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neoplasm or tumor, tissue composed of cells that grow in an abnormal way. Normal tissue is growth-limited, i.e., cell reproduction is equal to cell death. Feedback controls limit cell division after a certain number of cells have developed, allowing for tissue repair but not expansion. Tumor cells are less responsive to these restraints and can proliferate to the point where they disrupt tissue architecture, distort the flow of nutrients, and otherwise do damage.

Tumors may be benign or malignant. Benign tumors remain localized as a discrete mass. They may differ appreciably from normal tissue in structure and excessive growth of cells, but are rarely fatal. However, even benign tumors may grow large enough to interfere with normal function. Some benign uterine tumors, which can weigh as much as 50 lb (22.7 kg), displace adjacent organs, causing digestive and reproductive disorders. Benign tumors are usually treated by complete surgical removal. Cells of malignant tumors, i.e., cancers, have characteristics that differ from normal cells in other ways beside cell proliferation. For example, they may be deficient in some specialized functions of the tissues where they originate. Malignant cells are invasive, i.e., they infiltrate surrounding normal tissue; later, malignant cells metastasize, i.e., spread via blood and the lymph system to other sites.

Both benign and malignant tumors are classified according to the type of tissue in which they are found. For example, fibromas are neoplasms of fibrous connective tissue, and melanomas are abnormal growths of pigment (melanin) cells. Malignant tumors originating from epithelial tissue, e.g., in skin, bronchi, and stomach, are termed carcinomas. Malignancies of epithelial glandular tissue such as are found in the breast, prostate, and colon, are known as adenocarcinomas. Malignant growths of connective tissue, e.g., muscle, cartilage, lymph tissue, and bone, are called sarcomas. Lymphomas and leukemias are malignancies arising among the white blood cells. A system has been devised to classify malignant tissue according to the degree of malignancy, from grade 1, barely malignant, to grade 4, highly malignant. In practice it is not always possible to determine the degree of malignancy, and it may be difficult even to determine whether particular tumor tissue is benign or malignant.

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A malignant tumor originating in glandular or ductal epithelium and tending to produce acinic structures.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Fetal adenocarcinoma of the lung in a 25-year-old woman.
Fetal adenocarcinoma consists of glandular elements with tubules composed of glycogen-rich, nonciliated cells that resemble fetal lung tubules (Figure 11).
(68,69) Natatani et al (68) found mutations in exon 3 of the [beta]-catenin gene in 3 of 4 blastomas, but not in 5 carcinosarcomas with a high-grade fetal adenocarcinoma component.
The carcinoma component is most often squamous cell carcinoma, followed by adenocarcinoma, adenocarcinoma mixed with squamous cell carcinoma, and finally, large cell carcinoma, whereas the sarcoma component is most commonly rhabdomyosarcoma, followed by osteosarcoma mixed with chondrosarcoma, and osteosarcoma alone (Figure 2, E).7 The final subtype, pulmonary blastoma, is a biphasic tumor composed of a primitive epithelial component resembling well-differentiated, fetal adenocarcinoma and a primitive mesenchymal stroma that may contain rhabdomyosarcoma, osteosarcoma, or chondrosarcoma (Figure 2, F).