the epithelial islets of the pancreas in...

EPITHELIAL ISLETS 0¥ THB PANCREAS IN TELEOSTEI. 379

The Epithelial Islets of the Pancreas in Teleostei.By

John Keiuiie, D.Sc, F.R.M.S.,Assistant in Zoology, Aberdeen University.

With Plates 2G, 27, & 28.

Introductory and Historical.

THE question of the anatomical and functional natureof the islet-like groups of cells occurring within the pancreasof vertebrate animals has been studied by a large number ofinvestigators since attention was first directed to them byLangerhans in 1869. These inquiries have for the most partbeen confined to the higher vertebrates, and summaries oftheir results have already been given by other writers(Laguesse, 1894; Oppel, 1900). Notwithstanding the some-what extensive literature of the subject, there is so muchdisagreement as to the real nature of these bodies that furtherinquiry was desirable. Oppel (13) wrote in 1900, "Was dieBedeutung der intertubularen Zellhaufen anlangt, so istdieselbe, so viel auch daruber geschrieben wurde und so vieleAnsichten auch daruber bestehen, noch nicht ganz erklart."

Although much has been written regarding these cell-Tlie present research has been carried out in several laboratories and upon

material obtained from various sources. Cordial thanks are due to Prof.Mclntosh, F.R.S., for the valuable privilege of the use of the Gatl.y MarineLaboratory, St. Andrews, and to Professors Heincke and Ehrenbaum, ofHeligoland, not only for the free use of the laboratory there, with its abundantsupply of material, but also for their friendly treatment during my stay in theinstitute under their charge. I acknowledge also the assistance of a grantfrom the Carnegie Trustees in defraying part of the cost of material and pre-paration for publication of this research.

380 JOHN RENN1E.

groups, it appears that very little has been done in theinvestigation of the lower vertebrates. Indeed, until a shorttime ago, there appears to have been some doubt as to theirexistence in cold-blooded animals. According to Laguesse(7), " Les ilots de Langerhans paraissent constants chez lesMammiferes et les Oiseaux, leur existence est douteuse chezles Vertebres inferieurs. . . . Les auteurs ne les signalent pasen general chez les Vertebres inferieurs; Lewaschew les acherches et ne les a jamais trouves chez les animaux a sangfroid; Harris and Gow ne les ont pas vus chez les Reptiles,mais pretendent les apercevoir chez la grenouille ; Von Ebnerles y a decrits egalement. Enfin, quelques points des descrip-tions d'Ogata et de Platner pourraient s'interpreter en faveurde leur existence." No reference is made to their possibleoccurrence in fishes. Indeed, Harris and Gow, whomLaguesse quotes, expressly state that in consequence of theirdoubt as to the nature of the so-called pancreas in fishesthey did not investigate the group. In the following year,however, Laguesse recorded the existence of cell-islets in thepancreas of Crenilabrus; in 1898, Massari (12) describedthemintheeel (Anguilla vu lgar i s ) ; and in 1899 Diamare(3), in an important paper, established their occurrence in sixdifferent species of Teleostei, as well as in all the other verte-brate divisions. I shall have occasion later on to refer to thework and opinions of these writers.

In the investigation of which the present paper is a partialaccount attention has been limited to the bony fishes, of whichabout twenty-five different species have been studied. Ingeneral these islets are fewer in number and proportionatelylarger than in mammals. Owing to the diffuse condition of thepancreas in most Teleostei, they may, even when minute, beidentified by the unaided eye. They are usually of a palecolour,and, being somewhat thicker, are seen distinctly againstthe more translucent sheet of the surrounding pancreaticalveoli. Wherever a careful search was made, either macro-or microscopically, these bodies were found, and hence it mayreasonably be concluded they constitute a common feature of

EPITHELIAL ISLETS Otf THE PANCREAS IN TELEOSTEI. 381

this group. This is all the more probable since they appearto possess some functional activity.

As a result oE pursuing specially the study of the con-ditions in bony fishes, I have, inter alia, discovered theexistence of a "principal islet" (15). This has enabled meto offer a fresh suggestion as to the possible phylogeneticsignificance of these bodies in higher animals. I have, further,been able to confirm the opinion of Massari, Diamare, andothers regarding them as ductless glands with internal secre-tory function, and also to test experimentally the theory thatderangement of the function of these bodies leads to diabetes.A record of these experiments, which, are still iu progress,will appear later.

Names and Systematic Arrangement of theSpecies Examined.

Teleostei.

Physostomi:Cyprinidas—Cyprinus carpio.

Physoclys t i :Acanthopteri:

Scombriforines—Zeus faber, Agonus cata-phractus , Lophius pisca-torius, Cottus scorpius.

Gobiiformes—Cyclopterus lumpus, Callio-nymus lyra, Cyclogas terMontagui.

Bleniiformos—Anarrhichas lupus, Zoarcesviviparus, Pholis gunnel -lus, Chirolophis galer i ta .

Auacanthini:Gadidas—G-adus virens, G. asglifinus, etc.,

Onos mustela.Ophidiidse—Ammodytes tobianus.Pleuronectidas — Hippoglossus vulgaris ,

Pleuronectes platessa, etc.

382 JOHN RENNIB.

Lophobranch i i :Syngnathus acus, Nerophis sequorcus,

Siphonostoma t.yphle.

General Relations of the Islets.

The following account indicates the general relations andmacroscopic appearance of the bodies observed. In mostinstances, particularly in those cases where a " principalislet" is stated to exist, numerous specimens were examined.

Cyprinus carpio.—The islets observed in this speciesare among the smallest found. The pancreas is diffuse, andthey appear in sections of it in different regions of the body-cavity. In some instances they lie alongside the zymogenoustissue, but in most instances they are surrounded by it, and donot possess a limiting capsule.

Zeus faber.—Here thereexists a " principal islet," which,in specimens of about 25 cm., is as large as 5 mm. in length.There are also smaller forms in the neighbourhood of thepylorio cteca which may be dissected out, and also numerousmicroscopic ones within the interctecal pancreatic masses.The principal islet lies within a small mass of zymogenoustissue, which is attached to the base of the gall-bladder (seePI. 26, fig. 1). It and the smaller ones near the casca were foundovoid in form. The intercsecal examples which are investedby more compact masses of zymogenous tissue, are rounded,oval, or irregular in outline. In serial sections they are seento vai*y a good deal iu this respect, owing to their beingclosely surrounded by the irregularly arranged pancreaticalveoli. The large forms have a more or less distinct limitingcapsule; such a structure is not present in the smaller oneswithin the compact masses of the pancreas.

