Metaplasia (Greek: "change in form") is the reversible replacement of one differentiated cell type with another mature differentiated cell type. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly In Developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized Cell type. A cell type is a distinct morphological or functional form of cell. The change from one type of cell to another is generally caused by some sort of abnormal stimulus. In simplistic terms, it is as if the original cells are not robust enough to withstand the new environment, and so they change into another type more suited to the new environment. If the stimulus that caused metaplasia is removed or ceases, tissues return to their normal pattern of differentiation. Metaplasia is not synonymous with dysplasia and is not considered carcinogenesis. Dysplasia (from Greek roughly "bad formation" is a term used in Pathology to refer to an abnormality in maturation of cells within a tissue The term carcinogen refers to any substance Radionuclide or radiation that is an agent directly involved in the promotion of Cancer or in the fatation of its propagation It is also contrasted with heteroplasia, which is the abnormal growth of cytologic and histologic elements without a stimulus.
Heteroplasia and dysplasia are synonyms.
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Causes
When cells are faced with physiological or pathological stresses, they respond by adapting in several ways; one of these cellular adaptations is metaplasia. It is a benign (i. e. non-cancerous) change that occurs as a response to chronic physical or chemical irritation, such as cigarette smoke that causes the mucus-secreting ciliated simple columnar respiratory epithelial cells that line the airways to be replaced by simple squamous epithelium, or a stone in the bile duct that causes the replacement of the secretory columnar epithelium with simple squamous epithelium (Squamous metaplasia). Squamous metaplasia refers to Benign ( non-cancerous) changes in the epithelial linings of certain organs within the body Although metaplasia is an adaptation that replaces delicate cells with hardier ones that are more likely to be able to withstand the stresses that the epithelium is faced with, it is also accompanied by a loss of epithelial function, and is considered undesirable; this undesirability is underscored by the propensity for metaplastic regions to eventually turn cancerous if the irritant is not eliminated. Specialised epithelial cells are already differentiated, and cannot simply transform their morphologies to change from one cell type to another. Metaplasia, then, does not occur as a result of any change in the pre-existing epithelial cells but rather as a result of reprogrammed stem cells present in the organ's connective tissue that are nudged along a different pathway of differentiation by cytokines, growth factors and other substances in the cell's environment. Cytokines are a category of signalling Proteins and Glycoproteins that like Hormones and Neurotransmitters, are used extensively in cellular The term growth factor refers to a naturally occurring Protein capable of stimulating cellular growth proliferation and Cellular differentiation. In a nutshell, metaplasia occurs by stem cells that reprogram differentiation of cells rather than by transdifferentiation.
Therapy
The medical significance of metaplasia is that in some sites cells may progress from metaplasia, to develop dysplasia, and then malignant neoplasia (cancer). Thus, at sites where metaplasia is detected, efforts are made to remove the causative irritant, thereby decreasing the risk of progression to malignancy. Malignant (from the Latin roots mal- = "bad" and -genus = "born" is a medical term used to describe a severe and progressively worsening disease The metaplastic area must be carefully monitored to ensure that dysplastic change does not begin to occur. A progression to significant dysplasia indicates that the area could need removal to prevent the development of cancer.
Examples
Barrett's esophagus is an abnormal change in the cells of the lower esophagus, thought to be caused by damage from chronic stomach acid exposure. Barrett's esophagus (UK Oesophagus (sometimes called Barrett's syndrome, CELLO, columnar epithelium lined lower oesophagus or colloquially as Barrett's
Metaplasia of the cervix, which occurs in cervical erosion, can be detected by a cervical smear test. The cervix (from Latin "neck" is the lower narrow portion of the Uterus where it joins with the top end of the Vagina. Cervical erosion (also known as a cervical ectropion) is a partial or complete absence of the squamous epithelium of the cervix The Papanicolaou test (also called Pap smear, Pap test, cervical smear, or smear test) is a screening test used in Gynecology The normal endocervical columnar epithelium is replaced by a squamous epithelium in an area termed the transformation zone. This is a normal physiological event that takes place around puberty. The stimulus is believed to be a change in the vaginal environment, which becomes acidic.
The following table lists some common tissues susceptible to metaplasia, and the stimuli that can cause the change:
| Tissue | Normal | Metaplasia | Stimulus |
|---|---|---|---|
| Airways | Columnar epithelium | Squamous epithelium | Cigarette smoke |
| Urinary bladder | Transitional epithelium | Squamous epithelium | Bladder stone |
| Oesophagus | Squamous epithelium | Columnar epithelium | Gastro-esophageal reflux |
Notes
- The AMA Home Medical Encyclopedia, Random House, p. The airways are those parts of the Respiratory system through which air flows to get from the external environment to the Alveoli. In Anatomy, the urinary bladder is a hollow muscular, and distensible (or elastic organ that sits on the Pelvic floor in Mammals It is the Transitional epithelium (also known as urothelium is a type of tissue consisting of multiple layers of Epithelial cells which can contract and expand The esophagus or oesophagus (see American and British English spelling differences) sometimes known as the gullet, is an organ in 683
- Robbins and Cotran - Pathologic Basis of Disease, 7th Edition, Saunders, p. 10
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