The cervical mucosa measures about 2 to 3 mm in thickness and differs dramatically from the rest of the uterine endometrium in that it contains large, branched glands (Fig. 23.19 and Plate 98, page 884). It also lacks spiral arteries. The cervical mucosa undergoes little change in thickness during the menstrual cycle and is not sloughed during the period of menstruation. During each menstrual cycle, however, the cervical glands undergo important functional changes that are related to the transport of spermatozoa within the cervical canal. The amount and properties of the mucus secreted by the gland cells vary during the menstrual cycle under the influence of the ovarian hormones. At midcycle, the amount of mucus produced increases 10-fold. This mucus is less viscous and appears to provide a more favorable environment for sperm migration. The cervical mucus at other times in the cycle restricts the passage of sperm into the uterus. Thus, hormonal mechanisms ensure that ovulation and changes in the cervical mucus are coordinated, thereby increasing the possibility that fertilization will occur if freshly ejaculated spermatozoa and the ovum arrive simultaneously at the site of fertilization in the uterine tube.
Blockage of the openings of the mucosal glands results in the retention of their secretions, leading to formation of dilated cysts within the cervix called Nabothian cysts. Nabothian
FIGURE 23.18 • Development of the placenta. This schematic drawing shows growth of the uterus during human pregnancy and development of the placenta and its membranes. Note that there is a gradual obliteration of the uterine lumen and disappearance of the decidua capsularis as the definitive placenta is established. (Modified from Williams J. Am J Obstet Gynecol 1927;13:1.) cysts develop frequently but are clinically important only if numerous cysts produce marked enlargement of the cervix.
The transformation zone is the site of transition between vaginal stratified squamous epithelium and cervical simple columnar epithelium.
The portion of the cervix that projects into the vagina, the vaginal part or ectocervix, is covered with a stratified squamous epithelium (Fig. 23.20). An abrupt transition between this squamous epithelium and the mucus-secreting columnar epithelium of the cervical canal, the endocervix, occurs in the transformation zone that during the reproductive age of the woman is located just outside the external os (Plate 98, page 884). Before puberty and after menopause, the transformation zone resides in the cervical canal (Fig. 23.21).
Metaplastic changes in this transformation zone constitute precancerous lesions of the cervix. Metaplasia (Gr.
í, change in form) represents an adoptive and reversible response to persistent injury of the epithelium caused by chronic infections. It results from a reprogramming of epithelial stem cells that begin to differentiate into new cell lineage. Within the cervical canal (endocervix) it is manifested as a replacement of the simple columnar epithelium with fully mature stratified squamous epithelium (Fig. 23.22). The cervical epithelial cells are constantly exfoliated into the vagina. Stained preparations of the cervical cells (Papanicolaou [Pap] smears) are used routinely for screening and diagnosis of precancerous and cancerous lesions of the cervix.
The developing fetus is maintained by the placenta, which develops from fetal and maternal tissues.
The placenta consists of a fetal portion, formed by the chorion, and a maternal portion, formed by the decidua basalis. The two parts are involved in physiologic exchange of substances between the maternal and fetal circulations.