Agonus cataphractus.—In this species, occupying aposition between the gall-bladder and the spleen, within asmall mass of pancreatic tissue, is the principal islet. Asimilar body occurs close to the pyloric caeca. Both arecontained within a firm connective-tissue capsule.

EPITHELIAL ISLETS OP THE PANCREAS IN TELEOSTEI. 383

Lophius piscatorius.—In this species the number ofislets which can be seen in a naked-eye examination is rela-tively large. The pancreas is diffuse, lying for the most partin the mesenteric area between the intestine anteriorly andthe spleen. When this area is spread out (see PL 27,fig. 2), theislets, being opaque, may be easily observed. The " principal"lies a short distance in front of the spleen, several others arescattered between the cystic duct and the intestine. Abouthalf-way along this duct, between it and the intestine, thereoccurs with great frequency a fairly large islet, and, asalready noted (15), several near the pylorus, amongst whichis the second largest in size in this species. In appearancethey are most frequently quite white, sometimes the minutevessels on their surface show as fine red streaks, and atothers they are so distended with blood as to give the organ adark ruddy hue. This variable appearance doubtless hassome relation to the fact that re t ia mi rab i l i a are numerousin and ai'ound the organ. The principal islet is in largespecimens frequently of relatively great size. It is flattened,circular as a rule in outline, and ellipsoid in vertical section.In one case it measured 14 mm. in diameter, and about 5 mm.across its thickest part. It not infrequently in adults is aslarge as the supra-renal of a rabbit. The islet near thepylorus I have found 8 mm. in diameter. They are sur-rounded by a loose capsule of areolar tissue.

Cot tus scorpius.—The pancreas in this species is in theform of narrow bands adhering to the intestine, andoccupying the intercascal spaces. One of these bands liesnear the spleen. Immediately above this organ the principalislet may be seen with great distinctness even in smallspecimens as a pale, somewhat angular mass faintly streakedwith blood-vessels. The portal vein passes close to it, andthe main branches of the cceliaco-rnesenteric artery passventralwards a little distance in front. In a specimen 22 cm.long the islet measured 3 mm. in diameter.

Cyclopterus lumpus.—Here there is a principal isletsituated slightly anterior to the spleen. Its position and

VOL. 4 8 , PAET 3 . NEW SEH1ES. 28

384 JOHN KENN1E.

relations are very similar to those described for Cot tusscorpius . It is very pale and of relatively large size; iularge specimens it is about 1 cm. in length. The amountof pancreatic tissue around it is very slight.

Cal l ionymus lyra.—Several islets have been observedin this species, the largest—the principal—lying close to theportal vein on the right side of the fish where that vesselenters the liver. As in other cases, ifc is slightly anterior tothe spleen, and in the same portion of the mesentery. Theothers are all in the same region, but lie nearer the intestine;they are whitish in appearance and very small.

A n a r r h i c h a s lupus.—Besides a principal islet, severalothers—never a large number—have been observed in theanterior region of the abdominal cavity. Except in the caseof the principal, constantly occurring forms or largeexamples were not made out. • The principal is usuallyovoid in shape; in specimens of about 40 cm. its longestdiameter is 9 mm. It is usually of a pale red colour,and lies in a thin sheet of pancreas in a portion of themesentery well forward under the right lobe of the liver,and quite close to the mesenteric artery, from which vesselit is very easily injected.

Phol i s gunnellus.—The situation of the principal isletin this species has already been indicated iu my preliminarynote. Further, in sections of the abdominal viscera in thisregion may be seen a fair-sized islet close to the intestineat the pylorus, a common position for these bodies.

Chirolophis galer i ta .—In a position very similar to thatof the principal islet in Pho l i s there is an islet in thisspecies. Only two examples were examined, and it wasfound in both. It is well forward under the right lobe of theliver, between it and the stomach, near the portal vein. Itwas found oval in form, enclosed in a firm capsule, and waseasily separated from the surrounding tissue. In a specimen15 cm. long it measured 2 mm.

Gadus virens.—Islets are situated in the interceecalpancreatic tissue. They do not appear to be very numerous,


but some are of fairly large size; they are circular, elliptical,or irregular in outline. They do not possess a speciallimiting capsule, but are surrounded by the ordinary con-nective tissue of the pancreatic alveoli.

G-adus Eeglefiuus.—The islets here occupy the sameposition as in the preceding species. Their relations to thezymogenous elements are also similar.

Cyclogas ter Moutagui.—Four specimens of this smallspecies were examined, and in a position corresponding tothat of the principal, an islet was in each case found. It is,of course, very minute, but may be found on the right sideof the fish slightly anterior to the spleen and near to thepyloric casca.

Zoarces v iv iparus .—In this species the principal isletoccupies a position similar to that in P h o l i s gunne l l u swithin the triangular area already referred to, which isslightly larger than in the related genus; the islet has avariable position, lying in some cases close to the hepaticartery and in others lower down in the angle between themesenteric artery and the portal vein. There are present, iusome instances at least, one or two smaller islets nearerthe gut and within the area bounded by the vessels alreadynamed.

The pancreas is of the commonest type, viz. diffuse, and issometimes greatly obscured by the presence of fatty tissue.Hence, although the islets are definitely separated from thezymogenous elements by a firm capsule, they may be more orless concealed by this tissue, and not so readily observed asin other instances. They are ovoid or spherical, and inmedium sized adults the larger is about 2 mm. in length.The capsule is usually pigrnented.

Onos mustela.—Islets exist within the pancreatic tissuewhich is found alongside the pyloric cteca. I noted inparticular a large example of elongated irregular outline.The islets are in close relation to the ordinary pancreatictissue, and do not have any special limiting capsule.

