Clusters of endocrine-like cells in the Proventriculus of three small passerines
Abstract
The proventriculi of three small passerines have been studied by electron microscopy. Based on morphology, location of cells and size of granules, ten different types of gastric endocrine-like cells GELCs in the proventriculus were identified. GELCs were observed singly and in clusters in the glandular tubules of the proventriculi. Clustered GELCs in connective tissue, with a blood and nerve supply, is reported for the first time in the avian proventriculus. All GELCs were closely associated with capillaries and many had unmyelinated axons in close proximity. There was a considerable range in the size, electron density and distribution of the granular vesicles within the GELCs suggestive of colocalization and storage of differennt hormones in the same cell. Although most types of GELCs were round to oval, some had elongated cytoplasmic processes that contained numerous vesicles. The GELCs also varied in that some had specific membrane associations with adjacent cells while others did not. The identification of several types of cell suggests that there is considerable local hormonal control of activity within these regions of the avian digestive system. The study attempts to provide a baseline for future ultrastructural, immunocytochemical studies on the content of the granules in these cells.
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Introduction
Morphological and histochemical studies have described gastric endocrine-like cells GELCs, including argentaffin and argyrophilic cells, in the proventricular epithelium. 1,2,3 Immunocytochemical studies have identified several types of GELCs in the gastrointestinal tract of the domestic chicken including the proventriculus. 1,3 Other immunocytochemical studies have identified GELCs in the common finch, pigeon, quail, duck, gull, kite4 , New Holland honeyeater5 ; ostrich6 and three other honeyeaters: brown honeyeater, singing honeyeater and spiny-cheeked honeyeater. 7
GELCs tend to be oval with long or short cytoplasmic process, adjacent to the basement membrane and usually covered by epithelial cells. Some, the open type of GELC, make contact with the lumen via long cytoplasmic process or with other GELCs while the closed type of GELC make contact with parietal or chief cells. 2,4,5,7
GELCs in the gastrointestinal tracts of birds and mammals have been reported as individual cells8 , located between glandular or duct cells and the basement membrane. 9
Andrew1 identified two types of enterochromaffin cells based on whether the secretory vesicles had a maximum diameter of 290 nm or 510 nm but there is relatively little information on the ultrastructure of GELCs in birds.
Conclusion
This study has identified 10 possible types of endocrine-like cells based on their location, and their ultrastructural appearance. The possible endocrine function of these cells is supported by the presence of dense–cored vesicles of varying size and appearance in the cytoplasm. The presence of polymorphic secretory granules within the cytoplasm of some GELCs portrays their multihormonal function. The study has revealed the presence of clusters of endocrine-like cells in the glandular epithelium, interglandular and periglandular connective tissue.