**2. General aspects of neuroendocrine gastrointestinal system**

The terms "neuroendocrine tumors," "carcinoid tumors," and "endocrine tumors" are widely used when referring to tumors of the digestive tract. These designations can be found simultaneously even as part of the same classification. Neuroendocrine tumors were firstly called as "carcinoid tumors" by the German pathologist Oberndorfer, in 1907 [1]. And until now, the term carcinoid has been used ostensibly, colloquially, and even in almost all current classifications [2, 3].

The term "apudoma" may also be considered as being a synonym of carcinoid tumor. The denomination of "APUD cells" was proposed by Pearse in the late 1960s as an acronym of *amine precursor uptake and decarboxylation* [4]. The term APUD summarizes some of the most important characteristics of these cells, which are (a) a high amine content and/or amine precursor uptake, (b) amino acid decarboxylase activity, and (c) characteristic ultrastructural pattern. Initially, the term "apudoma" was used mainly from the clinical point of view to designate those tumors of symptomatic patients due to the pathological secretion of bioactive products and afterward also for clinically asymptomatic tumors originating from APUD cells [5–7]. As we can see, unlike the roots of the other designations for these tumors, the term "apudoma" has been derived from consistent morphologic and biochemical basis.

Apudomas are derived from the APUD cell line and so they have characteristic ultrastructural pattern recognized due the presence in the tumor cells of secretory granules where the regulatory peptides are located as well as the biogenic amines that they produce [8, 9]. Normally, the APUD cells are rich in amino acid decarboxylase, which gives them the ability to capture 5-hydroxytryptophan and dihydroxyphenylalanine and produce, respectively, serotonin and dopamine. Although this property has not been demonstrated in all the cells morphologically characteristic of this system, this biochemical link may occur in all of them whatever their specific function is. This gives these cells a familial, morphological, and biochemical bond, which extends to a greater or lesser degree to the apudomas. In addition, these morphological and biochemical characteristics give to the term "APUD" a biologically more specific designation compared to the other denominations. Thus, the term "apudoma" is less vulnerable to temporal changes than those currently used, namely, "carcinoid," "endocrine tumor," and "neuroendocrine tumor." The latter denomination is being adopted in this article because of its extensive use in modern classifications and also throughout the medical literature indexing.