Ammodytes t obi an us.—The pancreas here is of the

386 JOHN EENNIE.

extended type, stretching the whole length of the intestine intwo narrow bands, a condition which is common in smallslender bodied fishes. A principal islet was not found bydissection, but on sectioning the entire viscera in the usualregion of its occurrence several fairly large islets of irregularoutline were found (fig. 6).

Hippoglossusvulgar is ,Pleuronectespla tessa , etc.—In the Pleuronectidse examined the position of the principalislet is the same, and they may therefore in this section bereferred to collectively. It is the same as in Zeus, viz.within a small pancreatic mass attached to the gall-bladder.In the larger forms, e. g. Hippoglossus, it is very apparentas an ovoid mass of a ruddy colour. Here the pancreaticinvestment is slight. In smaller forms the outline of theislet is not so apparent, being masked by the surroundingzymogenous tissue. Smaller islets exist in other parts of thepancreas; thus in both Hippoglossus and in Pleuronectesa fairly large one occurs with very great frequency, if notwith absolute regularity,, near the origin of the pyloric c£eca.

Syngnathus acus, Nerophis asquoreus, Syphonos-toma typhle.—In these Lophobranchs the pancreas consistsmainly of two well-defined bands following the blood-vesselsalongside the gut. On one of these portions where theportal vein crosses to the liver (the usual position, in fact)the principal islet occurs as a small, somewhat flattened, ovoidbody, whitish in colour, slightly pigmented, and about 1 mm.in length in adult specimens. Its position in the threespecies is identical; it is found most readily by dissectionfrom the right side, lying between the mesenteric artery andthe portal vein (fig. 3).

In my previous paper I enumerated certain species in whicha principal islet existed, and in the foregoing statement suchparticulars of its position in the several instances are givenas will enable its occurrence to be verified. In the presentpaper additional examples are quoted, and it is more thanprobable that the list could be extended. For, after thegeneral relations of the body became known, I failed to find

EPITHELIAL ISLETS OP THE PANOEEAS IN TELBOSTEI. 387

it in few fresh species, and these were of small size; where itis liable to be missed when sought for by macroscopic methods.It will be seen that the position is practically the same in allcases. It lies a short distance in front of the spleen in themesenteric fold between the portal vein and the mesentericartery. In a certain number this islet, though still ina position agreeing generally with the foregoing, has a relationwhich enables its situation to be even more exactly stated.It is in close proximity to the gall-bladder, and has theappearance of a compact nodule attached to its base orposterior wall.

In the following species—Agon us oataphractus,Lophius piscator ius , Pholis gunnel lus , Anarrh i -chas lupus, Zoarces v iv iparus , Onos mustela ,Ammodytes tobianus , Hippoglossus vulgaris , andPleuronectes platessa—I found an islet at the pylorus.It was always smaller in size than the " principal." This isthe one referred to in my preliminary note. Although I can-not at present say that it is constant, its presence here in somany different species is of interest and an indication at anyrate that this is a common position. In one particular instance,Lophius piscatorius, I endeavoured to obtain some evidenceon this question of constantly occurring islets. This species isthe one in which I have found the largest number of thesebodies. I examined many hundreds of specimens and I noted—

1. That the "principal," the largest islet (see Pi. 27, fig. 2),was present in every case.

2. That an islet at the pylorus (PI. 27, fig. 2, Is. 1), and theislet marked " Is. 2 " occurred very frequently, although theywere not found in every case.

3. That there seemed to be considerable variation in thenumbers of the others.

These facts are in complete accordance with the suggestionI have already made. Here constant and varying isletsexist, whose relations may be compared to those of thyroidand accessory thyroids or spleen and accessory spleens.1

1 In this connection it is worthy of note that I found accessor; spleens of

388 JOHN RENNIE.

The principal islet, where it exists, is on this view to beregarded as a distinct organ, the others as supplementarybodies of similar function. In those cases, viz. certainfishes and all higher animals, where it is presumably absentthis organ in the course of phylogeny has disappeared andthe supplementary bodies have increased in numbers andimportance.

Histology.

The material made use of for histological purposes wasfixed immediately after death, in either corrosive sublimateor Bles's fluid. After washing (in the case of the sublimate)and dehydration, the tissue was embedded in paraffin and cutserially with a Cambridge rocking microtome. The sectionswere stained with hfematoxylin and eosin and examined undera Beck microscope possessing a y - oil immersion objectiveas well as lower powers.

In agreement witli the results of other investigators, thetissue of these islets was found to stain a lighter tintthan the surrounding pancreatic alveoli. Even when smallthey are very noticeable as paler areas in the pancreaticmass; they are frequently, however, more massive than theadjacent organ, from which they are definitely separated bya capsule. They are an epithelial tissue consisting ofvery small polyhedral or cylindrical cells well supplied withblood-capillaries. A common size of cell is about 10 fx acrossthe narrow diameter.

In a number of instances there was noted a difference inthe staining capacity of different areas within the islets.This feature has been noted by other observers, particularlyMassari (12) and Diamare (3). The latter has interpreted itas indicating a difference in functional state of differentparts of the islet; he regards such appearances as the accom-paniment of different phases of the same kind of cell.

fairly frequent occurrence in Lophius. These appear to be common also inthe skate, where two or three minute examples may be seen in a single fish.

EPITHELIAL ISLETS OF THE PANCREAS IN TELEOSTEI. 389

Although some of the material examined by me might beinterpreted similarly (e.g.in Zeus faber) in several instances,of which I give a detailed account, the contrast in size,form, structure, arrangement, and relation to the capillariesof the cells of the two regions of the islet, as well as theirdifferent staining capacities, appear to me so marked thatI am not prepared to accept Diamare's explanation assatisfactory.

In the descriptions which follow I have not considered itnecessary to detail the appearances in each of the speciesalready enumerated; I have selected such as together illus-trate fully the essential structure of these bodies as a whole.

Zeus faber.—In this species, apart from a slightly lessintimate relation of islet and acini in the case of the principalas compared with the others, the histological structure ofthese bodies is similar in every respect. They are fairlynumerous in the intercascal pancreatic masses, where they lieclose to the zymogenous tissue, and are without any specialcapsule. Some are of distinctly irregular outline. Theyconsist of polyhedral cells, smaller than the cells of thepancreas; their walls are well defined, and the cytoplasmexhibits a delicate meshwork of fibrils. The nuclei areirregularly circular or oval, and show a large nucleoluswhich always stained a dark red. Chromatin net-knots werealways observed, as also the nuclear membrane. Apart fromsize and the possession of a capsule of areolar tissue whichmay send in supporting partitions, the principal islet exhibitsthe same structure as the others. The pancreatic acini,however, in the examples sectioned, did not completely en-velop the islet.

