Paper - The bi-lobed form of the ventral pancreas in mammals

The bi-lobed form of the ventral pancreas in mammals. Am. J. Anat., 12. 389-400.

Lewis FT. The bi-lobed form of the ventral pancreas in mammals. (1911) Amer. J Anat. 12(3): 389-400.

Lewis FT. The bi-lobed form of the ventral pancreas in mammals. (1911) Amer. J Anat. 12(3): 389-400.

THE BI-LOBED FORM OF THE VENTRAL PANCREAS IN MAMMALS

FREDERIC T. LEWIS Harvard Medical School, Boston, Mass.

TWELVE fiGURES

Among some pig embryos sectioned for class use, the Writer, in 1907, found one in which the intestine was nearly encircled by pancreatic tissue. This unusual condition Was due to the presence of a large left lobe of the Ventralpancreas. The specimen was referred to Dr. Thyng for further study, and he described it briefly in vol. 7 of this Journal (1908, p. 493). He suggested that the annular pancreas of man may have a similar origin. Quite apart from this possibility, which Will be considered later, the condition in the pig is of interest because of its frequent occurrence, and because it represents a more symmetrical development of the ventral pancreas than is normal. The great variation in the ventral pancreas, even in its earliest stages, renders its description difficult, and accounts for the conflicting results of careful investigations.

It is generally agreed that as soon as the ventral pancreas becomes an easily recognized structure, it is a median ventral outgrowth from the ductus choledochus near its intestinal orifice. Preceding this stage, it is said to be represented by a pair of knoblike epithelial buds, situated one on either side of the hepatic diverticulum or primitive ductus choledochus. The paired stage is shown in fig. 1, from a pig embryo of 3.6 mm. It has not been previously recorded for the pig, but corresponds closely With the condition found in a sheep embryo of 4.5 mm. by Stoss and clearly figured for the same species by Choronshitzky. It has been observed in the rabbit, rat, guinea-pig and cat by Helly, and has been‘ figured by Debeyre in a human embryo of 4.5 mm. In Debeyre’s specimen, however, the buds are ill-defined.

In a pig embryo of 4.5 mm. (fig. 3), the ventral pancreas is a median mass of cells in the angle between the duetus choledochus and the intestine. On the left side, this mass connects with a lateral outgrowth (ac) which extends through six sections, terminating below in the section figured. On the right side, there is a corresponding outgrowth extending through four sections, but it is separated by one section from the median mass. If these lateral outgrowths correspond to the buds shown in fig. 1, it appears that the median mass has arisen in the interval between the buds, on the lower or posterior side of the duetus choledoehus. Frequently the median mass has been described as continuing upward on either side of the bile-duct, producing symmetrical lateral swellings. This condition was found by J ankelowitz in a human embryo of 4.9 mm. It was observed by Brachet iI1 rabbit embryos of ten and one-half to eleven and one-half days and he described it as follows:

“The ventral pancreas is formed at the expense of a semicircular fold embracing the posterior circumference of the duetus choledochus and the adjacent parts of its lateral walls It really represents two outgrowths which have fused.”

Hammar, in the following year (1897), described the ventral pancreas of the rabbit in the same way. He wrote:

“ It appears on the duetus choledochus as a thickening or outpocketing which semicircularly embraces the caudal and lateral surfaces of the duct. As soon as it projects freely, it appears as a single caudally directed outgrowth of the duetus elioledochus.” But he concluded that the description of the ventral pancreas as paired was “not applicable to the rabbit, dog and probably other mammals.” i ‘

Choronshitzky (1900) described this condition in sheep embryos in almost the same terms, yet he regarded the ventral pancreas as. paired.

“The caudal or ventral wall of the duetus choledochus, between the two ventral pancreatic buds, may be semicircularly thickened or outpocketed so that, to a certain extent, a connection is formed between these buds. . . . . In the next stage we see a single Ventral pancreas which has arisen through the confluence of the two ventral pancreatic buds.”

Helly (1901), after studying rabbit, rat, guinea—pig and eat embryos, concluded that the ventral pancreas is paired, but that its parts do not fuse. He writes as follows:

“The Ventral buds develop from the lateral walls of the ductus choledochus. They arise quite separate from one another; in

figs. 1 and 2 Transverse sections of a pig embryo of 3.6 mm. (Harvard EmbryoIogical Collection, Series 1406, Sections 119 and 128). X 75 diam.

fig. 3 Transverse section of a pig embryo of 4.5 mm. (H. E. (3., Ser. 1404, Sect. 104). X 75 diam. a, b, buds which give rise to the ventral pancreas, P. v. c, (1, plates which give rise to the dorsal pancreas, P. d. 2;, lateral proliferation.

no case could I observe a fusion between them. The left ventral bud undergoes degeneration.”

