20 SOURCES OF KINETIC AND METABOLIC HORMONES 



with large nuclei and secretory granules in the cytoplasm ; but the 

 latter may vary with the phase of the secretory cycle. This will 

 affect both the microscopic appearance of the cells (Fig. 2-2) and 

 their reaction to histochemical tests which can be applied to the 

 secretion. Some of these cells appear to release their secretion, or 

 neurohormone, where it can only diffuse through the closely 

 adjacent tissue without reaching the circulation. It is then difhcult 

 to distinguish the action experimentally from that of a normal 

 motor nerve, especially as the release of secretion is probably 

 accompanied by electrical changes in the axon similar to those 

 accompanying the nerve impulse. The distinction between such 

 cells and ordinary neurons seems only to be one of degree, for it 

 depends upon the presence or absence of ''granules". This in 

 turn depends rather arbitrarily upon the limits of resolution of the 

 ordinary light microscope. Since the abundant fine granules of 

 adrenaline, which stain brown with chromates, can be readily seen 

 with the light microscope in cells of the adrenal medulla, there 

 seems little reason to doubt that the minute quantities of the same 

 substance, secreted at sympathetic nerve endings, could also be 

 seen by using the greater magnification that can now be achieved 

 by the electron microscope. Yet these nerves are not usually con- 

 sidered to be neurosecretory. Other neurosecretory cells have 

 simple axon endings that discharge their secretion into blood 

 vessels, thereby clearly acting as a source of a vascular hormone. 



The secretion is formed as granules or droplets either in the 

 cytoplasm immediately surrounding the nucleus, or within the 

 nucleus itself. Thence the granules have been seen to move slowly 

 along the axon and are probably carried in the axoplasm current, 

 which flows at a rate of about 3 mm per day (the movement of 

 endoneural fluid is about 20 times as fast). They accumulate at 

 the unbranched ends of the axons, which become swollen and 

 are often aggregated together to form a storage-and-release organ 

 at the point where the hormone is passed into the blood. Such 

 structures have been called neurohaemal organs (Carlisle and 

 Knowles, 1953). The secretion can sometimes be detected for a 

 short distance even after its discharge into the blood vessel. 



It is probable, however, that the visible secretion often acts as 

 a "carrier", to which is attached the chemical substance that acts 



