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Grosser O. Lewis FT. and McMurrich JP. The Development of the Digestive Tract and of the Organs of Respiration. (1912) chapter 17, vol. 2, in Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.

XVII. The Development of the Digestive Tract and of the Organs of Respiration: Introduction | Early Entodermal Tract | Mouth and Its Organs | Oesophagus | Stomach | Small Intestine | Large Intestine | Literature | Liver | Pancreas | Pharynx and its Derivatives | Respiratory Apparatus | Figures | Literature
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Frederick Thomas Lewis
Frederick Thomas Lewis (1875-1951)

Development of the Pancreas

By Frederic T. Lewis.

Historical Note

The main duct of the human pancreas, figured by Wirsung in 1642, opens into the duodenum in common with the bile-duct. The regular occurrence of an independent accessory duct, opening into the duodenum somewhat nearer the pylorus, was recognized by Santorini in 1775. Meckel (1817) observed this accessory duct in several embryos. It was situated above and to the left of the bile-duct. Meckel mistook it for the only duct of the pancreas, and concluded, therefore, that " the bile and pancreatic ducts at first are quite separate from one another, but gradually they come together and unite." Kolliker (1879), in describing a rabbit embryo, stated that the pancreas is divisible into " two distinct glands which perhaps should be interpreted as an upper and a lowed' pancreas, such as are found in the chick." Not until 1888 was the similar condition observed in a human embryo. Phisalix then recorded that in a 10 mm. specimen the pancreas is represented by two separate outgrowths,—" one, superior and larger, the duct of which will become the accessory duct; the other, inferior and smaller, which corresponds with the canal of Wirsung." The upper gland is now known as the dorsal pancreas and the lower one as the ventral pancreas.

Early Development

The two pancreases arise almost simultaneously shortly after the formation of the hepatic diverticulum, but, from the first, the dorsal pancreas is the larger. Both have been found in embryos of 3 and 4 mm. The failure of Fol (1884) to record a ventral pancreas at 5.6 mm., Mall (1891) at 7 mm., and Janosik (1909) at 6.1 mm., must be attributed to imperfect description or to abnormal embryos. But the fact that Volker (1903) in a 3 mm. specimen, and Keibel and Elze (1908) in an embryo of 4 mm., describe only a dorsal pancreas, may indicate that the dorsal pancreas arises first. Bremer (1906), however, found only a ventral pancreas at 4 mm., represented by two knobs of intestinal epithelium, one immediately below the hepatic diverticulum (Fig. 288, D), and the other nearer the yolk-stalk. It is doubtful whether these intestinal outgrowths represent a nQrmal stage in pancreatic development.

The dorsal pancreas is at first a "stomach-like" enlargement of the digestive tube. It is somewhat flattened laterally, and has a convex dorsal border which merges anteriorly with that of the intestine. Posteriorly the transition is more abrupt. The posterior part of the dorsal pancreas at 4.9 mm. is shown in Fig. 307, A. At 7.5 mm. (Fig. 307, B) the dorsal pancreas is separated from the intestine by a slight constriction, and the notch on the lower side is characteristically deeper than on the upper side. At 9.4 mm. (Fig. 308) the constricted part is prolonged into a short duct. The distal portion has also elongated, and its surface presents nodular swellings, which are the beginnings of branches.

The ventral pancreas in its early stages is lodged in the inferior angle formed by the hepatic diverticulum and the intestine. It is a small epithelial mass, continuous above with the diverticulum, and uniting dorsally with the intestinal epithelium. This condition is seen in a series of four models made by Keibel and Elze, descriptions of which have not been published. They are from embryos of 4, ca. 4, 5.3, and 6.75 mm. respectively. The same condition is shown in Fig. 307, in embryos of 4.9 and 7.5 mm. In the latter the lower part of the ventral pancreas has become free from the duodenum. Subsequently it becomes entirely separate from the intestine, as in the 6.8 mm. embryo modelled by Piper (1900) and in the 8 mm. specimen modelled by Felix (1892). With the elongation of the bile-duct it becomes widely separated from the duodenum, as in the 9.4 mm. embryo (Fig. 308).

