13-12-2007, 21:09
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חבר מתאריך: 02.12.05
הודעות: 1,298
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בגדול....
נוזל חם שבא במגע בצורה כרונית עם רירית הוושט גורם לנזק מתמשך, אם תרצה כוויה כרונית, ברגע שרקמה ניזוקה בצורה כרונית היא עלולה לעבור שינויים בתכונות שלה (או יותר נכון התאים שמרכיבים אותה עוברים שינויים). השינויים יכולים להיות שפירים (METAPLASIA) או ממאירים (ANAPLASIA).
בהרחבה על השינויים הממאירים (באופן כללי):
Differentiation and Anaplasia
The terms differentiation and anaplasia apply to the parenchymal cells of neoplasms. Differentiation refers to the extent to which parenchymal cells resemble comparable normal cells, both morphologically and functionally. Well-differentiated tumors are thus composed of cells resembling the mature normal cells of the tissue of origin of the neoplasm (Fig. 8-5) . Poorly differentiated or undifferentiated tumors have primitive-appearing, unspecialized cells. In general, benign tumors are well differentiated .
Malignant neoplasms, in contrast, range from well differentiated to undifferentiated. Malignant neoplasms composed of undifferentiated cells are said to be anaplastic.
Lack of differentiation, or anaplasia, is considered a hallmark of malignant transformation. Literally, anaplasia means "to form backward," implying a reversion from a high level of differentiation to a lower level. There is substantial evidence, however, that cancers arise from stem cells present in all specialized tissues. The well-differentiated cancer (Fig. 8-7) evolves from maturation or specialization of undifferentiated cells as they proliferate, whereas the undifferentiated malignant tumor derives from proliferation without maturation of the transformed cells. Lack of differentiation then is not the consequence of dedifferentiation.
Lack of differentiation, or anaplasia, is marked by a number of morphologic and functional changes. Both the cells and the nuclei characteristically display pleomorphism--variation in size and shape (Fig. 8-8) . Cells may be found that are many times larger than their neighbors, and other cells may be extremely small and primitive appearing.
As compared with benign tumors and some well-differentiated malignant neoplasms, undifferentiated tumors usually possess large numbers of mitoses, reflecting the higher proliferative activity of the parenchymal cells.
Many normal tissues exhibiting rapid turnover, such as bone marrow, have numerous mitoses, and non-neoplastic proliferations such as hyperplasias contain many cells in mitosis. More important as a morphologic feature of malignant neoplasia are atypical, bizarre mitotic figures sometimes producing tripolar, quadripolar, or multipolar spindles (Fig. 8-9) .
Before we leave the subject of differentiation and anaplasia, we should discuss dysplasia, a term that literally means disordered growth. Dysplasia is encountered principally in the epithelia, and it is characterized by a constellation of changes that include a loss in the uniformity of the individual cells as well as a loss in their architectural orientation. Dysplastic cells also exhibit considerable pleomorphism (variation in size and shape) and often possess deeply stained (hyperchromatic) nuclei, which are abnormally large for the size of the cell. Mitotic figures are more abundant than usual, although almost invariably they conform to normal patterns. Frequently the mitoses appear in abnormal locations within the epithelium. Thus, in dysplastic stratified squamous epithelium, mitoses are not confined to the basal layers and may appear at all levels and even in surface cells. There is considerable architectural anarchy. For example, the usual progressive maturation of tall cells in the basal layer to flattened squames on the surface may be lost and replaced by a disordered scrambling of dark basal-appearing cells. When dysplastic changes are marked and involve the entire thickness of the epithelium, the lesion is considered a preinvasive neoplasm and is referred to as carcinoma in situ (Fig. 8-11) . Although dysplastic changes are often found adjacent to foci of invasive carcinoma and in long-term studies of cigarette smokers, epithelial dysplasia almost invariably antedates the appearance of cancer, dysplasia does not necessarily progress to cancer. Mild-to-moderate changes that do not involve the entire thickness of epithelium may be reversible, and with removal of the putative inciting causes, the epithelium may revert to normal.
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