Microanatomical observations on the gall bladder in kuttanad duck (Anas platyrynchos domesticus)

The study was aimed at observing the normal histological characteristics of gall bladder of Kuttanad ducks (Anas domesticus) using 72 ducks of various ages from day-old to 22 weeks. Gall bladder was a small sac lying on the visceral surface of liver. Its weight increased from 0.03 by day-old to 1.26g by 22 weeks. Thickness of wall increased from 13 μm to 169 μm by this age. Layers of gall bladder from inner to outer were: mucosa, muscularis, perimuscular connective tissue layer and serosa. There was no muscularis mucosa or submucosa. Mucosa consisted of simple, tall, columnar, homogeneous epithelium and lamina propria. Cytoplasm of epithelial cells was weakly acidophilic and at the free edge, appeared to be thickened probably due to the presence of microvilli. Nuclei were large, basal and oval. Lamina propria consisted of a layer of connective tissue with elastic and muscle fibres which was strongly folded into villus-like projections when the gall bladder was contracted and was much thinner with only small folds when the bladder was distended. Beneath lamina propria, the muscularis consisted of interlacing fascicles of smooth muscle fibers and abundant intervening connective tissue, forming a thin irregular and decussating fibromuscular coat. These muscular layers provided numerous spaces between their fibres, which were filled up with reticular, elastic or collagenous fibres and fibroblasts. Smooth muscle was found in longitudinal, transverse and oblique directions. Outer, circular or oblique layer was more constant and gave off strands passing into base of the villi. Longitudinal layer occurred only irregularly, as large bundles of fibres lying at the base of the villi. The perimuscular layer of dense connective tissue contained blood and lymphatic vessels (for abundant water reabsorption from bile) and nerves, and it formed an adventitia (fibrosa) on the surfaces in contact with liver. Other surfaces were covered with a serosa, which was moderately thick and vascular in some areas but thin and avascular in others. It was composed of coarse collagen fibres interspersed with small number of elastic fibres. Gall bladder provides a storage site for bile synthesized in liver and also concentrates it owing to ion-transporting activities of the epithelium lining the lumen. Lipids reaching the duodenum signal the release of polypeptide hormone, cholecystokinin from endocrine cells of mucosa into blood. Cholecystokinin has receptors in wall of gall bladder, which result in contraction of smooth muscle and release of bile via bile duct on to duodenum.