Paper - Some observations on the foetal vessels of the human placenta with an account of the corrosion technique (1938)

From Embryology

Bacsich P. and Smout CF. Some observations on the foetal vessels of the human placenta with an account of the corrosion technique. (1938) J Anat. 72: 358-364.1. PMID 17104704

Some Observations on the Foetal Vessels of the Human Placenta with an Account of the Corrosion Technique

By P. Bacsich and C. F. V. Smout

Department of Anatomy, University of Birmingham

Introduction

The literature dealing with the angio-architectonics of the foetal vessels in the placenta is surprisingly small. Indeed, textbooks of anatomy, embryology, and obstetrics make the briefest references to the subject, yet it is impossible to understand the physiology of the circulation in this organ without having first of all a clear idea of the anatomical relationship of its blood vessels.


The human placenta commences to function during the third month, and, according to most investigators, shows well-marked areas of degeneration at the end of pregnancy. It belongs to the haemo-chorial type, which according to Grosser represents the highest degree of development, and it is a unique example of the intimate and efficient functional union of the tissues of two separate individuals.


We know that the development of the vascular system of other organs is regulated by functional influences and hereditary factors, while in the case of the placenta, the site of implantation in the uterus and the characteristics of the maternal circulation also play an important role.

It is obvious that a clear conception of the foetal vessels in the placenta cannot be obtained by an ordinary dissection, and we believe that ‘misconceptions are bound to arise from X-ray photographs taken after the vessels have been injected with an opaque material, for this method does not show the anastomoses of the blood vessels or their stereoscopic arrangement. On the other hand, the corrosion technique provides specimens which may be seen and handled, and, accordingly, is obviously superior.

Technique

It is essential that the material should be obtained fresh, not more than 24 hours after death, and it should be unfixed.

If blood vessels are to be injected, they should be first washed through with acidulated water, in an endeavour to haemolyse any blood clot which might block the lumen of a vessel.

After this, it is an advantage in certain organs, e.g. the placenta, to distend the vessels by pumping in air, but great care must be exercised to prevent their rupture. The blood vessels of more delicate organs, such as the kidney, will certainly not withstand such severe treatment, and here the use of air is contra-indicated.

The next step is to choose a suitable injection material, and for this purpose we have used celluloid dissolvea in acetone. We used the celluloid from old X-ray and “‘ciné”’ films, from which the gelatine layer had been removed with hot water. We find celluloid just as good as its purer form celloidin, and it has the further advantages of dissolving more readily and of being considerably cheaper. The strength of the solution depends on the material to be injected, e.g. for the bronchial tree, the solution should be thick, say about 20%. Ifa weaker solution is used, not only the fine bronchioles but also the alveoli are filled and so the whole structure of the bronchial tree becomes obscured. On the other hand, for demonstrating the fine branches of an organ’s vascular tree, we injected pure acetone first, followed by an injection of weak celluloid, the strength of which was gradually increased to 20 %. It is important to bear in mind that the acetone readily diffuses through the tissue spaces, whereupon the celluloid, now freed from acetone, solidifies. Thus if, at first, too thick a solution is used, the finer branches of the blood vessels will not be injected at all, because the celluloid will have solidified before reaching them.

For small organs the injection was done with a glass Record syringe, but for large organs a large metal syringe with a screw drive piston was used. Special nose pieces were made for the metal syringe with a stopcock at the end to maintain pressure after removing the syringe. Recently, American workers have driven in the injection mass by means of compressed air regulated by a mercury manometer, and with characteristic American appreciation of laboursaving devices, they describe a mechanical shaker for dissolving the celluloid in the acetone, but we are not sure that these more elaborate methods are justified by the results.

Undoubtedly the great secret of success is patience, because the injections require a considerable time—6-12 hours for small organs, and 1-2 days for large organs, a fairly even pressure being essential throughout.

When more than one system is to be injected in any organ, the injection mass must be coloured, and the choice of the colour medium is one of the most difficult problems in the technique. The selection of the stain or pigment depends upon the macerating medium employed—the two questions are inseparable. There are two groups of colour media: (1) stains soluble in acetone, e.g. Sudan III, Scarlet Red, the colours mentioned by Narat and others, and Berlin Blue, Prussian Blue, Asphalt, Alkanin and Crystal Violet, mentioned by Morison and others; (2) pigments insoluble in acetone, e.g. Cobalt Blue, Chrome Yellow, Cinnabar, Victoria Blue, as used by Morison.

Using the acetone-soluble dyes, we found that after injecting the veins and arteries in the same organ with different colours, a certain diffusion always took place which confused the result. Therefore, to inject more than one system in a specimen, we preferred to use acetone-insoluble pigments in the form of artists’ oil colours, in which the pigment is so finely divided that no precipitation occurs even when the injection solution stands for several weeks.

After completing the injection it is necessary to immerse the specimen for about 24 hours in water, as this facilitates the complete removal of the acetone and so enables the celluloid to harden.

The final stage is the maceration, and four possible methods are available.

