Transformation zone
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"cervical transformation zone"
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cervical transformation zone WHO screening definition
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Cervical transformation zone (TZ) is the area of the cervix where the glandular, columnar epithelium of the endocervix is being replaced by squamous epithelium through squamous metaplasia.
- It lies between the original squamocolumnar junction (SCJ) and the new, active SCJ.
- Its location changes with age and hormonal state: it is usually more visible on the ectocervix during puberty and pregnancy, and tends to recede into the endocervical canal after menopause.
- It is the key area sampled in a Pap test and assessed at colposcopy because HPV-related cervical intraepithelial neoplasia (CIN) most often begins here.
Berek & Novak’s Gynecology, p. 835.
WHO similarly defines it as the cervical region in which columnar epithelium has been replaced, or is being replaced, by metaplastic squamous epithelium in its cervical screening guideline.
What are the differences between TZ types 1, 2, and 3?
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"cervical transformation zone" AND colposcopy
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WHO transformation zone type 1 type 2 type 3 cervix definitions
| TZ type | Location | Is the whole TZ and SCJ visible? | Practical implication |
|---|---|---|---|
| Type 1 | Entirely on the ectocervix | Yes | Easiest to assess by VIA/colposcopy and usually accessible for ablative treatment if other eligibility criteria are met. |
| Type 2 | Has an endocervical component, but remains visible | Yes | The examiner can still see the full TZ and SCJ, though it extends partly into the cervical canal. |
| Type 3 | Extends into the endocervical canal | No - the upper limit/SCJ is not fully seen | The lesion/TZ cannot be fully assessed visually. This often requires colposcopic evaluation and may affect suitability for ablation. |
Memory aid:
- TZ 1: outside and visible
- TZ 2: partly inside but visible
- TZ 3: inside and not fully visible
A type 3 TZ is more common after menopause, when the SCJ tends to retract into the endocervical canal. These definitions are consistent with the WHO cervical pre-cancer guideline.
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