Cervical canal. Coloured scanning electron micrograph (SEM) of the epithelium of the cervical canal through which the uterus is linked to the vagina. This epithelium is composed of columnar cells, which may be ciliated or, as shown here, secretory. Secretory cells are characterised by a great number of microvilli (tiny projections) on their surfaces. These cells secrete mucus in response to estrogen hormones and it is maximal at the time of ovulation. During the menstrual cycle the mucus is highly viscous but at mid-cycle it becomes more hydrated in order to ease sperm migration. Magnification: x1470 when printed 10 centimetres wide.