The differently staining patches already spoken of wereusually observable in preparations of islets from this species.Examination with high powers showed that this difference isdue mainly, though not altogether, to the staining capacityof the nuclei in the respective areas. The chromatin ismore abundant in the nuclei which stain more darkly. Atthe same time, the cytoplasm of these cells appears to take

390 JOHN RENNIE.

up a deeper tint than the cells of the lighter areas, thoughno structural differences were made out in this region of thecell. Capillaries are only fairly numerous in this type(fig. 4).

Phol i s gunnellus.—The two islets referred to in anearlier part of this paper possess the same structure, exceptthat the principal has a very definite capsule not apparent inthe other, and its relation to the pancreas is less intimate.The structure is similar to that described for Zeus, but thecells are smaller and more cylindrical. Cell-walls are lesseasily seen, but are present. In parts the capillaries run innearly parallel rows, and the cells are arranged in bandsbetween. The cytoplasm shows a delicate network, which, incontrast to that visible in the neighbouring acini, is veryfine. Differently staining areas, as noted in Zeus, were notobserved.

A n a r r h i c h a s lupus.—The principal islet is a largebody with a well-defined capsule which sends in numeroussupporting partitions (fig. 5). It is surrounded by a verythin band of zymogenous tissue, from which portions pene-trate within the islet. This feature is not uncommon in thecase of large islets in fishes; Diatnare noted it in some of thespecies examined by him, and I have met with severalinstances, as will be seen. The tissue of the islet proper pre-sented similar features to that seen in Zeus. Light anddark areas were observable, and they were respectivelytraceable through series of sections. In the main the nucleiof the lightly staining areas were seen to be irregular inoutline, while those of the dark areas were circular or oval.There was also a difference in size, the former beinglarger.

In Onos mustela a large islet of very irregular out-line was examined. It occurred in a mass of pancreatictissue adherent to the intestine at the pylorus. In somesections it appeared as two separate bodies, but examinationof the series showed a connection. This islet had no limitingcapsule, it was invested by the connective tissue of the

EPITHELIAL ISLP1TS OF THE PAN0REA.S IN TELEOSTGI. 391

surrounding zymogenous elements, which were here massedand not spread out in a thin sheet as is more common insmall fishes. Here, again, darkly and lightly staining patcViesof irregular outline were present, whose cells exhibiteddifferences in arrangement and structure. No cell-wallswere seen in the case of the darkly staining elements; but,from the very close arrangement of the nuclei, if walls werepresent the cells must have been of an elongated fusiformtype. These cells are arranged in bands between thecapillaries, and one could count half a dozen nuclei on anaverage in a row across a band between two capillaries. Nonetwork could be made out in the cytoplasm ; the nucleardetails were similar to those already given. These bandswere not so definite in some parts as in others, and the arrange-ment resembles more the irregular grouping characteristicof the lighter staining areas of the islet. Although this wasthe case, it was noticeable that the nuclei of the irregularlyarranged dark cells were more numerous than those of thelight and also more crowded together. The lightly stainingpatches showed a more open appearance owing to the cellsbeing larger; they were seen to be polyhedral in form. Thenuclei did not differ much from those of the dark areas (fig.13). In Cyclopterus lumpus the tissue of the principalislet is in every way similar to that here described. There is,however, a capsule around the body, outside of which a slightlayer of pancreatic tissue is present.

Lophius piscatorius.—The islets are surrounded by acapsule of rather open areolar tissue. In those examined Iobserved no indication of an arrangement of the cells inbands as is apparent in many species; the tissue was quiteuniform. Capillaries appeared abundant, and in the con-nective tissue around the islet as well as within it I notedthe presence of retia mirabilia upon the vessels (fig. 10).As many as four were seen in a single section; the com-ponent vessels had distinct walls, and were united by asurrounding and interlacing connective tissue. The cells ofthe islet are of the usual polyhedral type, with cytoplasm

392 JOHN EENNIB.

granular or fibrillate, the nuclei with distinct nucleolus andchromatin network. Distinctively dark and light areaswere not observed.

Zoarces viviparus.—As may be seen from fig. 11, therelation between pancreas and principal islet is extremelyslight. This is one of the cases where, were it not from theknown relations as revealed in other species, it would bedifficult to relate the body to its proper category. It issurrounded by a fairly thick capsule, upon which there is adeposition of pigment. I sectioned one throughout its wholelength, and found that it was penetrated by no large blood-vessels, though capillaries were abundant. The greater partoE the space within the capsule is occupied by bands ofdarkly staining cells. The parts between these bauds, whichwind irregularly, are occupied by cells which stain lightlyand are of different form from the others. The columns ofdarkly staining cells are more richly supplied with capillariesthan are the cells occupying the spaces between. Theformer are narrow, cylindrical, or fusiform, with very finelygranular contents, and measure about 10 fi across the narrowdiameter. The nucleus is oval, and stains very darkly,being filled with numerous minute chromatin granules. Itfrequently almost fills the width of the cell, and is about 9 /j..The cells of the lightly staining areas are irregularly poly-hedral in form, their cytoplasm shows a network of Gbi-ilsrather than granules, while the nucleus has an irregularoutline and contains fewer chromatin granules than that ofthe darkly staining form (tig. 12).

Ammodytes tobianus .— Besides small islets, two fairlylarge examples of irregular outline were found in this species.They were all completely enveloped in pancreatic tissue, anddid not possess a limiting capsule. The capillaries werevery abundant, so much so that in many parts they wereseparated from each other by the width of only a single cell.In consequence of this arrangement the cells appeared incolumns in certain parts, they stained more darkly than therest of the islet, and were of different form (fig. 6). The


contrast between the two was in fact marked. The darklystaining cells were columnar, with finely granular cytoplasm ;nuclei almost uniformly oval with a distinct nucleolus andgranular chrornatin. The lighter staining cells were irregu-larly polygonal, their cytoplasm not so granular, and theirnuclei of very irregular outline. A few of these are givenin fig. 7. A freqnent form is that with a deep cleftbetween two portions. There are fine fibrils of chromatinwith net-knots and also distinct nucleolus visible. Thecontrast here is very different from the appearance in Zeus ,and I do not think can reasonably be attributed to differencesin functional state of the same kind o£ cell. They appear tome to constitute two interlacing tissues.

