Paracrine agents

The ECL cells comprise approximately one-third of the endocrine cells in the oxyntic (acid-secreting) mucosa of most vertebrates. They are small, 8- to 10-pmdiameter cells that contain numerous electron-translucent cytoplasmic vesicles, many of which have an electron-dense core. ECL cells store histanrrine and contain histidine decarboxylase (HDC), the enzyme required for histamine synthesis. The obvious role of the ECL cells is to release histamine, which acts as a paracrine agent to stimulate the parietal cells. It has been demonstrated that the vesicles contain other regulatory compounds, such as chronxigranin and pancreastatin, but neither the specific compounds nor their functional significance has been defined. 

Growth factors are paracrine or endocrine agents, peptides or proteins, released by one cell to regnlate proliferation in adjacent cells (chapter 12) (Table 21.4). Normal cell proliferation requires the presence of growth factors but proliferation of tumonr cells reqnires either no growth factors or much lower concentrations. There are several different characteristics of tumor cells that enable them to respond to very low concentrations of growth factors ... 

Disturbances of the metabolism of bone tissue that usually exhibit not very specific clinical signs and symptoms are induced by the effects of both systemic and local agents. The role of hormones has already been described and defined many times, but news and new views keep appearing all the time. Without doubt, practically all systemic agents, hormones, but also drugs act on the bone tissue only through mediators. Their effect is provided by local, paracrine, and autocrine substances effective on the level of the cell, and by cellular components and on molecular level. Some of the hormones and local agents will be mentioned below. 

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