§ 2.21 ISOLATED CELLS IN THE GUT 45 



Since no recognizable endocrine glands or groups of secreting 

 cells have been found in the gut mucosa in those regions from 

 which the gastrointestinal hormones, such as gastrin and sec- 

 retin, can be extracted (§ 4.11), it must be concluded that all these 

 hormones arise from isolated cells (Table 5). So far, however, 

 these cells have not been found, despite intensive search; it is 

 possible that they may not be histologically distinct from other 

 cells in the gut lining, such as those secreting mucus, but it seems 

 more likely that they will eventually be revealed by more sensitive 

 or selective staining techniques. It has been suggested that 

 secretin may be produced in certain "argentafhne" cells, which 

 can be stained with silver; but this seems unlikely since similar 

 cells are abundant in the vermiform appendix from which no 

 secretin can be extracted (Grossman, 1950). 



The origin of the secretory cells certainly deserves further 

 investigation. The clues that would seem to be the most worth 

 following are the facts that (1) the action of this whole group of 

 hormones is kinetic, (2) all other kinetic hormones come either 

 from modified nerve cells or at least from the ectoderm and (3) the 

 action of these hormones in stimulating the secretion of stomach, 

 pancreas and intestinal glands is carried out in lower vertebrates 

 by the parasympathetic nerves in the vagus. The parasympathetic 

 nerves form part of the general visceral motor system and like the 

 sympathetic nerves have peripheral ganglia, connected to the brain 

 by preganglionic fibres. The latter are of central nervous origin ; 

 but there is considerable doubt as to the origin of the peripheral 

 cells, though the neural crest has been plausibly postulated. If so, 

 crest cells might be expected to migrate to the intestine from the 

 cranial region to form ganglia; and it seems possible that some of 

 them might also form argentaffine cells, and others secretory cells. 

 Although there is evidence that the argentaffine cells of the gut 

 can differentiate in grafts of chick intestinal epithelium, even if 

 this is separated from the embryo before any trunk neural crest 

 material is formed (Van Campenhout, 1946), it is not so clear that 

 cranial neural crest cells were excluded in these experiments. On 

 the other hand, there is one other curious feature about all the 

 hormones secreted by the gut wall in mammals: unlike kinetic 

 hormones derived from neurosecretory cells, they are not secreted 



