Can proventricular mesenchyme promote differentiation of endocrine cells in gizzard endoderm?

Can gastric endoderm change the regionally specific inducing ability of presumptive small intestinal mesoderm?

This study was designed to establish the source of gut mesoderm's ability to induce regional pattern in the endoderm. The most obvious possibility is induction by the endoderm through epithelial-mesenchymal interaction. To test this experimentally, reciprocal quail/chick combinations were prepared of early proventricular endoderm (that is already known to be regionally determined) and presumptive small intestinal mesoderm. The combinations were cultured for 7 days to allow for 'programming' of the mesoderm by the endoderm. After removal of the proventricular endoderm the mesoderm was combined with young gizzard endoderm. It is known that gizzard endoderm can be provoked to develop in either a proventricular or a small intestinal direction by association with the appropriate mesoderm. Thus, by combining intestinal mesoderm 'programmed' by association with proventricular endoderm with gizzard endoderm, the subsequent differentiation of the gizzard endoderm would indicate whether or not the inducing ability of the intestinal mesenchyme had been altered. In addition to such experimental grafts, three types of control graft were prepared. The results of the experiment, based on the morphology of the grafts and the immunocytochemical analysis of selected endocrine cell types, showed that in the majority of cases the gizzard endoderm developed the features of small intestine, not those of proventriculus. This indicates that at the stages studied, endoderm does not act to program mesoderm with which it is associated. If this does occur, it must take place at an earlier stage, i.e., before the time of explantation of the presumptive small intestinal mesoderm (1.25 days of incubation).

Gut endocrine cells in birds: an overview, with particular reference to the chemistry of gut peptides and the distribution, ontogeny, embryonic origin and differentiation of the endocrine cells

This review deals with gut endocrine cells in birds. It focuses on both morphological and developmental aspects of these cells, which were included members of Pearse's APUD series. They comprise many cell types, which, in birds as in mammals, produce serotonin and a range of regulatory peptides. The chemical structure of most avian gut peptides has been established. These peptides and their functions are outlined here. The types and distribution of avian gut endocrine cells are detailed and compared with the situation in mammals. In birds, ultrastructural work has been limited to certain types of gut endocrine cell and not as widely applied as in mammals. However, immunocytochemistry has found widespread application in studies on birds: the hatching chick and also the adult chicken and certain other species such as the quail and duck have been studied. Gut endocrine cells showing immunoreactivity for the following peptides/serotonin have been identified: somatostatin, pancreatic polypeptide (PP), peptide YY, glucagon, secretin, vasoactive intestinal peptide, gastrin, cholecystokinin (CCK), neurotensin, motilin, gastrin-releasing peptide, substance P, enkephalin and serotonin. The colocalization of different peptides (including chromogranins) and of peptides and serotonin in the same gut endocrine cells is reviewed: notable amongst such associations are glucagon with PP and gastrin/CCK with neurotensin in the same cells. On morphological grounds cells have been identified as endocrine in avian gut from at least 9 days of incubation. Immunocytochemical studies show the majority of the various types first to appear between 12 to 14 days of incubation, with substantial numbers being recorded from 17 days onwards. Experimental studies on chicken and quail embryos have determined the embryonic origin of gut endocrine cells: evidence is unequivocal that such cells arise from the endoderm, not the neural crest, other ectoderm or the mesoderm. Studies on avian embryos have also contributed to our knowledge of mechanisms controlling the differentiation of gut endocrine cells: evidence shows that gut mesenchyme plays an important role in provoking (or inhibiting) the development of gut endocrine cells and there are indications that the endocrine cell pattern in gut is established early and that an axially-derived factor may be important in this process. The kinds of genetic mechanism possibly involved are mentioned but full elucidation of the processes concerned is awaited. A better understanding of the formation of endocrine tumours of the gut should result from the findings.

Can a non-gut mesenchyme support differentiation of gut endocrine cells?