Hamburger (1892) found that the ventral pancreas in an embryo of 4 weeks; had an independent opening into the duodenum, and concluded that its common outlet with the bile-duct is formed later. This has not been confirmed. However, in the Bremer embryo the ventral pancreas may arise directly from the intestine, and in a 3 mm. specimen, according to Keibel and Elze, the ventral pancreas is an outpocketing found " just caudal to the bile-duct." A pair of ventral pancreases are found in many vertebrates, and have been reported in a human embiwo of 4.5 mm. (Debeyre, 1909). Felix (1S92) recorded that in a section of the upper part of the pancreas at 8 mm., the lumen was toward the right side of the epithelial mass. He considered that the lumen belonged with a right ventral pancreas which had fused with a left ventral pancreas, and that the latter was rep resented by the solid left portion of the section. Jankelowitz (1S95), in a 4.9 mm. embiwo, found a single lumen below, which bifurcated above, sending its branches respectively to the right and left sides of the hepatic diverticulum. He considered that this indicated a fusion of right and left constituents. Ingalls (1907) studied the same specimen, and states that he is inclined to agree with Jankelowitz, although there is " only a suggestion of the paired condition." Keibel and Elze (1908) again examined this specimen, and they state that "it is very questionable whether two outgrowths are present; to us there appears to be only one."

File:Keibel Mall 2 307.jpg

Fig. 307. — A, lateral view of the liver and pancreas of a 4.9 mm. embryo, from a model by N. W. Ingalls. X 65 diam. B, similar view of a model in which the hepatic trabeculse are not included, from a 7.5 mm. embryo. Modelled by F. W. Thyng. X 50 diam. D. chol., common bile-duct ; D. cyst., cystic duct ; D. hep., hepatic duct ; Div. hep\, hepatic diverticulum ; Ga., stomach ; Pane, d., dorsal pancreas ; Pane, v., ventral pancreas ; TV., trabecular ; Ves. fel. (vesica fellea), gall-bladder ; x, aberrant duct.

A double lumen is often seen in the ventral pancreas of older embryos (6.8 mm., Piper; 7.5 mm., etc.), but this condition, as noted by Helly, may be observed also in the impaired gall-bladder. It is associated with the formation of a lumen in a solid cord of cells.

Helly (1901 j and Kollmann (1907) have figured a pair of ventral pancreases which have not fused. In Kollmann's 7.5 mm. embryo they are cranial and caudal hi position, with the common hepatic duct between them. In Helly's 11 mm. embryo they are right and left. The left is much smaller and contains no distinct lumen. Helly believes that it degenerates without fusing with the righl pancreas, and that " in embryos scarcely older than four weeks it has wholly disappeared." In other embryos between 7.5 and 11 mm. in length, including seven specimens in the Harvard Collection, the ventral pancreas appears as a single outgrowth. Moreover, a well-defined pair is not recorded in Keibel and Elze's extensive series. Therefore the specimens described by Helly and Kollmann are properly regarded as exceptional.

A paired dorsal pancreas, such as Stoss described in sheep ernbryos, has been sought for in man, but has not been found (Felix, Helly).

Relative Position of the Dorsal and Ventral Outgrowths

Although the dorsal pancreas of most mammals enters the duodenum on the distal side of the bile-duct, that of man is normally on the proximal side, toward the pylorus, at all stages of development. In early stages the caudal border of the duct of the dorsal pancreas may be at a lower level than the cranial border of the hepatic diverticulum, as seen in Fig. 307 and in Keibel and Elze's models of the pancreas at 4 mm. Later there is an interval between them which varies in extent. Thus, from Keibel and Elze's model of a 5.3 mm. specimen the distance is found to be only 0.05 mm., whereas in a 5 mm. embryo figured by Tandler (1903) it is approximately 0.2 mm. The distance has increased to 0.5 mm. in a 22.8 mm. embryo (0.7 mm. in a 15 mm. specimen, Swaen), and in the adult, according to Letulle and Nattan-Larrier (1898), who examined 21 cases, it varies from 10 to 35 mm. (20 to 40 mm., Charpy, 1898).