(1) Commercial HCl either concentrated or diluted with one-sixth water is most generally used. By this method maceration is complete in from 1 to 6 days. This is certainly the quickest method, but unfortunately it bleaches most of the stains. We find, however, that Scarlet Red and Sudan III are acid-fast even after prolonged exposure in the concentrated acid. Narat and others claim to have obtained acid-fast dyes (Luxol Fast Blue AR, Orange E, Scarlet RR, Yellow, Red B, products of Du Pont de Nemours and Co.) but unfortunately we were unable to obtain these dyes.

(2) By ordinary macerating methods—this will not affect the dyes but it is very slow and not always completely successful.

(8) By pepsin and HCI digestion, but it is slow and expensive owing to the high price of pepsin.

(4) By 1% KOH in an incubator at 56° C. This is rather slower than the HCI but it has the advantage that most colours are unaffected.

When maceration is complete, the specimens are soaked in hot water for a short time. The adherent tissues are then washed away with a strong current of water. After this, the specimen is further washed in running tap water for 24 hours to remove all traces of HCl or KOH.

Finally, the specimens may be preserved in a formaldehyde-glycerin or glycerin-spirit mixture as suggested by various authors—on the other hand, we prefer dry mounting.

We also found it an advantage to varnish our dry mounted specimens with a solution of resin in a petrol-chloroform mixture (petrol 5 parts, chloroform 1 part). This preserves the specimen, and enhances its appearance by brightening the colours of the pigments used for the injection.

Observations

Our chief attention was directed towards the umbilical arteries, and these vessels were injected in fifty placentae. The relations of the umbilical vein were studied in ten cases and either the vein alone or both vein and arteries were injected in these specimens.

We considered three main principles in regard to the arterial system, viz.:

(1) the pattern of the vessels, (2) the area of placenta supplied by each artery, (3) any connexion between the two vessels and their branches.

(1) The umbilical arteries enter the placenta either marginally or centrally. Here the arteries break up into several branches which run towards the margin of that organ, giving off further branches along their course. All the large arterial branches are in the same horizontal plane. and they lie immediately below the amnion.

According to the pattern of the arteries, the placentae may be divided into The Foetal Vessels of the Human Placenta 361

two main types: (1) a disperse type (Text-fig. 1), in which the two arteries divide dichotomously, and rapidly diminish in calibre, resembling in their spider-like arrangement the spokes of a wheel; (2) the magistral type (Textfig. 2), in which the two arteries extend almost as far as the margin of the placenta before their calibre diminishes. Some of the placentae were definitely of a border-line type, and in these cases their classification needed more careful consideration.


Text-fig. 1. Disperse type of placenta.



Text-fig. 3. Asymmetrical type

Text-fig. 4. One umbilical artery is supplying of placenta. the periphery of the placenta and the other the centre.

(2) The areas supplied by each of the arteries showed great variation. In twenty-seven cases equal areas of the placenta were supplied by each umbilical artery, that is, they were symmetrical. In twenty-three cases the distribution was asymmetrical, e.g. one-third would be supplied by one artery and twothirds by the other (Text-fig. 3). In some cases, one artery supplied the peripheral part of the placenta while the other supplied the central part (Text-fig. 4).

It is important to emphasize the fact that whatever the relation of the areas supplied by the two arteries, these vessels were always of equal calibre in the cord.


(3) We found that there is a constant communication between the two arteries either immediately before or immediately after their entrance into the placenta. In forty-three of the specimens examined we found a wellmarked transverse communicating branch. Sometimes this was as wide as the main branches, and at other times it was considerably narrower. This branch usually arose before the arteries branched, but it happened sometimes that one or two small branches came from one or other of the umbilical arteries before they were united by the transverse branch (Text-fig. 5). In some specimens branches arose from the transverse branch and at times these were quite large (Text-fig. 6). In seven of our specimens the actual transverse branch was missing, but in these cases the umbilical arteries were fused at the entrance to the placenta and then separated later (Text-fig. 7).


Text-fig. 5. Small vessels arising from each umbilical artery before they are united by the transverse communicating branch.

Text-fig. 6. A branch arising from the transverse communication.

Text-fig. 7. Fused umbilical arteries.


In our specimens we could see no sign of a peripheral anastomosis and we do not believe that the two arteries communicate except by the transverse branch.

The umbilical vein accompanies the arteries, but it is wider in calibre and more deeply situated (Pl. I, figs. 1 and 2).

The capillaries lie in the cotyledons and communicate so freely with both the arteries and veins that they can be readily injected through either system. In a well-injected specimen (PI. I, fig. 3), the capillary network of an individual cotyledon resembles a ball of cotton-wool.

Discussion

In discussing the literature dealing with the subject, it is interesting to note that Spanner (1935), using corrosion specimens, first pointed out that all the main arterial branches of the placenta lie in a horizontal plane, while, in most of the other organs of the body, the arterial branches are arranged in three dimensions. As already indicated, we were able to confirm this statement.


