View source for Squamous metaplasia of the rectum/anus

'''Squamous metaplasia of the rectum/anus''' is a metaplastic change of the columnar epithelium of the rectum. 

It can histologically mimic [[anal intraepithelial neoplasia]] (AIN), especially. [[high-grade squamous intraepithelial lesion]] (HSIL).

==General==
*Present in the transformation zone between the columnar epithelium of the [[rectum]] and squamous epithelium of the [[anus]] - similar to the transformation zone of the [[uterine cervix]].<ref name=pmid20436833>{{Cite journal  | last1 = Pineda | first1 = CE. | last2 = Welton | first2 = ML. | title = Management of anal squamous intraepithelial lesions. | journal = Clin Colon Rectal Surg | volume = 22 | issue = 2 | pages = 94-101 | month = May | year = 2009 | doi = 10.1055/s-0029-1223840 | PMID = 20436833 }}</ref>

Note:
*The anatomy & histology of the anus covered in the ''[[anus]]'' article.

==Microscopic==
Features:
* Uniform cell spacing - no crowding - '''key feature'''.
* Nuclei are uniform size and round.
** +/-[[Nucleoli]] present.
*Distinct cell borders
* +/-Intercellular bridges (due to edema) - common.

Negatives:
* No mitoses/rare basal mitoses - think cancer/AIN if you see 'em.
* Usually no hyperchromatism (think cancer/CIN if you see it).

DDx:
*[[Anal intraepithelial lesion]].

===Images===
<gallery>
Image: Immature squamous metaplasia - anus -- very low mag.jpg | ISM - very low mag.
Image: Immature squamous metaplasia - anus -- low mag.jpg | ISM - low mag.
Image: Immature squamous metaplasia - anus -- intermed mag.jpg | ISM - intermed. mag.
Image: Immature squamous metaplasia - anus -- high mag.jpg | ISM - high mag.
</gallery>

==IHC==
*p16 -ve 
**Usually weak-to-moderate patchy, not full thickness.
*Ki-67 - scattered cells positive.

==See also==
*[[Squamous metaplasia]].

==References==
{{Reflist|1}}

[[Category:Gastrointestinal pathology]]
[[Category:Dermatopathology]]