P l eu ronec t e s p la tessa (figs. 8 and 9).—The principalislet lies within a small mass of pancreas which is attached tothe gall-bladder. The greater part of this mass, which is thatspoken of by Cole and Johnstone (1) as a "little nodularswelling," consists of islet; the zymogenous tissue forms asmall envelope around it. There is a definite capsule, but Idid not see any supporting trabeculaa. It is usually pene-trated by zymogenous elements, and where this is so, con-nective tissue surrounds these and separates them from theislet tissue. This penetration by pancreatic tissue is a featurewhich has already been noted in other instances, and, as inthose cases, it was here traceable as continuous with the sameelements around the capsule. The components of the islet arevery small cells richly supplied with capillaries. The capil-laries are not equally distributed throughout, but are moreabundant in the inner regions. In these parts the cellsoccur in columns or strands having a somewhat sinuousarrangement; they evidently in many instances surround andfollow the course of the capillaries. These cells stain darkly.In the spaces between these winding strands, and also inother parts of the islet, where, as already indicated, thecapillaries are not so numerous, cells, lightly stainiDg, aremassed. The arrangement is quite different, but besides adifference in staining capacity and relation to the capillaries,

394 JOHN EENNIB.

certain structural differences were made out with highpowers.

1. The form of the cell. In the darkly staining strandsthis was seen to be always more or less columnar, and wasprobably due to their position and arrangement between thecapillaries. The cell-walls could not be made out clearly.The lightly staining cells are irregularly polyhedral, their•walls could usually be traced with distinctness, and they areevidently larger than the columnar types.

2. The appearance of the cytoplasm. In the columnartypes this appeared diffusely and finely granular. The lightcells showed a fairly open network of fibrils.

3. The nuclei. In the columnar cells these appearedregularly ovoid or spherical, and smaller than those of thepolyhedral type. These latter were very variable and veryirregular in outline. Both kinds showed nucleoli and net-knots, the polyhedral cells showing these very clearly.Regarding those nuclear difference's, I do not think they canbe attributed to fixatives, since the irregular forms were foundonly among the polyhedral cells.

In Hippog lossus v u l g a r i s the conditions are similar tothose described for P leuronec tes .

In the Gadidse only small islets were examined, and inthese no special features were observed. The cells were allof one type, and similar to those of the lightly staining areasof those forms exhibiting two types. The islets were com-pletely surrounded by pancreatic tissue, and no capsule waspresent.

Lophobranchii.—Examples of three different genera ofthis group were examined. A principal islet only was found,and this was sectioned in each case. There is a distinctlimiting capsnle, which may be pigmented, of a differenttissue from that supporting the pancx-eatic acini. These latterare quite apart from the islet, and in fact are more closelyassociated with the mesenteric blood-vessels than with thisbody. Capillaries are numerous, but I did not find the cellsarranged in columns around these, nor did I find two types of


cell. The elements were small, and resembled the lightlystaining or polyhedral forms. They presented in their finerstructure no features of a special nature.

The conditions observed in Anar rh icas , Onos, Cyclo-p te rus , Zoarces, Ammodytes , P l eu ronec te s , and Hip-poglossus , it will be noted, are suggestive of a double tissuewithin the islet. No such appearance is observable in the otherforms examined. Massari describes a two-fold tissue inAngui l la , distinguishing the two kinds of cell as "chromato-phile" and " acbromatophile." Diamare has sought to refutethis view, his interpretation, as already indicated, being thatthe differences seen are indications of different functionalstates in the two regions, and that there is one tissue only.

It is satisfactory to note that on the point at issue there isa remarkable agreement even in some matters of minute detailas to the actual conditions. Diatnare's paper did not reachme until after my own observations were made, and althoughthe species examined were not the same, and his methods offixation and staining were different and more varied thaumine, all the appearances noted by him are to be seen on myown preparations. The question is largely a matter of inter-pretation of results.

The facts are briefly these. Tracts of more or lesscolumnar or fusiform cells wind through the islets, andaround and between these are slightly larger polyhedral cellsarranged in masses. The columns stnin more darkly than themasses. Diamare speaks of tracts showing intermediatestaining which force the suspicion that they do not representtwo different categories of cells. I myself found islets inwhich the contrast in the two types was less marked than inothers, but even here it is deserving of notice that the columnswere always darker than the masses. If a difference offunctional state be indicated by these appearances, we expectthe columns will at some t ime show the lighter stainingeffect, and also that the polyhedral cells will be found in thedarker phase. Such conditions were not found by me.Diamare, further, makes much of the fact that the one type

396 JOHN EBNNIE.

Could be seen to be continuous with the other, sometimes adark column merged with a light, or dark cells occurred onone side of a capillary and light ones on the other. I am notsure that there is much in this, but in any case he does notappear to have observed the contrast in grouping of the twosets as noted by me, e. g. in P l eu ronec t e s , nor the relativedistribution of the capillaries of the two regions. In thisspecies 1 found the capillaries more abundant in the innerregions, and here the columnar cells were most noticeable,while the other type occupied along with the capillaries thespaces between and also the surrounding areas. In Zoarcesthe arrangement suggested by the grouping was that ofcolumns of cells with an interstitial tissue. It is true that thecolumnar arrangement is due probably to the abundance ofthe capillaries, whose course the cells follow; but if they areall one tissue I have been unfortunate, as also has Massari, inseeing preparations which exhibited the columnar cells in onephase only, and that different from the rest of the islet.Diamare himself compares the appearance of an islet inMotel la to the supra-renals of birds, where the cortical andmedullary substances interlace.

I examined some very small islets in Ammodytes andPleuronec tes where thei'e was only alimited number of cellsvisible in a section. They were so small that a difference offunctional state between different parts was scarcely to belooked for, and yet the two types were apparent (fig. 6, 8m.is, and Is. 3).