Migrations of the dorsal pancreas in relation to the bile-duct have been described. His (1885) figured the dorsal pancreas on the pyloric side of the bileduct in embryos of 5.7 and 10 mm., but at 11.5 mm. he placed it opposite the bile-duct, and at 1*2.5 and 13.S mm. it is shown on the caudal side. This would necessitate a return to the pyloric side in subsequent stages. Thyng (1908) examined 18 embryos from 7.5 to 24 mm. in length, and failed to rind a single) instance of the caudal position figured by His. Janosik (1805 and 1909) and Volker (1902 and 1903) have held that the dorsal pancreas arises on the distal side of the bile-duct, and that it migrates anteriorly. This is denied by Helly (1904). Janosik's reconstructions (1909) begin with an embryo of 6.1 nun., in which the dorsal pancreas connects with the intestine " a little more distally than the bile-duct." This embryo, however, must be considered abnormal, since it shows "no trace of a ventral pancreas." In the next stage figured (8.7 mm.) the ducts are opposite. Doubtless the duct of the dorsal pancreas may occasionally open into the intestine caudal to the common bile-duct, as is the ease in an embryo of 11.5 mm in the Harvard Collection. Here, however, thei'e is an abnormal persistence of the adjacent portion of the right omphalomesenteric vein.

Union of the Dorsal and Ventral Pancreases

With the elongation of the bile-duct, which bends dorsally on the right side of the intestine, the ventral pancreas is brought into close relation with the dorsal pancreas (Fig. 308). Subsequently the ramifications of the two pancreases interlock, as shown in Fig. 309, from an embryo of 22.8 mm. In the model represented in the figure, the horizontal body and tail of the pancreas have been cut away at x. The pancreas at that point bends downward, forming the head of the organ, which in the adult terminates below in the uncinate process. The ventral pancreas forms a part of the head and more or less of the uncinate process; the dorsal pancreas forms the remainder of these parts, together with the entire body and tail.

File:Keibel Mall 2 308.jpg

Fig. 308. — Model of a part of the liver and the pancreas of a 9.4 mm. embryo (Harvard Collection, Series 1005). X 50 diam. D., duodenum; D.c, common bile-duct; D.cyst., cystic duct; D.h. hepatic duct; P. d., dorsal pancreas; P. v., ventral pancreas; Tr., hepatic trabeculae; V. /., gall-bladder.

File:Keibel Mall 2 309.jpg

Fig. 309. — Model of the head of the pancreas of a 22.8 mm. embryo (Harvard Collection, Series 871). X 50 diam. D. chol., common bile-duct; D. pane, d., duct of the dorsal pancreas; D. pane, v., duct of the ventral pancreas; Duo. duodenum; Pane, d., dorsal pancreas; Pane. v. ventral pancreas; x and y are explained in the text.

At 22.8 mm. the duct of the dorsal pancreas is a round stein, which passes into a flattened, plate-like duct, strongly curved upon itself. The cleft leading into its concavity is shown at y in Fig. 309. The convex surface of the flattened portion of the duct is beset with nodular branches, radiating in all directions. Distally the main duct again becomes round, and it may be followed as an axial structure through the tail of the gland. The duct of the ventral pancreas arises from the common bile-duct at some distance from the duodenum. It passes to the centre of a group of ramifications which nearly equal it in diameter. In this embryo, and in specimens of 14.5 and 16 mm., no connection could be found between the two pancreases. Moreover, the branches of either pancreas rarely anastomose among themselves.

File:Keibel Mall 2 310.jpg

Fig. 310. — Corrosion preparation of the pancreatic ducts of an adult. Prepared by Dr. S. T. Mixter. X 1/^diam. D. pane, d., duct of the dorsal pancreas; D. pane, v., duct of the ventral pancreas; x, anastomosis between the pancreatic ducts.

