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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., cancerscancer,
in medicine, common term for neoplasms, or tumors, that are malignant. Like benign tumors, malignant tumors do not respond to body mechanisms that limit cell growth.
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, 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 leukemiasleukemia
, cancerous disorder of the blood-forming tissues (bone marrow, lymphatics, liver, spleen) characterized by excessive production of immature or mature leukocytes (white blood cells; see blood) and consequently a crowding-out of red blood cells and platelets.
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 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.


An aberrant new growth of abnormal cells or tissues; a tumor.
References in periodicals archive ?
This unusual gross appearance may be attributed to the loss of tumor cells and rich hypervascular network.
The overall gross appearance of EGC and AGC in male or female patients was consistent with those of the total population for both Chinese and Japanese patients.
In summary, we have described an example of extreme hydropic degeneration in a uterine leiomyoma, associated with a highly unusual gross appearance.
The gross appearance of chordoma is an expansile soft-tissue mass with a lobulated appearance and cystic spaces filled with gelatinous material.
The pathologic definition of low-malignant-potential disease is the gross appearance of peritoneal carcinomatosis and a microscopic appearance that characteristically fails to reveal evidence of frank invasion by the tumor nodules.
The gross appearance of the hairy leukoplakia lesion may be variable, resulting in possible misdiagnosis.
When the woman is taking the Pill, you can clearly see the difference, based both on gross appearance -- as seen with the naked eye -- and under a microscope.
A,B: A polypoid urethral mass in distal urethra C: The gross appearance of mass after excision.
The tumor is often large, but the gross appearance appears to vary, with cystic, solid and necrotic areas being present, and a fibrous pseudocapsule often surrounds the tumor, demarcating it from the pancreatic tissue.
While the presence of RBCs in fluids is significant and should be reported in the gross appearance description, the RBC count has no clinical significance.
6) Despite their common cellular makeup, the clinical presentation and gross appearance of these forms differ considerably.