Although I incline to the view that we have here twodistinct tissues, from a consideration of the fact that in manyspecies this double nature is not evident, I do not think theyare likely to be of independent secreting function. The darkcells appear to stand in a relation intermediate to thecapillaries on the one hand and the light-cells on the other.It may be that they regulate the supply to the capillaries ofthe substance secreted by the light-cells, or they may effect afinal stage in its elaboration.


Relation of the Islets to the Pancreatic Acini.

From the foregoing account it will be seen that in bonyfishes these islets, though undoubtedly existing under con-ditions similar to those met with in higher animals, also verycommonly occur in distinctly less intimate morphologicalrelation to the pancreatic alveoli. It will have been notedthat various conditions have been observed, from that wherethe tissue of the islet stands in the same relation to the zymo-genons elements as the separate alveoli of the latter do toeach other (figs. 4 and 6) to cases where a thick investingcapsule exists around a large islet with no alveoli in contact,and only a very few of these in the surrounding parts, e. g.Syngnathus (fig. 3). Indeed, my attention in the firstinstance was confined to these latter bodies in such fishes asPholis gunnellus and Syngnathus acus, where theordinary panci'eatic tissue is in no more intimate relation tothem than it is to the portal vein or mesenteric artery, alongwhich vessels it extends as narrow bands. . Accordingly Ihesitated to relate these distinctly encapsuled and separateglands with the pancreatic "islet" of the usual type until Ihad found in various species bodies of identical structure insituations which left no further room for doubt. Amongstthese encapsuled glands there is included the body alreadynoted as a " principal islet." Diamare, without making anyreference to the question of regular occurrence, describes thisbody in Orthagoriscus mola, Rhombus laavis, andLophius piscatorius as a pancreatic islet. Indeed, noreasonable doubt can be raised as regards the homology ofthe principal islet.

A feature observed in the islets in Hippoglossus vul-garis, Pleuronectes platessa, and Anarrhichas lupus,viz. the penetration of these by zymogenous elements, had beenpreviously noted by Diamare in other species. These elementsmay appear continuous with the pancreatic tissue outside, oras detached alveoli suiTounded by the islet. In the latter

398 JOBN EENNIE.

instances, however, they could be seen in serial sections tobe continuous with the same tissue outside (figs. 5 and 8). Ido not consider it a feature of any morphological importance.Tn all the cases "where the peculiarity was noted the pancreasis of a very diffuse type, and in the spreading of its alveoliduring development, as is well known, it may invade orbecome attached to other organs of independent function. Acommon feature is the close envelopment of the leading blood-vessels throughout the body-cavity by long strands of pan-creatic tissue, and in several instances (Syngathus, Pleuro-nectes) such tissue accompanies the vessels within the liver,ramifying with these throughout the tissue of that organ.

Pancreatic elements penetrating the islets are supported byconnective tissue, which is continuous with such tissue beyondand around the islet. On account of this Diamare has arguedthat the capsule is of the nature of interstitial tissue of thepancreas, and that here, owing to the larger development ofthese islets, it has assumed a capsular form ; and he definitelyopposes the interpretation that the tissue enclosed by thecapsule is merely joined to the pancreas, and is not an inherentportion of it. The capsule, he says, in these cases is a" secondary " formation.

The view thus contested is, from evidence already partlysubmitted and partly to follow, one that I continue to hold.In the first instance we may recall the fact just referred tothat a pancreas, intra-hepatic, exists ; and if it can invade thetissue of an ox'gan undoubtedly distinct and having einbry-onically a separate origin, there is no argument for identityin the fact that pancreas is found sometimes within anotherorgan which has its rise from the same embryonic tissue,1 andwhich we may assume is from the first in closer proximity.Further, I find the capsule is best developed where, owing to

1 According to Laguesse (9) and, more recently, Pearce (14), they Lave Hiesame embryonic origin. This does not affect the present argument, for werecall such facts as the origin of thymusand thyroid from branchial epithelium,and (according to S. Vincent) the medulla of the supra-renal from sympatheticelements.—J. It.


the extremely diffuse condition of the pancreas, interstitialtissue can scarcely be said to exist, the organ deriving supportfrom the several other organs to which it is adherent, e. g.the larger blood-vessels (Lophobrauchs). In fact, just inproportion as the pancreatic tissue has a more or less massivearrangement, the capsule is more or less indefinite. That isto say, the capsule tends to disappear where the form of thezymogenous tissue approaches most nearly the common formin higher animals. This is well seen in those fishes where thepyloric casca have their interspaces filled with pancreas. Theislets observed in such cases had no capsule. Where theyhave come to be enclosed within pancreatic elements, thenecessity for a protecting capsule has ceased to exist.

In further support of the view which regards these isletsas independent organs, two other points appear to me worthstating. In many fishes the peritoneal membrane and blood-vessels are pigmented, and in such cases so also is the capsuleof these bodies, although I have never seen any pigment laiddown within or upon pancreatic tissue. As examples may bequoted Pholis gunnellus, Zoarces viviparus. In thislast I have found several islets within pigmented capsulesin a single fish. The second point has reference to the intra-hepatic pancreas of certain fishes. If the islets are inherentportions of the pancreas in such fishes, related directly to itsfunctions as a digestive gland, and, according to Jarotsky (6),who conducted an extensive series of experiments on whitemice in dieting and fasting, " they probably supply a sub-stance or substances representing a chemical stage ofdevelopment of a ferment or substances necessary to thecells producing it," we would expect to find islets in thenot inconsiderable part of the gland placed inside the liver.I have looked carefully for islets in this region in bothSyngnathus acus and Pleuronectes platessa, andhave found none.