In the adult the normal relations of the two glands are shown in the corrosion preparation, Fig. 310. In comparing this with Fig. 309, it will be noted that the duct of the dorsal pancreas appears to open into the duodenum at a higher level in Fig. 310 than in Fig. 309. This is associated with a shifting of the duodenum; in both cases the dorsal pancreas opens nearer the stomach than the ventral. In the adult the duct of the dorsal pancreas, shortly before entering the duodenum, receives a large branch which passes upward from the uncinate process. This branch is in front of the duct of the ventral pancreas. The latter, as in the embryo, lies at a deeper level, and is on the right side of the axis of the dorsal pancreas. The duct of the ventral pancreas forms a single large anastomosis with the duct of the dorsal pancreas, which is shown at x in Fig. 310. The continuous channel formed by the distal part of the duct of the dorsal pancreas, the anastomosis, and the duct of the ventral pancreas constitutes the "pancreatic duct" of the adult; the proximal part of the duct of the dorsal pancreas is the "accessory duct." According to Hamburger (1892) the anastomosis between the dorsal and ventral pancreases has formed in a "six weeks' embryo." In a reconstruction of this specimen he shows that the distal end of an unbranched ventral pancreas has fused with the dorsal pancreas, which has a nodular surface but no branches. In a 14 mm. embryo Keibel and Elze (1908) found the pancreases ' ' close together but not yet united. ' ' This is the largest specimen in their series in which the pancreases are separate, and an embryo of 12.4 mm. is the smallest in which they have united. At 14 and 15 mm. they are generally described as "fused." In such embryos the tubules interlock, and it requires a careful study of drawings of successive sections to determine whether there is a passage between the ventral and dorsal ducts. Ordinarily only a single anastomosis is produced, but Bernard (1856), in an abnormal adult specimen, has shown two connections. Charpy (1898) has found that the duct of the ventral pancreas may enter the dorsal duct at any point in its wall, — that is, on its superior, inferior, anterior, or posterior surface. Usually it appears to enter on the inferior surface. In one of the specimens figured by Charpy, the main duct draining the uncinate process passes upward to join the duct of the dorsal pancreas behind the duct of the ventral pancreas, instead of in front of it as in Fig. 310. This arrangement is abnormal and difficult to explain. Hasse (1908) has shown the normal relation of these ducts in the adult, but his inferences regarding their development are incorrect.

Vessels and Nerves

The dorsal pancreas in early stages is lodged between the right and left omphalomesenteric veins. These vessels form a transverse anastomosis immediately caudal to the pancreas, and branch abundantly around it (see Ingalls, 1908, pi. 2). Later, as described in Chapter XVIII, portions of these veins give rise to the portal vein. The vena ported reaches the inferior border of the pancreas in the notch between the body and head; it then passes behind the dorsal pancreas and curves forward, with the bile-duct, to enter the liver. In a 10 mm. embryo (Phisalix) the dorsal and ventral pancreases are completely separated by the portal vein; at 16 mm. (Fig. 311) they have come together and have partly surrounded the vein. The splenic branch of the portal vein develops early, and may be recognized in a 9.4 mm. embryo. It passes along the dorsal surface of the tail of the pancreas, which it drains. Some of its branches, and its opening into the portal vein, are indicated in Fig. 311.

Although the pancreas is at first in close relation with the portal vein, it does not give rise to a portal or sinusoidal circulation, and thus it differs strikingly from the adjacent liver. Its afferent blood supply is from the splenic and hepatic branches of the cceliac artery, and from small branches of the superior mesenteric artery as it accompanies the portal vein across the inferior border of the pancreas (Fig. 311). At 37 and 42 mm. the pancreatico-duodenal arteries form a loop, which connects the hepatic and superior mesenteric arteries and supplies the head of the pancreas.

In a 42 mm. embryo lymphatic vessels are abundant in the connective tissue around the pancreas, but they do not extend among the tubules. Lymph-glands have not developed. They are present in close relation with the pancreas in a 99 mm. embryo, but they may arise in some much younger stage.

File:Keibel Mall 2 311.jpg

Fig. 311. — Section through the stomach, pancreas, and a part of the liver, from an embryo of 16 mm. (Harvard Collection, Series 1322). y>X 40 diam. ; ,A. mes. sup., superior mesenteric artery; tB. oment., omental bursa ;'_Gaster, stomach; Lien, spleen; V. p., portal vein. (Other labels as in preceding figures.)

At 42 mm. the coeliac plexus of nerves sends branches toward the head of the pancreas, and some of them extend among the pancreatic tubules. Within the pancreas there are a few conspicuous groups of nuclei which, from their association with nerve fibres, are presumably ganglionic.

The Outlets of the Ducts

In entering the duodenum the common bile-duct passes obliquely through the duodenal musculature, and is directed caudally. In its transit across the muscle, in embryos between 20 and 40 mm., it is joined by the duct of the ventral pancreas, and the pancreatic duct is always on its lower or caudal side (Figs. 309 and 311). At 22.8 mm there is still no duodenal papilla at the outlet of the bile-duet, but Helly (1900) states that an elevation is present at 28.5 mm. In this embryo he finds that sphincter muscles have developed around the bile and pancreatic ducts, but in specimens of 44 mm, in the Harvard Collection, the ducts are surrounded only by concentric mesencliyma and by the duodenal muscle through which they pass.

