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|Classification and external resources|
Micrograph of an adenocarcinoma showing mucin containing vacuoles. Pap test.
Adenocarcinoma is a cancer of an epithelium that originates in glandular tissue. Epithelial tissue includes, but is not limited to, the surface layer of skin, glands, and a variety of other tissue that lines the cavities and organs of the body. Epithelium can be derived embryologically from ectoderm, endoderm or mesoderm. To be classified as Adenocarcinoma, the cells do not necessarily need to be part of a gland, as long as they have secretory properties. Well differentiated adenocarcinomas tend to resemble the glandular tissue that they are derived from, while poorly differentiated adenocarcinomas may not. By staining the cells from a biopsy, a pathologist can determine whether the tumor is an adenocarcinoma or some other type of cancer. Adenocarcinomas can arise in many tissues of the body due to the ubiquitous nature of glands within the body. While each gland may not be secreting the same substance, as long as there is an exocrine function to the cell, it is considered glandular and its malignant form is therefore named adenocarcinoma. Endocrine gland tumors, such as a VIPoma, an insulinoma, a pheochromocytoma, etc., are typically not referred to as adenocarcinomas, but rather, are often called neuroendocrine tumors. If the glandular tissue is abnormal, but benign, it is said to be an adenoma. Benign adenomas typically do not invade other tissue and rarely metastasize. Malignant adenocarcinomas invade other tissues and often metastasize given enough time to do so.
A diagnosis of adenocarcinoma which is not further described, known as adenocarcinoma not otherwise specified or adenocarcinoma NOS, is significant because it displays when a cancerous process is present. However, it is not very useful for treatment decisions and prognosis, as these are determined by the tissue from which the tumour cells arose, i.e. the tissue of origin; an adenocarcinoma of the colon has a different prognosis and treatment than an adenocarcinoma of the ovary.
Cancer for which a primary site cannot be found is called cancer of unknown primary.
Symptoms of this cancer can be pain in the abdomen, weight loss, weakness, and fatigue. As the tumor increases in size, it can block the passage of digested food. The tumor can cause obstruction. Obstruction is when the intestine is blocked and nothing can move through. This causes pain with severe nausea and vomiting.
The tumor can also cause perforation. This is when all the contents of the small intestine spill into the abdominal cavity. The symptoms for perforation are sudden severe pain, nausea, and vomiting. However, this is only on rare occasions.
Sometimes a tumor will start to bleed into the intestine. Slow bleeding will lead to a low red blood cell count. This is called anemia, which causes weakness and fatigue. Rapid bleeding can cause the stool to become black and tarry from digested blood. This can make the patient feel lightheaded or even pass out.
Examples of tissues where adenocarcinomas may arise:
The vast majority of colorectal cancers are adenocarcinomas. This is because the colon has numerous glands within the tissue. Normal colonic glands tend to be simple and tubular in appearance with a mixture of mucus secreting goblet cells and water absorbing cells. These glands are called glands because they secrete a substance into the lumen of the colon, this substance being mucus. The purpose of these glands is twofold. The first is to absorb water from the feces back into the blood. The second purpose is to secrete mucus into the colon lumen to lubricate the now dehydrated feces. This is crucial as a failure to lubricate the feces can result in colonic damage by the feces as it passes towards the rectum.
When these glands undergo a number of changes at the genetic level, they proceed in a predictable manner as they move from benign to an invasive, malignant colon cancer. In their research paper "Lessons from Hereditary Colorectal Cancer", Vogelstein, et al., suggested that colon cells lose the APC tumor suppressor gene and become a small polyp. Next, they suggested that k-Ras becomes activated and the polyp becomes a small, benign adenoma. The adenoma, lacking the "carcinoma" attached to the end of it, suggests that it is a benign version of the malignant adenocarcinoma. The gastroenterologist uses a colonoscopy to find and remove these adenomas and polyps to prevent them from continuing to acquire genetic changes that will lead to an invasive adenocarcinoma. Volgelstein et al. went on to suggest that loss of the DCC gene and of p53 result in a malignant adenocarcinoma.
Grossly, one will see a mass that looks of a different color than the surrounding tissue. Bleeding from the tumor is often apparent as the tumor tends to grow blood vessels into it in a haphazard manner via secretion of a number of angiogenesis promoting factors such as VEGF. Histologically, tumours resembling original structures are classified as well differentiated. Tumour cells that have lost any resemblance to original tissue, both in appearance and structure form, are denoted as poorly differentiated tumour cells. Regardless of the grade, malignant tumors tend to have a large nucleus with prominent nucleoli. There will also be a noticeable increase in the incidence of mitoses, or cell divisions.
- Most cervical cancer is squamous cell cancer, but a sizeable minority of cervical cancers are adenocarcinomas.
- pancreas (≥90% of pancreatic cancers are ductal adenocarcinomas.) 
The term adenocarcinoma is derived from 'adeno' meaning 'pertaining to a gland' and 'carcinoma', which describes a cancer that has developed in the epithelial cells.
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