Postmenopause Vulva and Vagina Changes (part two)

A woman’s vagina consists of 3 layers of tissue: the epithelium (composed of squamous cells), the lamina propria, and the muscularis (inner circular and outer longitudinal smooth muscle). The epithelium undergoes mild changes during the menstrual cycle. The lamina propria is replete/supplied with tiny blood vessels that become engorged with blood during sexual arousal, leading to lubrication. The smooth muscle of the muscularis enables the vagina to dilate and lengthen during penile penetration. Relaxation of that muscle leads to arousal. These 3 layers of tissue may function in an interrelated way. It is hypothesized that the blood vessels in the lamina propria that allow for lubrication are dependent on growth factors, and that the growth factors are derived from the muscularis. Postmenopausal atrophy (weakness) of vaginal tissues may be due to decreased synthesis these growth factors resulting in diminished number of critical blood vessels in the lamina propria.

Controlled studies employing a rat model of vaginal atrophy demonstrated the effects of different dosages of estrogen, progestin, testosterone, and various combinations of these hormones on various physiologic and anatomic outcome parameters. These measures included organ wet weight, vaginal blood flow, epithelial height, muscularis thickness, and vaginal innervation. In each study, the effects on the vagina of different dosages of the hormone being tested were compared between rats that had undergone sham ovariectomies and rats that had actually had both ovaries removed. The hormones (or saline) were delivered through a pump inserted into the back of the neck of each animal. A Doppler probe inserted into the vagina was used to record blood flow after electrical stimulation of a nerve next to the vagina. All animals were then euthanized and vaginal tissue removed for biochemical or histologic studies. In those animal studies, removal of the ovaries reduced the wet weight of the uterus that rose with administration of estrogen because the uterus is a very estrogen-sensitive organ, much more so than the vagina.

Increased vaginal blood flow is an indicator of sexual arousal. Genital swelling and lubrication are responses to increased clitoral and vaginal perfusion; increased length and diameter of the vaginal canal and clitoral corpora cavernosa; engorgement of the vagina wall, clitoris, and labia major and minora; and transudation of lubricating fluid from the vaginal epithelium. In the animal studies just described, blood flow to the vagina was greatly reduced in the oophorectomized rats compared with the intact rats.

Contrary to what one might expect, subphysiologic (lower) doses of estradiol increased vaginal blood flow in oophorectomized (removal of ovaries) rats more than either physiologic or supraphysiologic (higher) doses. Ovariectomy deprived the rats of estradiol, causing the vaginal epithelium to thin down to a single layer. Subphysiologic doses of estradiol increased the thickness of the vaginal epithelium the most because the oophorectomized rats had more estrogen-alpha receptors in the epithelium than the intact animals. A small amount of estradiol delivered to tissue with many estrogen-alpha receptors produced a huge response. Thus, estradiol regulates estrogen receptors through a negative feedback system. The more estradiol that is available, the fewer estrogen receptors there are. The muscularis, the muscle that enables the vagina to lengthen and widen during sexual arousal, also atrophies without estrogen. In postmenopausal women who do not take hormone therapy, the vaginal epithelium, lamina propria blood vessels, and muscularis all decrease. Like the epithelium, the muscularis responds to estradiol by increasing in thickness.

Sourced From:
BIOLOGY OF FEMALE SEXUAL DYSFUNCTION
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