Squamous metaplasia is a term used to describe cellular non-cancerous changes in the epithelial linings of certain organs such as the bladder, cervix and lungs. Metaplasia occurs when constant stress or irritation causes a reversible maturation process which converts one differentiated epithelial cell type into another. In the case of squamous metaplasia, varying types of epithelial cells are converted or replaced with squamous epithelium as an adaption mechanism.
The change of cell type may result in reduced epithelial function. Once the abnormal stimuli are removed, the metaplastic cells revert to their original form and function. Physiological stressors that are not removed in metaplastic regions may lead to dysplasia or pre-cancerous cell changes. Dysplastic cells can develop into cancerous neoplastic cells if the stressor or irritant is not removed in time.
Epithelium is composed of layers of epithelial cells such as columnar, cuboidal and squamous. Columnar epithelial cells are an elongated column-based shape that makes up the lining for the endocervix, intestines and stomach. A columnar cell whose nuclei are of a different height is called a pseudostratified epithelial cell. Cuboidal epithelium is made of square-like cells commonly found in the exocrine gland and the kidney tubules. Squamous epithelial cells are thin and flat which in turn creates a smooth epithelium.
The cervix is a good example of the squamous metaplasia process and resulting neoplastic cell change. Columnar epithelial cells are originally found in the endocervix, which is the cavity of the cervix. Estrogen and constant exposure to acidic vaginal pH levels are triggers of the squamous metaplasia process involving the endocervix’s epithelium. In response to the pH irritant the fragile columnar epithelial cells begin to be replaced with sturdy squamous cell type.
Squamous metaplasia of the endocervix also occurs as a result of merging with the neighboring ectocervix region. An ingrowth of squamous cells from the ectocervix begins to populate in the endocervix areas, replacing the original columnar epithelial cells. Cancerous neoplasia of the cervix results when carcinogens such as the human papillomavirus (HPV) act as an irritant or stressor to metaplastic epithelial cells.
A similar metaplastic process takes place when chronic cigarette smoke irritates the pseudostratified epithelial lining of the lungs. Cigarette smoke is the stressor which adapts the pseudostratified cells into the much stronger squamous cells. Even squamous cell types can go through a squamous metaplastic change often seen in the bladder. The bladder’s trigone, or inner triangular area, houses squamous epithelial cells that make up the region’s lining. When the bladder’s adult squamous cells are subjected to chronic inflammation, the cells undergo a metaplastic change.