosities toward the clitoris 
      • Assists in clitoral erection
    • Bulbospongiosus: 
      • Wraps around the lateral vagina like a pair of parentheses
      • Provides lateral structure and tightens on the penis during intercourse
    • Superficial transverse perineal muscle:
      • Extends medially from the ischial tuberosities toward the perineal body, which is a strong central tendon of the perineum
      • Provides structure and support between the vagina and rectum
  • Clinical relevance: The bulbospongiosus and superficial transverse perineal muscles often tear during labor and delivery and should be intentionally repaired.

Supporting Structures

The pelvic diaphragm provides structure to the pelvic cavity and surrounding structures. In addition, 3 primary levels of vaginal support are all connected through endopelvic fascia.

Pelvic diaphragm

The pelvic diaphragm is the deepest layer of the pelvic floor.

  • Functions:
    • Provide structure to the pelvic floor separating the internal pelvic cavity from the external perineum
    • Resist intraabdominal pressure
  • Structure:
    • Starts at the pubic bones, extends posteriorly towards the coccyx
    • “Wraps around” the urethra, vagina, and rectum to provide lateral support
    • Perineal body: the central tendon and site of attachment for the pelvic floor muscles
  • Muscles of the pelvic diaphragm:
    • A group of muscles collectively known as the levator ani muscles:
      • Puborectalis: the innermost muscle, which originates from the pubic bone and forms a sling around the rectum; in direct contact with the lateral vagina and the lateral and posterior rectum
      • Pubococcygeus: the middle muscle, which connects the pubic bones to the coccyx
      • Iliococcygeus: the most lateral layer, which connects the ischial spine to the coccyx
    • Coccygeus
  • Other muscles of the pelvic floor:
    • Piriformis
    • Obturator internus
Muscles of the pelvic floor

Muscles of the pelvic floor

Image by BioDigital, edited by Lecturio

Primary levels of vaginal support

  • Level 1: 
    • The most proximal level of support
    • Consists of the uterosacral ligaments, which suspend the uterus and upper vagina to the sacrum and lateral pelvic wall
    • Clinical relevance: Loss of level 1 support leads to uterine prolapse into the vagina.
  • Level 2: 
    • Lateral attachments along the length of the vagina
    • Paravaginal attachments to the endopelvic fascia of the levator ani muscles surrounding the vagina
    • Clinical relevance: Loss of level 2 support leads to anterior vaginal wall and bladder prolapse (cystocele).
  • Level 3:
    • The most distal level of support
    • Consists of the perineum and perineal muscles
    • Supports the distal ⅓ of the vagina
    • Clinical relevance: Loss of level 3 support leads to urethral hypermobility and incontinence anteriorly or posterior vaginal wall prolapse (also known as a rectocele) posteriorly.

Neurovasculature

Circulation

  • Arterial supply: 
    • Superior vagina: 
      • Vaginal artery (a branch directly off of the internal iliac artery)
      • Vaginal branch of the uterine artery (a different branch of the internal iliac artery)
      • Vessels run primarily along the lateral sides of the vagina.
    • Inferior vagina: internal pudendal arteries
    • Vulva: 
      • Majority of the structures are supplied by the internal pudendal artery (a branch of the internal iliac artery).
      • Lateral structures are supplied by the external pudendal artery (a branch of the femoral artery).
  • Venous supply
    • Vagina: 
      • Veins form the vaginal venous plexuses around the vagina.
      • The veins drain into the uterine vein → internal iliac vein
    • The vulval drains via external pudendal veins → great saphenous vein
    • The clitoris drains via the dorsal veins of the clitoris.
Neurovasculature of the perineum

Neurovasculature of the perineum

Image by Lecturio.

Lymphatic drainage

  • Upper and middle vagina: external and internal iliac nodes
  • Lower vagina and vulva: superficial and deep inguinal nodes
Lymphatic drainage of the vulva

Lymphatic drainage of the vulva

Image by Lecturio.

Innervation

  • Sensory innervation:
    • Vulva:
      • Pudendal nerve branches
      • Ilioinguinal nerve branches
      • Genitofemoral nerve branches
    • Vagina: 
      • Pudendal nerve
      • Pelvic splanchnic nerves (S2–S4): afferent fibers of the upper vagina 
  • Motor innervation: vulva and vagina: pudendal nerves (S2–S4)
  • Autonomic innervation:
    • Sympathetic: uterovaginal plexus (off the inferior hypogastric plexus)
    • Parasympathetic: pelvic splanchnic nerves (S2–S4)
  • Clinical relevance: A pudendal nerve block can be used in childbirth to numb the perineum and lower portion of the vagina.

Microstructure

Vaginal microstructure

The vagina is a fibromuscular tube lined by mucosa made up of 3 layers:

1. Outer adventitia:

  • Inner layer of dense connective tissue
  • Outer layer of loose connective tissue

2. Middle muscularis layer:

  • 2 indistinct layers of smooth muscle:
    • Outer longitudinal layer
    • Inner circular layer connected by oblique interlacing fibers
  • Longitudinal fibers are continuous with superficial muscle fibers of the uterus.
  • Extensive vascular plexuses surround the muscular layer.