In the adult, according to Letulle and Nattan-Larrier (1898), the pancreatic duct may empty into the bile-duct, in the same way as in the embryo. More often the two ducts reach the duodenal surface independently, either at the base of an ampulla or at the summit of a nipple-like projection.

The duct of the dorsal pancreas, in embryos between 20 and 40 mm., has a longer course within the duodenal wall than the duct of the ventral pancreas. Consequently, although the branches of the ventral pancreas extend close to the outlet of its duct, they are almost entirelv outside the duodenal musculature, whereas +' 7 branches of the dorsal pancreas are regularly found in the submucosa. Helly (1900) states that an outpocketing of the dorsal duct within the wall of the duodenum is present in embryos of 12.5 and 14.5 mm. At 37 and 42 mm. distinct knobs and diverticula are present. Some of them branch and give rise to pancreatic tissue. According to Helly, this explains why true pancreatic tissue is so often found in the papilla of the dorsal duct {papilla minor) of the adult, but almost never occurs in the papilla of the ventral duct {papilla major).

The large pancreatic ducts, both within the duodenal wall and outside of it, give rise to diverticula and mucous glands. Helly has determined that they occur in both papillae of an 80 mm. specimen, as very small outpocketings. In a 90 mm. embryo, and in all later stages, he finds that the mucous glands are easily recognized.

The Development of the Alveoli

Kolliker (1861) described the pancreas of a four-weeks embryo as consisting of a simple wide and hollow duct, with branches, each of which has a lumen in its more slender proximal part, but terminates in a solid, pearshaped bud. Similar buds arise in later stages, not only in the terminal branches, but also along the sides of the main ducts. The duct from a 42 mm. embryo shown in Fig. 312, A, presents early stages in their development. At a there is a group of cells with crowded nuclei and darkly staining basal protoplasm. At b a similar group is seen at the bottom of an outpocketing of the lumen, and at c there is a larger mass which causes a basal bulging. These structures apparently give rise to the darkly staining knobs which are abundant in the 42 mm. embryo and in younger stages. Three of them, from a 55 mm. embryo, are shown in Fig. 312, B. Often they appear to be solid, but sometimes a slender lumen may be found within them, as shown at e. Considered as terminal parts of the gland, these buds may be called alveoli. They contain central cells which apparently persist as the central cells of the adult. The stalks of the alveoli become elongated, forming branches of the duct, and the alveoli subdivide. Thus in the adult, as seen in the model by Maziarsky (1902), the pyriform alveoli may be cleft nearly in two ; some of them show lateral buds. The extension of the lumen between and into the secreting cells of the alveoli, which has been shown by the Golgi method to occur in the adult (Dogiel, 1893), has not been studied embryologically.

The Development of the Islands. — The youngest human embryo in which the islands of the pancreas have been observed is a specimen of 54 mm. (Pearce, 1903). None are present in embryos of 42 and 44 mm. in the Harvard Collection, nor in the head of a pancreas at 55 mm. Weichselbaum and Kyrle (1909) find none at 50 mm. They appear first in the distal part of the pancreas. Thus, Pearce found none in the head at 90 mm., and in an embryo "believed to be of the third month . . . numerous islands are scattered through the tail and body, while for the first time a few are seen in the head." Krister (1904) found them larger and more numerous in the splenic end in an embryo of the 17th week, and this accords with Opie's conclusion that in the adult the islands are almost twice as numerous in sections from the tail as in those from other parts.

File:Keibel Mall 2 312.jpg

Fig. 312. — Sections of pancreatic tubules; A, from an embryo of 42 mm. (Harvard Collection, Series 838) ; B, from an embryo of 55 mm. X 350 diam. a-e , early stages in the formation of alveoli.

The islands in an embryo of 99 mm. (Fig. 313, b) already resemble those of the adult. In sections stained with haematoxylin and eosin, they appear as pale areas, composed of anastomosing solid cords or rows of cells. Capillary blood-vessels extend among the cords, and their endothelium comes into close relation with the cells of the island. The presence of epithelial stalks connecting the islands with the ducts, as shown on the right of Fig. 313, has been observed by Pearce, Kiister. and Weichselbaum and Kyrle.

