Immature squamous metaplasia

Topic Completed: 1 April 2017

Minor changes: 23 May 2020

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PubMed Search: Immature squamous metaplasia

Related topics: Metaplasia

Jaya Ruth Asirvatham, M.B.B.S.
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Cite this page: Immature squamous metaplasia. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/cervixmetaplasiaimmaturesquamous.html. Accessed July 13th, 2020.
Definition / general
  • Squamous metaplasia refers to the physiologic replacement of endocervical surface and glandular epithelium by a newly formed squamous epithelium from the subcolumnar reserve cells
  • This process starts with the appearance and proliferation of reserve cells, first forming a thin layer of immature squamous cells without stratification or glycogen (immature squamous metaplasia), then differentiating into mature stratified squamous epithelium, which contains glycogen from the intermediate cell layer onwards
  • Most immature squamous metaplasia has no cytologic atypia (typical IM), while a small percentage can show mild atypia (so-called atypical immature squamous metaplasia or AIM)
  • The term AIM was introduced in 1983 to describe cervical lesions with uniform full thickness basal proliferation with high nuclear density but without sufficient criteria for CIN III (now HSIL)
  • Currently, it is debatable whether AIM terminology should be used in diagnosis
  • The diagnosis of AIM has poor intra- and inter-observer reproducibility
Microscopic (histologic) description
  • Immature squamous metaplasia is composed of a monotonous population of cells with a high N:C ratio that lack cytoplasmic maturation in the superficial layers
  • Typical IM has minimal cell crowding, no cytologic atypia, uniformly distributed fine chromatin, and smooth nuclear contours
  • Mitoses are uncommon; if present, are typical and confined to basal layers
  • No papillarity, koilocytosis or p16 immunoreactivity
  • AIM represents a heterogenous group of lesions, and is characterized by absent or minimal maturation of metaplastic squamous epithelium, absent or minimal koilocytosis, and mild nuclear pleomorphism
  • Mitoses are rare, and confined to the lower third of the epithelium
  • Ancillary studies (HPV, Ki-67, p16, CK17) may be helpful to further classify AIM as either benign or dysplastic
Microscopic (histologic) images

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IM: CK17 staining

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Atypical immature metaplasia

Cytology description
Positive stains
  • CK17 stains the cytoplasm and p63 stains the nuclei of subcolumnar reserve cells, proliferating basal and suprabasal cells in immature squamous metaplasia
  • Expression of CK17 decreases and disappears as the metaplastic epithelium matures
  • Studies of CK17 utility in cervical dysplastic lesions have generated conflicting results (Table 1 - Histopathology 2007;50:629, Mod Pathol 2014;27:621)
Negative stains
  • Typical IM and mature squamous metaplasia is consistently negative for p16, in contrast to CIN
Differential diagnosis
  • Important to exclude HSIL/CINIII with a metaplastic growth pattern, especially when nuclear enlargement or hyperchromasia is present in the parabasal cells or surface atypia and nuclear crowding are observed
  • Reactive and inflamed cells can be distinguished from CIN when the background cell population shows uniform nucleoli, nucleoli and lack of crowding
Additional references
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