3. Inner mucosal layer:

  • Nonkeratinized stratified squamous epithelium: 
    • Allows for the normal “wear and tear” experienced by the vagina
    • Cells are ultimately shed from the surface of the mucosa into the vaginal lumen.
    • The basal layer is constantly dividing and replacing the epithelial cells above.
  • Epithelial cells contain glycogen:
    • Superficial cells accumulate glycogen, which is secreted into the vaginal lumen when the cells are exfoliated into the vagina.
    • Natural vaginal bacteria (particularly Lactobacillus spp.) break down glycogen in desquamated epithelium to produce lactic acid.
    • The acidic environment creates a barrier to infection.
  • Estrogen promotes the storage of glycogen:
    • Glycogen content significantly ↑ at puberty
    • Prepubescent and menopausal women have less glycogen → less acidic environment 
  • No glands are present in the vaginal mucosa; lubrication is achieved by:
    • Cervical glands via mucus secretion
    • Transudation (i.e., “vaginal sweating”) of serous fluid
    • Secretions from Bartholin glands
  • Contains vaginal rugae (numerous transverse folds functioning as friction ridges to stimulate the penis)

Vulvar microstructure

  • Labia majora:
    • Longitudinal skin folds
    • Contain smooth muscle and adipose tissue
    • Pubic hair on the lateral surface, no hair on the medial surface
    • Sweat and sebaceous glands on both surfaces
  • Labia minora:
    • Contain sweat and sebaceous glands
    • No hair on either surface
  • Clitoris:
    • Contains 2 cylinders of erectile tissue (corpora cavernosa)
    • The glans also contains erectile tissue.
    • Very large number of sensory nerve fibers

Clinical Relevance

Vulvar and vaginal disorders

  • Pelvic Organ Prolapse (POP): prolapse of the vaginal wall and the pelvic organs behind the wall (e.g., bladder or rectum) through the vaginal orifice. Pelvic Organ Prolapse occurs when the vaginal support structures are weakened and unable to support the internal organs against intraabdominal pressure. Major risk factors include age, parity, and obesity.
  • Female sexual dysfunction: disorders in any part of the sexual response cycle, including desire, arousal, orgasmic, and pain disorders. One particularly notable pain disorder is vulvodynia, a chronic pain syndrome of the vulva without an identifiable cause often severely limiting the ability to engage in sexual activity.
  • Vulvovaginitis: an acute inflammation of the vulva and the vagina due to an infectious etiology. The 2 most common forms are candidiasis (caused by Candida yeast species) and bacterial vaginosis (a polymicrobial bacterial infection).
  • Bartholin gland abscess: Bartholin glands can become occluded and develop cysts, which can then become infected. The resulting abscess typically presents as a painful mass in the vestibule at the location of the Bartholin gland (5 o’clock or 7 o’clock when viewing the vaginal orifice as a clock face). The abscess is treated with incision and drainage. A small catheter is often placed to allow for continued drainage and to decrease the risk of recurrence. Occasionally, surgical management is required.
  • Vulvar and vaginal cancer: relatively uncommon types of cancer in the female reproductive tract. Risk factors for vaginal cancer include HPV infection and exposure to diethylstilbestrol (DES) in utero (a medication commonly prescribed for pregnancy complications until the early 1970s). Lichen sclerosus is a significant risk factor for vulvar cancer.

Obstetric clinical relevance

  • Obstetric lacerations: spontaneous tears to the perineum, vagina, or cervix occurring as a result of trauma due to the infant passing through the vaginal canal during delivery. Frequently, the bulbospongiosus and superficial transverse perineal muscles are lacerated and should be intentionally repaired to restore anatomic structure and function.
  • Pudendal nerve block: opioid injection into the area surrounding the pudendal nerve providing excellent pain relief to the perineum and lower vagina. The pudendal nerve block can provide anesthesia during perineal laceration repairs without an epidural. The pudendal nerve block does not cover the uterus and provides no relief against contraction pain.

Related anatomical structures

  • Pelvis: consists of the pelvic girdle, pelvic cavity, pelvic floor, and all the viscera, vessels, and muscles contained in the pelvis. The pelvic cavity houses various gastrointestinal and urogenital structures. 
  • Uterus: a pear-shaped, hollow organ composed of smooth muscle functioning to nourish the developing fetus until the end of the pregnancy. The uterus is also responsible for expulsion of the infant.

References

  1. Ulfelder H., Robboy SJ. The embryologic development of the human vagina. Am J Obstet Gynecol. 1976 Dec 1;126(7):769–76.
  2. Drake, R.L., Vogl, A.W., & Mitchell, A.W.M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA:  Churchill Livingstone.
  3. Bui, T., and Bordoni, B. (2021). Anatomy, Abdomen and Pelvis, Inguinal Lymph Node. In StatPearls. Retrieved July 28, 2021 from https://www.ncbi.nlm.nih.gov/books/NBK557639/ 
  4. Miranda, A.M. (2018). Vaginal anatomy. In Medscape. Retrieved July 28, 2021 from https://emedicine.medscape.com/article/1949237-overview 
  5. OpenStax College, Anatomy and Physiology. Chapter 11.4: Axial muscles of the abdominal wall and thorax. OpenStax CNX. Retrieved July 28, 2021 from https://philschatz.com/anatomy-book/contents/m46485.html 
  6. Rogers, R.G., and Rashokun, T.B. (2021). Pelvic organ prolapse in women: epidemiology, risk factors, clinical manifestations, and management. In Eckler, K. (Ed.) UpToDate. Retrieved July 28, 2021 from https://www.uptodate.com/contents/pelvic-organ-prolapse-in-women-epidemiology-risk-factors-clinical-manifestations-and-management

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