The general structure of the islands, and of the glomeruli which they contain, is well shown in a model prepared by Miss Dewitt. from the pancreas of an adult. Miss Dewitt (1906) failed to find any arteries connecting with the vessels of the islands, contrary to Laguesse (1900) and others. She regards the blood-vessels of the islands as venous, " with abundant capillary connections with the surrounding interalveolar capillary plexus," and describes them as sinusoids. In their development they are quite different from the portal sinusoids of the liver, but they resemble them histologically. Laguesse has described groups of red corpuscles as occurring normally between the endothelium and the cells of the islands.

An earlier stage of the islands than that shown in Fig. 313 has been described by Pearce. In a 54 mm. embryo he found them represented by small groups of from ten to fifteen cells, directly connected with the sides of the ducts. He described them as having round and lightly staining nuclei, with relatively abundant protoplasm which stains deeply with eosin. At this stage the islands are not penetrated by blood-vessels. Weichselbaum and Kyrle describe the islands in an 80 mm. embryo as solid buds, directly connected with the ducts, but composed of paler cells.

File:Keibel Mall 2 313.jpg

Fig. 313. — Section of the pancreas from a 99 mm. embryo. X 350 diam. a, pancreatic tubules; b, islands; c, blood-vessel; d, alveolar bud.

They are partly surrounded by capillaries. The difficulty of distinguishing these developing islands from the alveolar buds is apparent in sections and in the published figures. Laguesse admits this difficulty, but he concludes that certain formations in sheep embryos, apparently comparable with those seen in Fig. 312, are the first stages in the development of the islands.

In later stages the islands become detached from the epithelial tubes. Some of them have separated in an embryo of 130 mm. (Weichselbaum and Kyrle). Von Hansemann (1910) found them all detached at 210 mm., and concluded that the islands arise from mesenchyma ; but in another embryo of the same length, and also at birth, Weichselbaum and Kyrle found that some islands are still in connection with the ducts. They believe that new islands may arise throughout life by budding from the ducts. The detached stalks of the older islands may be recognized in the small dark cells which have been described at the periphery of certain islands. Occasionally they show a lumen, and pathologically they may give rise to retention cysts (Weichselbaum and Kyrle). Kiister, in embryos of 24 and 32 weeks, has found stalks ending blindly in the islands.

Usually the islands are considered to possess " an anatomical identity as definite as that of the glomeruli of the kidney" (Opie), but some believe that alveoli may be transformed into islands and islands into alveoli. A review of the literature of this subject is presented by Laguesse (1906). Their embryological development does not accord with the idea that they represent a phase of glandular activity, and the presence of mitotic figures indicates that they are not degenerative structures.

The Pancreas at Birth

In sections of embryos from 270 to 320 mm. in length, Weichselbaum and Kyrle find that the groups of alveoli are not only more compact than in earlier stages, but there is no longer such abundant connective tissue between them (cf. Fig. 313). Nevertheless the pancreatic connective tissue at birth, as compared with that in the adult, is relatively very abundant. It extends around individual alveoli, and forms broad septa between the clusters which are connected with the terminal ramifications of the ducts. These groups of alveoli, bounded by connective-tissue septa, become compact in the adult and constitute the lobules, which are ill-defined and may show secondary subdivisions.

"Islands are more numerous, as pointed out by Kasahara, in the pancreatic tissue of the fetus and of very young children than in that of the adult. . . . The organ being much smaller in the fetus, the same number of islands, though themselves smaller, are closer together and therefore appear more numerous in sections" (Opie). Kiister likewise found that, in relation to the alveoli, the islands are decidedly more numerous at birth than in the adult. His measurements show no difference in the size of the islands, but Miss Dewitt finds that they are smaller, on the average, at birth than in the adult.

Concerning the position of the islands, Opie states that, "though an island is often situated in the centre of a more or less clearly defined lobule, no constancy of position is discoverable." Pearce considers that the islands at first lie free in the connective tissue, but that later, in the fifth and sixth months, "glandular elements surround and inclose the island, and it then occupies the centre of the lobule." But, as noted by Weichselbaum and Kyrle, here and there, at birth, an island occurs at the periphery of a lobule, or in the interlobular connective tissue near a duct. Moreover several may be found within a single lobule.

The cells of the islands at birth lack distinct outlines; thev are crowded with fine granules which do not react to osmic acid (Stangl, 1901). Stangl states that fat appears in the cells of the islands at the end of the first year, but this has been denied by Symmers (1909). The cells of the alveoli at birth show the zones characteristic of the adult. In the outer zone they sometimes contain small scattered fat drops (Stangl). Histologically no differences have been established between the dorsal and ventral pancreases, at birth or in preceding stages.