The view I now bring forward may be stated as follows:—Theconditions observed in various Teleostei force the conclusionthat here " islet " and pancreas are distinct organs. In certain

VOL. 4 8 , PAKT 3 . NEW SERIES. 29

400 JOHN KENNIE.

genera, e.g. Lophius, Pholis, Zoarces, Syngnathus,the "islet" tissue has no more intimate relation to pancreasthan to other neighbouring organs. Diamare, indeed, pointsout that the "islets" are glands of a more primitive typethan the pancreas, which represents an advance in the evolu-tion of organs. What he fails to appreciate is the fact thatthe more highly developed organ, in its most primit ivestate, is distinct from the still more primitive internalsecreting gland. The compact pancreas, I consider, is afurther development, in which the association of the twotissues is strengthened, so that they become virtually oneorgan, although there is no evidence but that they are still ofindependent function. This association is due to the fact thatthey arise from the same embryonic tissue. The results ofPearce (14) on the development of the islands iu the humanembryo are of interest, and their bearing on this point worthquoting. He does not agree with Laguesse and Renaut thatthe islands arise from "peculiar cells with rich eosinophilicprotoplasm, comparable to the parietal or oxyntic cells of thegastric tubules." He finds that the pancreas develops asbranching glandular processes, which become tubular later.The islets develop as side branches of these processes, and,from a careful study of the paper, I consider it clearlybrought out that the island is formed from the " branchingglandular process " before the remainder of it is trans-formed into acini. Thus the interesting point seems tobe established that "island" is an earlier formation thanacinus; that is, the phylogenetic order is paralleled inontogeny.

Certain observers, investigating the pancreas of mammals,have concluded that the islets exhibit transitional forms indi-cating a change of islet tissue into gland lobuli. Levvaschew(11) claimed that irritation caused the groups to becomemore numerous and larger, and that various transitionsbecame apparent. Statkewitsch (16) asserted that some ofthe lobuli of the pancreas underwent such changes duringfasting that they assumed the islet form. On the other hand,

EP1THEMAL ISLETS OF THE PANCREAS IN TELEOSTE1. 401

Diamare has, as the result of similar experiments, failed tofind any appearance which might be taken as representingtransitional forms, and his histological methods and resultsseem beyond reproach. Laguesse, who held the opinion thatthroughout life there was a repeated transformation of islettissue into zymogenous and vice versa, has (according toPearce), in deference to Diamare's work, in large measureabandoned this view. I have never observed any appear-ances which might be regarded as transitional, but in anycase the facts already adduced are entirely opposed to suchinterdependence as is here described. It is, indeed, quitepossible that under such unnatural conditions as those of theexperiments of Lewaschew or Statkewitsch the pancreaticlobuli underwent degeneration, or possibly reverted to thecondition of the cellular "processes" of Pearce, which,although distinct in appearance from the islets, might well bemistaken for transitional forms.

The fact is not without interest that hitherto observershave failed to find anything like epithelial blood-islets withinthe pancreas in elasmobranchs. It is possible, assuming thatthey do not exist within this organ, that their function iscarried out by certain of the other ductless glands in thesefishes. In elasmobranchs both interrenal and supra-renalglands exist, while in teleosts adrenals, regarded as corre-sponding to the interrenals, are the only forms. May not oneor other of the glands in the former group carry on thefunction of the missing " islets " ?

The Funct ion of the I s le t s .

Amongst the later investigators there appears to be agree-ment concerning the functional nature of these bodies. Theyare regarded as blood-glands with internal secretion. This isthe opinion of Laguesse, who until recently held the some-what peculiar view that they are alternately " endocrineislets" and " esocrine glands," the change being repeatedduring life. Other investigators, e. g. Dogiel (2), have held

402 JOHN RENNIE.

them to be functionless effete portions of the pancreas, orembryonic remains. Others, again, regard them as contri-buting to the production of the pancreas secretion, e. g.G-iannelli ed Giacomini (4) and Jarotsky, already quoted.

The facts as far as observed by me seem to point clearly toan internal secretory function. These bodies are ductlessglands; they are all well supplied with capillaries, and insome cases these are very abundant. In some, structures forregulating the flow of blood through their tissues are present;and this, taken in conjunction with the different appearancesmet with in the cytoplasm and nuclei, leaves little doubt butthat they are active organs. Whether two types of cellexist in certain instances or not, the irregularly polyhedrallightly staining forms occur in all. Reviewing these, it isnoted that the nuclei occurred with regularly spherical oroval outline, and also very irregular in form. In the latterthe chromatin was not so abundant; the cytoplasm, too, wasmore open and less granular. Such like differences Diamarealso noted and correctly, I think, interpreted as indications ofdifferent functional states. They correspond, according toBaum ('Deutsch. Zeitschr. f. Thiermed. u. vergl. Pathol.'xii, 1886), with resting and active conditions respectively ofgland-cells.

In a future paper I hope to give an account of certainexperiments with extracts of these " islets," the results ofwhich, as far as at present obtained, appear to indicate thepresence in them of substances possessing some physiologicalactivity.

Conclusions .

The occurrence of epithelial islets of the pancreas is wide-spread in Teleostei.

In many of these there is an encapsuled islet (" principalislet "), of relatively large size and of constant occurrence,whose relation to the pancreatic tissue is frequently extremelyslight. In some species it was the only body of this naturefound.

EPITHELIAL ISLETS OF THE PANCREAS IN TBLEOSTEI. 408

The smaller islets, which do not appear to be constant innumber (Lophius) , it is suggested, probably originated as"accessory bodies," but are now established as definite organs.

These islets are blood-glands which have entered into asecondary relation with the pancreas. This has been broughtabout in Teleostei mainly by the tendency of the diffusepancreas to envelop or invade other tissues. In the case ofthese so-called islets in the compact pancreas of Teleostei, andalso of higher animals, the closer relation is due to the commonembryonic origin of the two tissues. Here the islets form aconstituent part of the pancreas, although they maintaintheir function as an internal secretory gland. The primitivecondition, however, is that seen in Teleostei with diffusepancreas, where the islets are both morphologically andfunctionally distinct.

No evidence of transitional forms to support the view thatthe islets undergo metamorphosis into zymogenous tissue wasfound. The reputed changes of zymogenous elements to islettissue are possibly degenerative, or regressive to the " cellularprocess " condition of the embryo.

From internal histological evidence, these bodies are pro-bably functionally active. (Confirmatory of Diamare's Avork.)

REFERENCES.

