Most things that can go wrong with the cervix involve the membranes that line its canal or cover its vaginal surface.
The canal that runs through the cervix opens into the cavity of the uterus at the internal os and into the vagina at the external os (os is Latin for mouth). It is lined by a single layer of columnar epithelial cells that dips down into its wall to form glands. These cells are influenced by oestrogen and progesterone to form varying amounts and types of the cervical mucus that fills the canal. Where the cervix projects into the vagina, its outer wall (called the ectocervix) is covered by a tough stratified squamous epithelium many cells thick, the same as the vaginal lining. The point where the columnar epithelium that lines the canal meets the squamous epithelium covering the ectocervix is called the squamo-columnar (S-C) junction and is very important in cervical health.
At puberty, ovarian hormones stimulate growth of the cervix to its adult size and mucus production by the lining cells and glands of the cervical canal. From puberty on, the cervix undergoes constant changes with the rise and fall of hormones during each menstrual cycle and with the altered hormonal conditions of pregnancy and lactation. Its wall becomes slightly enlarged and softer at the time of ovulation due to an increase in tissue fluid between its cells, and the external os becomes more lax and open.
Throughout the reproductive years there are changes in the position of the S-C junction. At birth and before puberty the junction may be inside the canal, at the external os or on the ectocervix. Its position is thought not to change much during the first 10 years of life. At puberty, pregnancy and other times during the reproductive years the junction may move further out onto the vaginal surface of the cervix. This is called cervical eversion, because the canal lining appears to be ‘turned out’ onto the ectocervix.
The delicate columnar epithelium that lines the canal cannot survive when it is exposed to the acidity of the vagina, so it becomes transformed into the stratified squamous epithelium that covers the ectocervix. The surface of the cervix over which everted canal epithelium has been or is being transformed is called the transformation zone (TZ). During the transformation process (which can happen more than once during our reproductive years) the cervix seems to be more susceptible to anything that might cause unusual or abnormal cell growth and division.
This outward movement of the canal lining seems to be influenced by hormones and possibly other factors not yet identified. It is considered to be a normal process, but we don’t know why it happens in some women and not others. When this is understood we may find the answers to many puzzling questions about abnormalities of the cervix.
The lining of the cervical canal is transparent. When it is everted onto the ectocervix the blood vessels in the tissue beneath show through, so that the eversion looks red. It also appears rather knobbly compared with the smooth surface of the membrane covering the ectocervix.
In the past doctors called this eversion an ‘erosion’ or ‘ulcer’, because that’s what it looked like with the naked eye. It was thought to be abnormal (we now know that it isn’t) and was treated by burning or freezing the area to transform it to ectocervical epithelium. Most of these treatments were unnecessary: given time almost all eversions will be replaced naturally by a transformation zone.
Occasionally an eversion that is slow to transform may lead to chronic inflammation of the cervix.
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