Introduction and Gross Anatomy
The vagina is a functional organ of the female reproductive organ system. It extends from the vulva externally to the uterine cervix internally and is located within the pelvis, anterior to the rectum and posterior to the urinary bladder.
The vagina lies at a 90° angle in relation to the uterus and is held in place by endopelvic fascia and ligaments. It is a potential space that is easily distended.
The vaginal lining is called rugae, which are situated in folds throughout. These allow for distention, especially during childbearing and coitus. The structure of the vagina is a network of connective, membranous, and erectile tissues.
The sphincter urethrae and the transverse perineus muscles, perineal membrane, and pelvic diaphragm support the vagina. The term urogenital diaphragm is no longer used; formerly, it included the sphincter urethrae and the deep transverse perineus muscle, together with their inferior fascia, the perineal membrane. These muscles are innervated by perineal branches of the pudendal nerve. The pelvic diaphragm primarily refers to the levator ani and coccygeus muscles and is innervated by branches of S2-S4 from the sacral plexus.
Vasculature, nerve supply, and lymphatic drainage
The vasculature of the vagina is supplied primarily by the vaginal artery, a branch of the anterior division of the internal iliac artery. Several of these arteries may be found on either side of the pelvis to richly supply the vagina.
The nerve supply to the vagina is primarily from the autonomic nervous system. Sensory fibers arise from the pudendal nerve, and pain fibers are from sacral nerve roots.
Lymphatic drainage of the vagina is generally to the external iliac nodes (upper third of the vagina), the common and internal iliac nodes (middle third), and the superficial inguinal and perirectal nodes (lower third). [1, 2, 3]
Histologically, the vagina has 3 distinct layers.
The first layer is the mucosa. The epithelium of the mucosal layer is composed of stratified squamous cells, which contain a small amount of keratin. The lamina propria is composed of loose connective tissue that has a vast amount of elastic fibers, giving the vagina its capability to distend.
The second layer is muscular, mainly smooth muscle.
The final layer is the adventitia, which is also rich in elastic fibers. A large plexus of blood vessels is also present within the adventitia. 
Vaginal agenesis is complete absence of the vagina. This congenital defect is typically part of the Mayer-Rokitansky-Kuster-Hauser syndrome, characterized by an absence of the vagina and uterus (46,XX karyotype). This syndrome is thought to be a developmental accident rather than an inherited condition.
The vaginal lumen is formed when the paramesonephric (müllerian) ducts join the sinovaginal bulb at the paramesonephric (müllerian) tubercle. If these areas are not completely joined, a transverse vaginal septum forms. This septum may be partial or complete; different variations are possible; and the most common site of occurrence is at the junction of the upper third and lower two thirds of the vagina. This is sometimes referred to as a double vagina (see the image below), although the presence of 2 separate vaginas is also possible. If the septum is complete, the diagnosis is made when the female experiences primary amenorrhea with cyclic cramping and hematocolpos. Treatment of a vaginal septum is surgical, by resection of the septum.
Vaginal dysplasia, although rare, can result in cancer if left untreated. Diagnosis typically results from detection of atypical vaginal cells followed by colposcopy and vaginal biopsy.
Vaginal intraepithelial neoplasia (VAIN) can range from simple to complex dysplasia. Typically, if abnormal cells are ablated, the recurrence risk is low. The modalities used to ablate this tissue include laser and cautery. For removal of the affected tissue, or in the case of cancer, a partial, complete, or radical vaginectomy may be performed.
Vaginal candidiasis (also known as yeast infection) is a fungal infection and is caused by an overgrowth of the normal flora of the vaginal secretions, predominantly Candida albicans. This overgrowth can be caused by systemic antibiotic treatment for other infections or can be exacerbated by immune compromise (eg, HIV infection or AIDS) or diabetes.
Typical symptoms of vaginal candidiasis include internal vaginal itching and increased discharge (thick, white). External vulvar irritation may also be involved. Diagnosis is made by microscopic identification of pseudohyphae on potassium hydroxide slide preparation. Treatment is with either oral or vaginal antifungal medication, available both over the counter and by prescription.
Bacterial vaginosis is caused by an overgrowth of normal flora of the vaginal secretions, predominantly Gardnerella vaginalis. Bacterial vaginosis is characterized by increased vaginal discharge (thin, yellow/gray), strong malodor (fishy smell), and mild itchiness. Diagnosis is made by microscopic identification of “clue cells” on a saline slide preparation. Treatment is with oral or vaginal metronidazole, available by prescription only.
Trichomoniasis is a sexually transmitted vaginal infection caused by the parasitic organism Trichomonas vaginalis. Symptoms include increased vaginal discharge (thin, watery, yellow/green) and malodor. Diagnosis is made by microscopic identification of the motile organisms, trichomonads, on a saline slide preparation. Treatment is with oral metronidazole, available by prescription only.