Gizzard myoglobin contents and feeding habits in avian species.

In an attempt to consider physiological function of myoglobin (Mb), we determined Mb contents of gizzard smooth muscles with special reference to feeding habits in 85 avian species of 19 orders. The Mb content in 44 species of herbivorous birds was 7.52+/-3.81 mg/g wet muscle, which was significantly higher than the value of 2.34+/-1.74 mg/g in 41 species of carnivorous ones (P<0.001). Buffering capacity, as determined by in vitro titration method, was 37.3+/-5.5 slykes/g in gizzard smooth muscles of 75 species and 60.7+/-10.5 slykes in breast skeletal muscles of 77 species (P<0.001), which suggests a significantly higher dependence, almost comparable to cardiac muscles, of the gizzard muscular function on aerobic metabolism. Together with the fact that blood circulation in the gizzard is very low at resting, and might be further limited during activity, we conclude that the higher Mb content in gizzards of herbivorous birds is an adaptation, to allow storage and/or facilitated diffusion of oxygen, during process of high mechanical work required to grind down hard and fibrous vegetable food under the conditions of limited circulatory supply.

Chicken gizzard. The muscle, the tendon and their attachment.

The fine structure and the organization of muscle and connective tissue in the middle portion of the chicken gizzard (muscular stomach) has been studied by light and electron microscopy. The musculature is divided into long, well-defined bundles arranged circularly and concentrically and extending between the two tendons (tendinous aponeurosis). The muscle bundles are inserted onto the inner surface of the tendon at an angle of about 45 degrees. In addition to muscle cells (which are ultrastructurally similar to those of the small intestine) the musculature contains fibroblasts and interstitial cells and a small number of nerve bundles and capillaries. The gizzard tendons are very compact, made of parallel fascicles of collagen fibrils with interposed stellate tendon cells; ultrastructurally they are very similar to the tendon of skeletal muscles of this and other species. Their collagen fibrils range in size from 30 to 160 nm. The muscle cells that approach the tendon develop longitudinal invaginations of the cell membrane and then break into finger-like terminal processes heavily encrusted with dense bands. The membrane of the invaginations and the terminal processes are surrounded by a basal lamina material which embeds a conspicuous web of small collagen fibrils. The boundary between tendon and muscle is sharp, without interpenetration of the two tissues. A novel type of cell is found at the interface of muscle and tendon (junctional cells), filled with intermediate filaments and some rough endoplasmic reticulum and displaying a trace of a basal lamina.

[The system of blood vessels in the glandular and muscular stomach of the chicken (author's transl)]. / Das Blutgefässsystem des Drüsen- und Muskelmagens vom Haushuhn.

The vascular system of the stomach of the chicken was investigated by means of corrosion preparations as well as by light- and electronmicroscopic methods. The main vessels form a) an Intermuscular Plexus the outer longitudinal and inner circular layers of the muscle wall in the glandular stomach and b) a Subserous Plexus in the muscular stomach. In the glandular stomach the Interglandular Parietal Arterial Plexus I, situated below the Lamina muscularis mucosae I arises from the arteries which pass through gaps in the muscle wall. This Plexus I produces another plexus, the Interglandular Arterial Plexus II, located on top of the Lamina muscularis mucosae I. Both arterial plexuses are accompanied by venous plexuses, which convey the blood into a Subglandular Venous Plexus. -The vessels of the muscular stomach provide the predominant supply of blood to the muscle wall. The mucous membrane of the muscular stomach is vascularized far less than the one in the glandular stomach. -The hemodynamic relationships of the vessels of the hen's stomach and the significance of the fine structure of the subepithelial capillaries and venules for the exchange of materials are discussed.