Anomalies and Variations

The obliteration of the proximal end of the duct of the dorsal pancreas is probably not infrequent.

Charpy (1898) found the papilla minor closed in three-fourths of the thirty eases which he examined. Letulle and Nattan-Larrier (1898) state that the accessory duct is permeable throughout its extent, including the papilla minor, in only three out of twenty-one cases examined, and that usually it appears as a branch of the pancreatic duct. But Helly (1898) concludes that an open accessoiy duct is "by far the rule" (compare with Fig. 311), and Hamburger found it present in all of the fifty cases which he examined. In an embryo of 55 mm. Helly (1900) found, in place of a single papilla minor, two papillae of nearly equal size,, each of which contained a pancreatic duct. Letulle and Nattan-Larrier have recorded a similar case in the adult. The failure of the dorsal and ventral pancreases to unite, so that the duct of the dorsal pancreas persists as the main duct, opening at a normally situated papilla minor, has been figured by Charpy, and reported by Helly, Baldwin (1907), and others. In one of Helly's cases "an independent duct of Wirsung was not to be found." In a specimen figured by Bernard the duct of the dorsal pancreas persists as the main duct, although it has two small anastomoses with the duct of the ventral pancreas. As recorded in a previous section, the dorsal pancreas in an abnormal embryo of 11.5 mm. opens into the intestine lower down than the bile-duct. Charpy has figured an adult pancreas which shows this relation ; in this case the. ducts have not anastomosed.

Pancreatic tissue sometimes surrounds such adjacent structures as the portal vein, the bile-duct, and the intestine.

The encircling of the portal vein by a process of the dorsal pancreas has apparently not been observed in man, though characteristic of the rabbit and pig (Thyng). The common bile-duct occupies a groove in the head of the pancreas which is frequently converted into a canal of pancreatic tissue (see Helly, 1898). An annular pancreas, encircling the intestine, has been recorded by Ecker and by Symington (as cited by Thyng) and a case has been reported by Baldwin. An abnormal condition observed in a pig embryo of 12 mm. suggests that this anomaly may arise early in development, by an extension of the ventral pancreas dorsally on either side of the intestine.

Accessory pancreases are of frequent occurrence.

Among 150 autopsies, Symmers found three cases in which there were accessory glands of considerable size. Sometimes two occur in a single case (Opie, 1903). Gardiner (1907), who has reviewed the literature, finds that in nearly a third of the cases reported, the accessory pancreases are connected with the stomach. They occur also in the duodenum, jejunum, and ileum, and have been frequently found at the apex of a " true " diverticulum. These diverticula were at first considered to be remnants of the vitelline duct (Meckel's diverticula), but Neumann (1870) questioned this interpretation. Nauwerck (1893) reported a diverticulum 9 cm. long, tipped with an accessory pancreas, and situated 2.3 metres above the valve of the colon. In the same case he found another diverticulum, 3 cm. long, situated 80 cm. above the valve of the colon, and he regarded the latter as Meckel's diverticulum. Hanau (Brunner, 1S99) reports a duodenal diverticulum tipped with an accessory pancreas, and Weichselbaum (Gardiner, 1907) has described a similar pancreatic diverticulum of the stomach. It is evident that these diverticula are not vitelline remains, and yet it is not impossible that an accessory pancreas may be associated with a true Meckel's diverticulum. Wright (1901) has reported a case in which pancreatic tissue was excised from the umbilicus of a child of 12 years, who had an umbilical fistula since birth. The fistulous tract had apparently become separated from the intestine.

Accessory pancreases generally penetrate the muscularis, but they may be limited to the submucosa. The larger ones show lobules composed of typical pancreatic alveoli. Islands have been reported in numerous cases, including that of Wright. Sometimes, however, the islands are lacking, and the tubules may be duct-like rather than glandular.

The accessory pancreases develop from elongated epithelial buds, as observed in the wall of the stomach of a 19 mm. entbryo. Lewis and Thyng (1908) have frequently found similar buds along the intestine of pig embryos of 10-20 mm., but not in human embryos. They usually become detached and degenerate. It is possible that accessory pancreases sometimes develop in relation with the embryonic intestinal diverticula described in a previous section.


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