Fraser (1923) described the arteries of the human placenta as being similar in their arrangement to the spokes of a wheel. He did not allude to the fact later observed independently by two other authors, Burg (1929) and Schordania (1929), that all the placentae can be divided into two main groups, according to the pattern followed in their branchings. While these two authors give practically the same descriptions, the terms “disperse” and “ magistral” were introduced by Schordania. It is clear that if Fraser’s description is accepted, all the placentae examined by him must have belonged to the disperse type. These three authors, Fraser, Burg and Schordania, used X-ray photography in their researches, and our results on the corrosion specimens fully justify the descriptions given by the two latter workers.

Spanner was mostly concerned with the circulation in the intervillous spaces, and in his explanation of the foetal part of the circulation of the placenta, he attributes a special role to the finer branches of the umbilical veins, which, according to his microscopic investigations, contain a special “‘sluice-gate” and pressure-regulating mechanism. As we were not concerned with the microscopical characteristics of the foetal vessels of the placenta, we cannot deny the truth of his statements, but certainly on our corrosion specimens we were not able to observe the beaded appearance of the finer branches of the umbilical veins mentioned by Spanner as being seen on his venous corrosion specimens.

Schordania advanced the theory that placentae belonging to the magistral type are usually associated with a better-developed foetus, with greater weight, length and thoracic measurement, than those of the disperse type. He, as well as Fraser and Burg, agreed that the vascularity of the cotyledons shows regressive changes (white infarct) at the end of pregriancy. They also agreed that during pathological conditions of the mother the decrease of vascularity can develop at a much earlier stage, and may account for the death and abortion of the foetus. None of these authors advanced any theory regarding the physiology of the foetal circulation of the placenta.

Fraser, Hyrtl (1870) and Holl (1881) mentioned numerous anastomoses in the peripheral branches of the umbilical arteries, but we were not able to confirm this in our specimens, and in our opinion all the arterial branches distal to the transverse communicating branch are end arteries. This anatomical fact explains the occurrence of white infarcts.

Certainly our most interesting finding is the transverse communicating branch between the two umbilical arteries. With the exception of Burg, we are not aware that any previous writer has mentioned the presence of this branch, and Burg does not attribute any importance to it, while we believe it plays a most important role in regulating the circulation of this organ. The function of the circle of Willis is usually considered to equalize the pressure and establish an equal blood supply to all parts of the brain (see Cunningham and Buchanan). May we not apply the same functional role to this transverse communication? To support this view, when we remember that even the normal, 364 P. Bacsich and C. F. V. Smout

and certainly the pregnant uterus, carries out rhythmic contractions, it is obvious that during the contraction wave the blood pressure in the corresponding part of the intervillous space and cotyledons of the placenta is increased, and the presence of a pressure-equalizing or “buffer” system is well justified. This arrangement may also be compared with the arterial arcades of the intestines, where the continuous peristaltic movement is comparable to the contractions of the uterus.

The presence of the transverse communication between the two umbilical arteries also explains why these vessels are always equal in calibre, irrespective of the size of territory supplied by them, and therefore we must accept the conclusion that this transverse branch plays an active role in the distribution of the foetal blood in the placenta.

Summary

The foetal vessels of fifty human placentae were studied on corrosion specimens. A detailed account of the corrosion technique is given.

  1. According to the pattern made by the arteries, placentae can be divided into two groups: (i) a disperse type in which the blood vessels divide dichotomously, and (ii) a magistral type in which the two main arteries extend as far as the margin of the placenta and give off two branches of small size only.
  2. Whatever is the relation of the areas supplied by the two arteries (symmetrical or asymmetrical distribution) these vessels are always of equal calibre in the cord.
  3. There is a constant communication between the two arteries as they enter the placenta. Forty-three specimens showed a well-marked transverse communicating branch, while in seven specimens the two vessels were fused.
  4. Because of the absence of peripheral anastomoses, it is believed that all the vessels to the placenta are end-arteries.
  5. The authors believe that the transverse communicating branch establishes an equal distribution of blood and regulates the pressure in the placenta, counteracting as a buffer system the effects of uterine contractions,

References

Bure, E. (1929). Z. Geburtsh. Gyndk. Bd. xcv.

Fraser, J. (1923). Amer. J. Obstet. Gynaec. vol. v1.

Hyrtt, J. (1870). Die Blutgefdsse der menschlichen Nachgeburt, etc. Wien: W. Braumiiller.

Hott, M. (1881). S.B. Akad. Wiss. Wien. Bd. xvi.

ScHorpania, J. (1929). Arch. Gynaek. Bd. crim.

Srreve, H. (1935). Verh. anat. Ges. Jena, Bd. xiim.

Spanner, R. (1935). Z. ges. Anat. 1. Z. ‘Anat. EntwGesch. Bd. cv.

Narat, Lorr and Narat (1936). “On the preparation of multicoloured corrosion specimens.” Anat. Rec. vol. LxIv, no. 2.

Plate

EXPLANATION OF PLATE I

Fig. 1. Distribution of umbilical vein in the placenta.

Fig. 2. Arteries and vein of placenta injected with different colours to show their respective relations.

Fig. 3. Injection of capillaries in the placenta. Journal of Anatomy, Vol. LXXII, Part 3 Plate I



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