Neuroendocrine tumors may develop from neuroendocrine cells (NEC), and it has been proposed that pheochromocytoma, a rare catecholamine-producing neuroendocrine tumor, is derived from the chromaffin NECs in the adrenal medulla. In general, the conventional concept of a neuroendocrine system combines aspects of the nervous system with aspects of the endocrine system, highlighting the integration and cooperation between the two. Pulmonary NECs are regarded to be components of the pulmonary neuroendocrine system that consist of a specific group of airway epithelial cells. Chromogranin A, a neuroendocrine cell marker, is present in early human fetuses when the number of chromogranin A-immunoreactive cells surpasses that of insulin- and glucagon-containing cells. In humans, differentiation diagnostic tests can separate normal NECs from cancerous cells because prostatic neuroendocrine tumor cells may also express all of the aforementioned proteins NEC markers. The hypothalamus and the pituitary gland are the major centers of neuroendocrine integration in the body.