1. COLE and JOHNSTONE.—" Pleuronectes," 'L.M.B.C. Memoirs,' 1901.2. DOGIEL, A. S.—" Zur frage iiber die Ausliirgiinge des Pankreas des

Menschen," • Arch. f. Anat. u. Phys.,' Anat. Abt. Jabrg., s. 117—122,1893.

3. DIAMAKE, V.—" Studii comparativi sulle isole di Langerhans depancreas,"'Intern. Monatsschr. f. Anat. u. Phys.,' Bd. xvi, pp. 155—209.

4. GIANNELLI ED GIACOMINI.—" Richerche istologiche sub tubo digerentedei Rettili," ' R. Accad. d. Fisiocrit. in Siena,' 1896.

5. HAEEIS and Gow.—" Note upon one or two Points on the ComparativeHistology of the Pancreas," 'Journ. of Anat. and Phys.,' vol. xv,pp. 349—360.

6. JABOISKJT, A. J.—Uber die Veranderungen in der giosse und im Bau derPankreaszellen bei einigen Arten der Inanition," 'Arch. f. path. Anat.u. Phys.,' Bd. civi, Heft 3, s. 409—450, 1899.

404 JOHN' BENNIK.

7. LAGUESSE, E.—" Structure et developpement du pancreas d'apres lestravaux recents," 'Jonrn. de 1'Anat. et de la Phys.,' vol. xxx,pp. 591-608 and 731—783.

8. LAGUESSE, E.—" Sur le Pancr6as du Crenilabre, et particulieremeiit surle pancreas intrahepatique," ' Rev. Biol. du Nord de la France,' vii,No. 9, pp. 343—360,1895.

9. LAGUESSE, E.—"Sur ia formation des ilots de Langerhans dans lepancreas," ' Compt. rend, de la Soc. biol.,' Annee 45, ser. 9,pp. 819, 820,1893.

10. LANGEHHANS, P.—"Beitiage zur mikroskopischen Auatomie derBauchspeicheldriise," 'Inaug. Diss.,' Berlin, 1869.

11. LEWASCHETV, S.—"tiber eine eigenlumliche Yeranderung der Pankrcas-zellen warmbliitiger Tiere bei slarker Absonderungstliatigkcit derDriise," ' Arch, f.mikr. Anat.,' Bd. xxvi, s. 453—485,1886.

12. MASSAKI.—" Sul pancreas di pesei," ' Rend. II. Accad. dei Lincei,' vol.vii, pasc. 5, pp. 134—137,1898.

13. OrrEL, A.—'Lebrbuch der vergleichenden mikroskopisclien Anatomieder "Wirbeltiere,' Jena, 1900.

14. PEAKCE, B. M.—"The Development of tlic Islands of Langerlians in theHuman Embryo," 'Amer. Journ. Anat..' vol. ii. No. 4, Oct., 1903,pp. 445—455.

16. BESSIE, J.—"On the Occurrence of a Principal Islet in the Pancreas ofTeleostei," 'Journ. Anat. and Phys.,' vol. xxxvii, p. 375—378.

16. STATKEWITSCII, P.—" Uber "Veiauderungen des Muskel- und Driisenge-webes sowie der Herzganglien beim Hungern," 'Arch. f. exper. Path.u. Pharm.,' Bd. xxxiii, pp. 415—461.

D E S C E I P T I O N OF P L A T E S 26—28,

I l lus t ra t ing Dr. John Eennie 's paper on " The EpithelialIslets of the Pancreas in Teleostei."

REFERENCES TO ALL THE FIGURES.

Art. Artery. Cap. Capsule, ca. Capillury. c. t. Connective tissue, cy. d.Cystic duct. d. c. Darkly staining cells, g. h. Gall-bladder. Is. Islet tissue.Is.l and Is. 2. Islets in Lophius referred to in text. Is. 3. Separated portionof large islet in Ammo dytes. Int. Intestine. /.Liver. /. c. Lightly stainingcells, me. a. Mcsenteric artery, p. Pancreas, p. d. Pancreatic duct. pa. is.Pancreatic tissue within islet, po. v. Portal vein. pr. is. Principal islet.


py. e. Pyloric ca;ca. re. Kete mirabile. sm. is. Small islet in Ammody tes.sp. Spleen, si. Stomach, p. Vein.

PLATE 26.

Dissection of Zeus faber to show relation of principal islet to other organs.

PLATE 27.

Abdominal viscera of Lophius piscatorius, showing general distributionof the islets. The principal, which is always tlie largest, is seen directlyanterior to the spleen.

PLATE 28.

FIG. 3.—Principal islet in Syngnathus acus. X about 50 times.

FIG. 4.—Interciecal islet from Zeus faber. x 350. The centre portionthroughout the series stained more darkly than the rest of the islet. Notethe absence of a capsule.

FIG. 5.—Principal islet from Anarrhichas lupus. Here there is aslight penetration of its tissue by pancreas. The full thickness of the lattertissue in the proximity of the islet is shown. X 72 times.

FIG. 6.—Islet from Ammodyfces tobianus. X 350. This islet showswell the relation to pancreas wherever the latter is at all massive. Dark andlight cells are well contrasted. Capillaries are extremely abundant, but itshould be noted that in this fish a similar appearance, in this respect, is seenin other organs, e.g. the liver. Besides the main islet, which in this sectionappears in two portions, there is a very small one to the right near a largevein. A large pancreatic duct is present.

FIG. 7.—Dark and light cells from the section in fig. 6. x 810. Thenuclei («.) in the light cells appear similiar to those seen by Diamare also, anddescribed by him as " contorti."

FIG. 8.—Pyloric islet from Pleuronectes platessa. x 50. Itshows areas of dark and light cells, and also a considerable amount of penetra-tion of pancreas.

FIG. 9.—Portion of the principal islet of Pleuronectes, showingdifferent appearances of the dark and light cells, x 810.

FIG. 10.—Rete mirabile from capsule of Lophius. x 810.FIG. 11.—Principal islet from Zoarces viviparus. x 72.FIG. 12.—(a) Dark and (b) light cells from islet in fig. 11. x 810.FIG. 13.—Portion of islet from Onos mustela. x 810. Showing the

contrast between the two types of cell in (his species.

Med

Fig 2.

WSnulh del Mnthl,ithr!,ondl.m

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the epithelial islets of the pancreas in...

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