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METAPLASIA
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Metaplasia is the reversible change in which one adult cell type is replaced by another adult cell type of the same germinal layer. It is also defined as the transformation of one cell type to another cell type within the embryological limits. Metaplasia may involve epithelial or mesenchymal tissue. In metaplasia, one type of epithelium may be converted into another, usually less special type or one type of mesenchymal tissue into another type. While metaplasia is reversible, it is considered as a double edged sword, as it may lead to cancer.
Mechanism
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Metaplasia may arise from reprogramming of stem cells (Reserve cells in epithelium) or from undifferentiated mesenchymal cells present in the connective tissue. The stem cells may differentiate following changes in signals through cytokines, growth factors and extracellular matrix. The tissue specific and differentiation genes involved are bone morphogenetic protein, TGF- β etc. that induce chondro-osteogenic expressions. Some transcription factors involved in the cellular differentiation are Myo-D for muscle, PPAR-γ for adipose tissue, CBFA-1for osteoblast differentiation.
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Metaplastic changes may be caused by chronic irritation, nutritional deficiency, neoplasm etc.
I.Epithelial metaplasia
Squamous metaplasia: - It may occur due to many reasons like chronic irritation, nutritional deficiency etc.
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Chronic irritation from chemicals, carcinogens or other chemicals.
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Smoking: In lung of smokers, ciliated cuboidal and columnar epithelia of airways are converted into stratified squamous epithelium.
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Estrogenism: Stratified squamous metaplasia of prostrate or urinary tract.
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Calculi: Calculi of salivary gland, biliary calculi, pancreas etc.
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Nutritional deficiency: Vitamin A deficiency produces squamous metaplasia of esophageal mucous glands of chicken, transitional epithelium of urinary bladder, cuboid and columnar epithelial cells lining the eye and salivary gland ducts.
II Mesenchymal metaplasia
DYSPLASIA
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The term dysplasia is applied to the tissue malformed during maturation. There will be alteration in size, shape and orientation of tissue. The condition is mainly affecting the epithelium. Dysplastic changes are commonly found in the eye, skin, brain and skeletal system. The developmental defect involved complex interactions among three germinal layers.
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In dysplasia, there will be loss of uniformity of cells and their architecture. It is characterized by pleomorphism (Change in the size and shape of cells), abnormally enlarged hyperchromic nuclei, increased mitosis and disorderly arranged cells.
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Dysplasia when marked and in which all layers of stratified squamous epithelium are involved, it is called ‘preinvasive carcinoma, or ‘carcinoma in situ’. The condition is mild to moderate and reversed if the stimulus is removed.
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