Neuroendocrine cells are those cells which receive their stimuli in the form of nerve impulse and in response to the stimulus these cells release various neurotransmitters and hormones from the synaptic end of the cells. The main cells found to be responsible for the origin of neuroendocrine tumors are cells of kulchitsky[1]. These cells resemble the enterochromaffin cells of the intestine in structure and function, which produces intestinal hormones hence also known to be enterochromaffin-like cells. These are also known by another name as APUD cells. APUD stands for amine precursor uptake and decarboxylation cells which have the capability of taking up the amines and making or converting them into various amine derived products or hormones. These cells also have storage granules for storing amines and converting them into products on their own.
So the neuroendocrine tumors are defined as tumors which arise from the cells having both neural and hormonal control of the body.
Where Do Neuroendocrine Tumors Originate & Where Are Neuroendocrine Cells Found?
These neuroendocrine cells are known to be originated from neural crest cells which are found in the embryo during the development. The neural crest cells originate from the ectoderm in the embryo. The mechanism thought to be responsible for the presence of neuroendocrine cells on locations where these are not found generally is because of abnormal movement of the neural crest cells or sometimes slowing of the movement resulting in lagging behind of these cells at abnormal sites. Until recently these cells were thought to be derived of neural crest cells but the recent researches have found out the origin of these cells to be more related to embryonic endoderm then the neural crest cells[2].
The neuroendocrine cells are also present in various body parts normally and perform their function with efficiency. But sometimes due to the responsibility of various factors and syndromic associations these cells become uninhibited in their proliferation and their death cycle is abolished. These cells then create various neuroendocrine tumors which have high metastatic potential.
The neuroendocrine tumors originate from the locations where these cells are already located in normal individuals. The gastrointestinal tract is the most commonly found origin of the neuroendocrine tumors. The sites of origin in GIT are an appendix in intestines, stomach and proximal duodenum in the foregut, and sometimes hindgut also. The other gastrointestinal visceral organs known to be the origin for neuroendocrine tumors are pancreas, adrenal glands, liver, gall bladder, etc. The neuroendocrine tumors are found as different cancers in the gastrointestinal tract like carcinoids in the stomach, appendix, etc. In the pancreas, these are found in large variation because of the fact that pancreas is a high-end endocrine organ-like islet cell tumors(insulinoma), gastrinoma, glucagonoma, vipoma, somatostatinoma, etc.
Other sites where the origin of the neuroendocrine tumors is seen are breast, pituitary gland, thyroid gland, parathyroid gland, lungs, bronchus, genitourinary tract, peripheral nervous system, skin, etc. At various organs of origin the neuroendocrine tumors present as different cancers like in thyroid gland it is seen as medullary carcinoma, in lungs as large cell carcinoma, in the skin as Merkel cell carcinoma, in the peripheral nervous system as schwannomas, paraganglioma, etc.
Conclusion
Neuroendocrine tumors are found at normal as well as abnormal locations of these cells in the body. These cells are found all over the body in various forms and perform their function efficiently but whenever the normal process gets disrupted, it leads to the formation of such tumors. The most commonly affected organ by the neuroendocrine tumors is the gastrointestinal tract. To be more specific stomach and intestine are the most commonly involved organs by neuroendocrine tumors like carcinoids. The organ showing the most number of neuroendocrine tumors is pancreas and peripheral nervous system due to the presence of a different type of neuroendocrine cells at the same site. These are usually slow-growing tumors which produce systemic symptoms by the excess hormones secretions from them but are known to have high metastatic potential and poor prognosis.
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