A carcinoid tumor is a type of neuroendocrine tumor that typically originates in the gastrointestinal system. Histologically, it is composed of cells that resemble neuroendocrine cells and contain multiple granules. The granules contain a variety of proteins, including serotonin, chromogranin, and neuron-specific enolase. These granules are also responsible for the production of various hormones, including ACTH and VIP. Other histological features of a carcinoid tumor include a sheet-like growth pattern of small cells, a pseudocapsule surrounding the tumor, and a high mitotic rate.
In addition, a carcinoid tumor may contain areas of necrosis or calcification. The tumor is typically surrounded by a fibrous capsule, which can contain an accumulation of cells called the desmoplastic reaction. It is also common to find the presence of inflammatory cells, such as eosinophils and lymphocytes. Finally, a carcinoid tumor may be associated with a variety of other histological features, including vascular and parenchymal changes, hemosiderin-laden macrophages, and various types of cystic spaces.
Overall, these histological features are characteristic of a carcinoid tumor and can help to distinguish it from other types of tumors. It is important to note, however, that not all carcinoid tumors will display all of these features. Therefore, it is important for the pathologist to carefully evaluate the tumor to ensure an accurate diagnosis.