Читать книгу Ridley's The Vulva - Группа авторов - Страница 124

The menstrual cycle is under endocrine and paracrine influence. Pituitary gonadotrophins, hypothalamic releasing factors, and numerous growth factors regulate intraovarian events, which occur in a cyclical pattern unless interrupted by pregnancy.

The first phase is the follicular phase, where oestradiol increases to reach a peak before the surge in luteinising hormone and ovulation. The endometrial layer grows at this time.

At ovulation, a mature oocyte is released, and the follicular remnants form a corpus luteum which produces progesterone that prepares the endometrium for possible implantation. The second phase of the cycle is the luteal phase where progesterone predominates. There is a small second peak in oestrogen secretion about 5 days after ovulation, and if pregnancy occurs, the corpus luteum persists and maintains the hormone levels. If it does not, then it will regress, and then both oestrogen and progesterone levels fall rapidly, and as the endometrial layer is not maintained, it will be lost as the next menses.

The cervical mucus secretion varies daily but becomes transparent, watery, and thin around ovulation to allow entry of sperm into the higher reproductive tract. In the luteal phase, it thickens and has more of a barrier function due to its consistency but also from the immunoglobulins it contains. There are several alterations in the protein and enzyme content of the mucus, but mucin glycosylation at ovulation is the main change [27]. It is these changes in the cervical mucus that lead to a subjective feeling of wetness on the vulva around the time of ovulation. A profuse discharge can lead to an irritant dermatitis.

The vaginal epithelium is very responsive to the influence of ovarian hormones. Oestrogen causes a thickening of the epithelium and its differentiation into the well‐recognised basal, intermediate, and superficial layers characteristic of the reproductive years. The percentage of superficial cells present in a vaginal smear is an indicator of the amount of oestrogenic activity. Progesterone produces a relative decrease in the number of superficial cells while increasing the number of intermediate cells.

The normal vaginal flora is mixed, but lactobacilli predominate and use glycogen to produce lactic acid and therefore lower the vaginal pH. During menses, there is a reduction in lactobacilli and an increase in G. vaginalis [28]. The acidity of the vaginal environment may be reduced by the alkaline secretions of the cervical glands, particularly in the presence of a large cervical erosion, menstrual flow, and ejaculate.

The thickness of the vulval epithelium is stable throughout the menstrual cycle but there may be an increase in parakeratosis, indicating increased skin turnover, at mid‐cycle [29]. One study of the vestibular mucosa showed no difference in the number of cell layers in users of oral contraceptives or with the normal cycle. There was an increased distance between the dermal papillae and the surface, possibly due to larger keratinocytes in those on oral contraceptives. An increase in vasculature was also seen, which may explain why the mucosa can sometimes look more erythematous in those taking oral contraceptives [30].