This experiment was designed to find out if endoderm lacks an intrinsic ability to give rise to gut endocrine cells, and, if not, whether differentiation of endocrine cells can be supported by mesenchyme from a source outside the digestive tract. Heterospecific combinations of proventricular endoderm and flank mesenchyme from chick and quail embryos at 3.25-4 days of incubation were grown as chorio-allantoic grafts to a final incubation age of 21 days. Re-associated proventricular endoderm and mesenchyme served as controls. The proventricular endoderm induced some smooth muscle in the flank mesenchyme but the latter did not support as advanced glandular morphogenesis as did proventricular mesenchyme. Nevertheless, endocrine cells differentiated in experimental as in control grafts and at similar frequencies. The various types were distinguished immunocytochemically by their contained peptides; the range of types found was specific for the proventriculus. Hence it is concluded not only that the particular non-gut mesenchyme used does support differentiation of gut endocrine cells, but also that the determination of the progenitors of endocrine cells, and the selection of the range of types destined to differentiate in a particular part of the digestive tract under normal circumstances, occurs early in development--before 3.25 days of incubation in the case of the proventriculus.

Susceptibility of epithelia to directive influences of mesenchymes during organogenesis: uncoupling of morphogenesis and cytodifferentiation

Morphogenesis and functional cytodifferentiation are two major events in organogenesis, and normally they take place inseparably either in vivo or in vitro conditions. In this article, we reviewed a series of our recent results on mesenchymal-epithelial interactions in organogenesis of digestive organs, urogenital organs and the skin of avian and mammalian embryos, giving special attention to the importance of the responses of epithelia to the directive influences of mesoderms and also to the uncoupling of morphogenesis and cytodifferentiation, which has often been observed during the course of these studies.

Intestinal mesenchyme provokes differentiation of intestinal endocrine cells in gizzard endoderm

The gizzard (muscular stomach) of chicks is deficient in endocrine cells at hatching. It has previously been shown that proventricular types and proportions of endocrine cells can be induced in gizzard endoderm under the influence of proventricular (glandular stomach) mesenchyme. In order to test its capacity to form nongastric endocrine cell types, gizzard endoderm of 3.75- to 5-day chick embryos was combined with mesenchyme from the small intestine of 3.5- to 4-day quail embryos. The combinations were grown as chorio-allantoic grafts until they attained an incubation age comparable to that of hatching chicks. Controls comprised reassociated endoderm and mesenchyme of chick gizzard and of quail intestine. In the experimental grafts, morphogenesis was predominantly intestinal but some grafts showed gizzard-like features, particularly if the endoderm had been provided by older donors. All intestinal endocrine cell types, including those also found in the normal proventriculus (serotonin-, glucagon-, pancreatic polypeptide-, neurotensin- and somatostatin-immunoreactive cells) differentiated in experimental grafts, some even where morphogenesis was gizzard-like. Hence progenitors of not only gastric, but also intestinal, endocrine cells are indeed present in gizzard endoderm. The possibility that gizzard mesenchyme is inhibitory to endocrine cell differentiation is mooted. Motilin- and secretin-immunoreactive cells, which are characteristic of the intestine but not of the proventriculus of chicks at hatching, were respectively sparse or absent when the endoderm was derived from older donors. Thus the ability of gizzard endoderm to differentiate into nongastric endocrine cell types declines before its capacity to form gastric types. The unexpected appearance of gastrin-releasing peptide (GRP)-immunoreactive cells, a proventricular type not found in normal chick intestine, suggests that the intestinal mesenchyme, at least in this instance, was exercising a permissive role.

Differentiation of endocrine cells in chick allantoic epithelium combined with pancreatic mesenchyme

Allantoic endoderm of 3-day chick embryos was combined with pancreatic mesenchyme of 5-day embryos and cultured as chorio-allantoic grafts for a total of 14 days. Recombinations of endoderm and mesenchyme of the pancreas and of the allantois served as controls. The usual types of endocrine cells differentiated in the pancreatic controls, none in the allantoic controls. In experimental grafts simple columnar epithelium with goblet cells and a sucrase-positive brush border developed; a few insulin cells and endocrine cells typical of the intestine differentiated. Hence allantoic endoderm has endocrine potentiality not realised in vivo, where its own mesenchyme may be inhibitory.