In the Harvard Collection, there is rabbit embryo of eleven and one—half days (5.6 mm.) which appears to accord with Helly’s description, since the median mass is directly continuous with a swelling on the right side of the bile duct, and the swelling on the left side is smaller and perhaps free from the Ventral pancreas. But in two other rabbits of the same age, this relation is not seen; in them, the ventral pancreas appears to be exactly median, and it encroaches little, if at all, upon the sides of the bile duct.

over Helly’s interpretation is not applicable to the pig embryo shown in fig. 3.

While the ventral pancreas of the pig is passing through its transient paired stage, the dorsal pancreas is also represented by a pair of epithelial prolifcrations (fig. 2, c, d). These are platelike thickenings, and not round buds as in the case of the ventral pancreas. The plate on the right soon becomes larger than that on the left (fig. 3). At this stage (4.5 mm.), the dorsal wall of the intestine has become thickened and forms a part of the dorsal pancreas. Later, as in the 6.0 mm. embryo shown in fig. 11, the entire structure is an asymmetrical bi-lobed mass swinging toward the right. In embryos of 10 and 12 mm., the bi-lobed condition is generally clearly seen where the duct enters the duodenum (fig. 8), but anteriorly these lobes are lost in the compact mass which turns toward the right (fig. 7, P. d.).

The bi-lobed form of the dorsal pancreas of pig embryos has been recognized by Wlassow and Volker, but its paired origin has not been previously recorded. In the sheep, however, Stoss observed the parallel development of the dorsal and ventral pancreases, both of which he described as paired. Felix, Brachet, and Helly have denied the paired origin of the dorsal pancreas, which, indeed, is not apparent in the rabbit, or in other mammals in which the dorsal margin of the intestine is thickened from the first. But in pig embryos the parallelism is strikingly shown. Both the dorsal and ventral pancreases pass through a transient paired stage; both become median structures which are more or less bi-lobed, and both develop predominantly on the right side.

The ventral pancreas in pig embryos may extend wholly toward the right side, as seen in figs. 4 and 6, from embryos of 5.1 mm. and 10 to 12 mm. respectively. In the younger specimen, the upper part of the pancreas is subdivided by a groove into leftanterior and right—posterior divisions. If these lobes are due to the fusion of the primary buds, it is evident that the right bud has produced the greater part of the ventral pancreas. In the older specimen (fig. 6), there is no trace of a left lobe. But in other cases, as shown in figs. 5 and 7, from embryos of 6.0 mm. and 10 to 12 mm. respectively, both lobes are well defined; in both figs. 4 to 7 Models of the ventral pancreas in pig embryos. X 120 diam. fig. 4, 5.1 mm. (H. E. C., Ser. 1409). fig. .5, 6.0 mm. (H. E. (3., Ser. 918). figs. 6and7, '10to12'mm. D. ch.,ductuscho1edoohus. Duo., duodenum. Int., intestine. L. 03., right lobe of the ventral pancreas. L. 3., left lobe of the ventral pancreas. P. d., dorsal pmereas. Pr. 1)., ventral process of the dorsal pancreas.

cases the left lobe is longer but more slender than the right. The four models just described explain the contradictory conclusions of Wlassow and Volker. Wlassow states that “the ventral pancreas is Very distinctly two—lobed.” Volker finds that “in its origin it is not two—lobed, since it bends at once to the right.” Both forms were found by Hilton, who modelled the ventral pancreas in two embryos, the ages of which he estimated at 17% and 20 days respectively. Presumably they measured between 6 and 7 mm. Since the bi—lobed form occurred in the younger specimen, Hilton suggested that it might be an earlier stage.

The student collection at the Harvard Medical School, which includes serial sections of 150 pig embryos measuring from 10 to 12 mm., affords an unusual opportunity for determining the frequency of the bi—lobed form. In seventeen specimens (11.3 per cent), the left lobe is well developed. In seven additional cases, a small wing or projection from the main mass suggests a rudimentary left lobe. VVhen present, the left lobe varies in position. Usually it crosses the root of the ventral Inesentery, and it may terminate near the dorsal pancreas (fig. 8). In three embryos, the left lobe descends along the mesenteric attachment, and in one of these it terminates within the mesentery (fig. 9). F requently a nodule of cells, or a small cyst, is seen on the hepatic side of the mesentery (as shown at :3 in fig. 10). Because of the possibility that these may have been detached from the left lobe of the ventral pancreas, they were carefully examined.

Among 100 embryos, the nodules or cysts were found in 51 specimens. Often two or three occur in a single embryo. . They are generally located close to the peritoneal epithelium and frequently they are found at the summit of a connective tissue elevation, in fig. l(). This elevation does not always coincide with the line of mesenteric attachment. The nodules may occur at any point along the peritoneal covering of the gall bladder, and, near its tip, where the peritoneum invests it on all sides, they are found toward the ventral body wall. Occasionally they are seen deep within the connective tissue layer which surrounds the gall bladder, and often they are found in actual connection with the hepatic trabeculae at the sides of this layer. Some of them thereTHE VENTRAL PANCREAS 395

fore arise from the trabeculae, and others are detached outgrowths of the cystic duct or gall bladder, with which two of them were seen to be connected} After becoming detached, they may migrate along the peritoneum to the elevation where they are frequently found.

