In this article we will discuss about:- 1. The Lungs in the Vertebrates 2. The Liver in Vertebrates 3. The Pancreas and Bursa Fabricii in Vertebrates.
The Lungs in the Vertebrates:
The lungs develop from a rudiment which is a pocket-like evagination of the endodermal epithelium on the ventral side of the alimentary canal, just posterior to the branchial region. The pocket at first projects straight downward. At its tip it bifurcates, and the two branches grow out to the sides and backward. The unpaired medial part of the rudiment becomes the trachea, and the two branches give rise to the two bronchi and to the lungs themselves.
In the lower vertebrates, the lungs are developed as saclike expansions at the ends of the bronchi, the walls of which become folded to various degrees. In warm-blooded vertebrates, birds, and mammals, in which lung respiration attains the highest efficiency, the greater degree of differentiation of the air spaces in the lungs already becomes manifest in the earlier stages of lung development.
In mammals, the distal ends of the bronchi, as they grow out, become branched in a more or less dichotomous fashion, the branches representing the secondary, tertiary, etc., bronchi and the bronchioles. The alveoli are eventually developed on the terminal branches of this system.
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The unpaired part, the rudiment of the trachea, may elongate greatly by subsequent growth. Although the first visible rudiment of the lungs is ventral and unpaired, there is good reason to believe that the lungs are derived from originally paired and lateral rudiments.
By local vital staining in amphibians, the presumptive epithelium of the lungs has been found to lie in the neurula stage, in the lateral walls of the foregut, just posterior to the presumptive endoderm of the pharyngeal pouches. The presumptive lung endoderm later shifts downward toward the midline.
In the frog Xenopus (and possibly also in other frogs), the lung rudiments first become noticeable as two separate lateral pockets at a stage when the parts of the alimentary canal are not yet clearly separated from one another. When the gastric part of the gut becomes inflected downward, the lung rudiments are found just behind and ventral to the crest of the transverse fold separating the pharyngeal section of the gut from the esophagus. Following this, the part of the gut cavity connected to the lateral lung rudiments protrudes forward and becomes a di stinct pocket-like evagination.
This cavity is the rudiment of the trachea (which is very short in frogs). The cavity of the trachea, which is continuous with the cavities of the lung rudiments, becomes temporarily separated from the esophagus and later opens into the pharyngeal cavity. This new opening is the glottis. It does not coincide with the mouth of the original invagination which gave rise to the trachea.
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The lateral and independent origin of the lung rudiments in amphibians makes it probable that in the early history of the terrestrial vertebrates the lungs developed from the last pair of pharyngeal pouches, which failed to break through to the exterior and became adapted to the retention of air gulped in through the mouth. Thus, they became organs in which the oxygen could diffuse into the blood vessels supplying the organ.
The swim bladder of fishes is similarly a pocket, growing out from the endodermal wall of the alimentary canal posterior to the branchial region. In many fishes the cavity of the swim bladder remains permanently in communication with the esophagus, and air can be taken into the swim bladder through this canal.
It is fairly obvious that the unpaired lung of the lungfish Neoceratodus is the same organ as a swim bladder, except that it is adapted to respiration. Whether the rudiment of the swim bladder may be compared to a pharyngeal pouch has not been investigated.
The Liver in Vertebrates:
The liver in all vertebrates develops from the endodermal epithelium on the ventral side of the duodenum. In amphibians, the site of liver development is the anterior wall of the liver diverticulum. At the stage when the main parts of the alimentary canal begin to take shape, the anterior wall of the liver diverticulum bulges forward, so that the slit-like cavity of the diverticulum enlarges locally.
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This pocket-like enlargement of the gut cavity is the primary hepatic cavity. At the time when this occurs, in frogs, the liver diverticulum is still in communication with the space between the endoderm and the mesoderm. Next, the front wall of the hepatic rudiment becomes thrown into folds, which occlude most of the primary hepatic cavity, leaving only the most posterior part open.
At the same time, the original communication between the primary hepatic cavity and the duodenum is constricted and is gradually transformed into the bile duct. Simultaneously, the opening into the sub-mesodermal space becomes closed by a sheet of cuboidal endodermal epithelium, and the adjoining posterior remnant of the primary hepatic cavity becomes the cavity of the gallbladder.
The epithelial folds of the anterior wall of the liver rudiment soon break up into strands of cells, which for a short time may appear as tubules, with their lumen opening into the remainder of the primary hepatic cavity and thus also communicating with the rudiment of the bile duct and the gallbladder. The strands of liver cells become interwoven with blood vessels and sinuses produced by ramifications of the vitelline veins.
In the embryos of amniotes, the structure of the gut in early stages is very different from that of amphibians, owing to the absence of yolk in the endoderm. The liver rudiment accordingly also has a different appearance, but it develops in a corresponding position, namely, in the ventral wall of the gut, posterior to the section which gives rise to the stomach.
The first visible rudiment of the liver can be found as a pocket-like evagination on the anterior intestinal portal, at a stage when the opening from the yolk sac into the definitive gut is still quite wide. As the floor of the gut continues to close in an anteroposterior direction, the liver rudiment is later found some distance in front of the anterior intestinal portal, well within the foregut.
The endodermal cells then begin migrating forward from the original pocket-like evagination in the form of solid strands or cords of cells. The strands of liver cells form a meshwork and enclose the blood vessels, the vitelline (omphalomesenteric) veins and their ramifications, lying in the region posterior to the heart. From an interaction between the cords of liver cells and the blood vessels, the complicated structure of the adult liver eventually emerges.
The gallbladder is formed as a secondary hollow outgrowth at the posterior edge of the original hepatic rudiment. The liver increases in size very rapidly and soon becomes a large and massive organ, although the part of the duodenal wall from which it develops is relatively a very small one and even of that a large proportion is used up for the formation of the gallbladder and cystic duct.
The Pancreas and Bursa Fabricii in Vertebrates:
The Pancreas:
The pancreas develops from two rudiments, a ventral rudiment (or two ventral rudiments) and a dorsal one. The ventral rudiment develops from part of the ventral wall of the duodenum just posterior to and in close association with the rudiment of the liver. It may appear as a pocket-like evagination and soon becomes subdivided distally into a system of proliferating epithelial tubules. The original evagination becomes the duct of the ventral pancreas.
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The dorsal pancreatic rudiment in amniotes is also a pocket-like evagination of the duodenum, but it appears on its dorsal side slightly in front of the liver rudiment. In frogs, the dorsal pancreas develops from a section of the roof of the mid-gut, posterior to the rudiment of the stomach. Part of the roof becomes cut out by transverse crevices reaching from the cavity of the gut right into the sub-mesodermal space above the gut.
At the same time, that part of the gut wall which is destined to become the pancreas makes an abortive attempt to form a pocket-like evagination, but eventually the dorsal pancreatic rudiment in amphibians, as well as the ventral rudiment, is a solid mass which becomes transformed into a system of alveoli and ducts secondarily by rearrangement of cells.
The dorsal and ventral pancreatic rudiments may remain completely independent throughout life, as in the dogfish, but in amphibians and amniotes the two rudiments approach each other and fuse completely. The system of pancreatic ducts becomes reorganized in later life and is very variable in different vertebrates.
Bursa Fabricii:
In birds a dorsal diverticulum of the cloaca becomes the bursa fabricii, which is the source of antibody-producing plasma cells.