Although most of the nodules and cysts are of hepatic origin, it is still possible that some of them are derived from the pancreas. In two specimens, nodules indistinguishable in structure from that shown in fig. 10 were found on the intestinal side of the mesentery. It is quite improbable that these came from the liver. The largest cyst observed, which is shown in fig. 12, occurred in a 20 mm. embryo. It occupies the same position as the nodule in fig. 10. In structure, it closely resembles the mesenteric cyst found in another pig embryo of 20 mm., which was figured by Lewis and Thyng in Vol. 7 of this Journal (p. 509). The mesenteric cyst was probably derived from an accessory pancreas, and it is possible that the cyst shown in fig. 12 is of pancreatic origin. That the left lobe of the ventral pancreas may extend across the ventral mesentery to the hepatic side, is shown in fig. 11, from an embryo of 6.0 mm. Moreover, a constriction suggests that the terminal part of this lobe may become detached. In an embryo of 8.0 mm., VVlassow found the left lobe extending still further across the mesentery, as seen in fig. 3 of his publication. Although corresponding conditions have apparently not been observed in the adult pig, it is of interest to note that in two cats, Heuer found lobes of the pancreas extending along the cystic and common bile ducts to the gall bladder.

At the Ithaca meeting of the American Association of Anatomists, Dresbach demonstrated a very interesting specimen of ‘pancreatic bladder’ in the adult cat. In such cases, which have been reported by Mayer, Gage, and Miller, the ductus choledochus is accompanied by another duct, which terminates in an expansion resembling the gall bladder, to which it is closely applied. Moreover its microscopic structure, as found by Dresbach, is

‘In addition to the slender outgrowths referred to, the gall bladder presents rounded outpocketings, which are being studied in this laboratory by Dr. H. Bernstein.

figs. 8 and 9 Transverse sections of pig embryos of '10 to 12’ mm. X 75 diam. V. f., gallbladder. V. 1)., portal vein. V. u-m., umbilical Vein. For other abbreviations, see figs 4 to 7.

“essentially like that of the gall bladder and its duet.” Thus the cases suggest an extreme subdivision of the gall bladder and hepatic diverticulum; but Miller is doubtless correct in correlating them with embryonic conditions in the pancreas. He considers that they are degenerate left lobes of the Ventral pancreas, terminating in cysts, and having little or no pancreatic tissue along their ducts.

‘When the left lobe tends to encircle the intestine, instead of extending into the Ventral mesentery, it gives rise to a Very different anomaly—the annular pancreas. It may fairly be said that the embryo shown in fig. 8 already possesses such a pancreas.

figs. 10 to 12. Transverse sections of pig embryos. X 75 diam. fig. 10, 10 to 12’ mm. fig. 11, 6.0 mm. E. C., Ser. 1705, Sect. 252). fig. 12.20.0mm. (H. E. (3., Ser. 59, Sect. 977). P. 1)., appendage of the left lobe of the ventral

pancreas. :0, epithelial nodule or cyt. For other abbreviations, see figs. 4to 7.

although at this stage the circuit of the intestine is not quite complete. Baldwin has recently reviewed the cases reported in man, to which he adds another. In explaining the anomaly, he assumes that “ordinarily the left half of the ventral pancreas atrophies,” and concludes that the anomaly is due, either to a persistence of the left half, or to an excessive growth of the right half, which takes place ventral to the duodenum and extends to the left. He does not decide between these two possibilities. Since then, Miss Cords has reported another case. She does not refer to right and left halves, or lobes, but considers that in her

case the entire ventral pancreas has grown around the duodenum to the left. Fortunately she has published excellent figures of the ducts, which show clearly the embryonic relations, since there is no anastomosis between the ducts of the dorsal and ventral pancreases. The duct of the ventral pancreas is seen to bifurcate. Its right branch, soon subdivided, corresponds to the normal right lobe. Its long left branch, which has led to the anomaly, corresponds closely to the left lobe which we have described in the pig embryo (figs. 7 and 8). It is perhaps more like one of the pig embryos not figured, in which the right lobe is rudimentary and the median stem appears continuous with the left lobe.

Lecco, who studied the annular pancreas in the adult and sought in human embryos for an explanation of the anomaly, justly criticizes Miss Cords’s work as follows:

“A glance at the familiar figures in Keibel and Elze’s N or'mentafel, as well as the examination of the embryos placed at my disposal, shows what a great distance separates the two pancreases, making a fusion of the two, ventral to the duodenum, seem highly improbable.”

He adds that in Baldwin’s paper he finds no evidence for either of the explanations proposed by Baldwin (cited above). The evidence, indeed, can be found only in abnormal embryos having annular pancreases in process of development. Baldwin apparently did not examine embryos, and Lecco studied only normal ones; but in certain of the pig embryos described above, the anomaly is almost complete, and it is due to a left lobe of the Ventral pancreas.

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