LORI.equine

Allantoic cysts

Keywords: cysts, allantoic, mesoderm, equine, chorion

Accumulations of fluid are occasionally seen on the allantoic surface of the placenta. They vary from microscopic in size to more that 15 cm in diameter.  Figure 1 shows examples of allantoic cysts. They contain clear interstitial fluid and fluctuate on contact. Allantoic cysts can also be seen on ultrasonography.

Figure 1. Allantoic cysts viewed from the allantoic surface of the placenta. Copyright for the larger image belongs to Dr Amy Clark. Email: aclark@essexequine.net. The author has not been able to ascertain the origin of the smaller image. Image size: 1103 x 849 px

Figure 2 shows accumulation of water adjacent to placental veins after it was introduced into the umbilical vein at faucet pressure. This probably illustrates the potential for the allantoic membrane to detach from the allantois; a precursor to the formation of allantoic vesicles. With some concern that this observation was due to excessive water pressure coupled with venous autolysis, the author was interested to see that the same phenomenon has been described by Ginther (1992) using air injection adjacent to placental vessels.

Figure 2. Accumulation of fluid adjacent to veins in the placenta after infusing water at faucet pressure into the umbilical vein. Image size: 2000 x 1419 px

Ginther (1992) stated that the air in his observations had entered the exocelom between the allantois ad chorion and separated the two membrane systems. This author however, suggests that it is more specific to ascertain that water or air (as the case may be) enters the exocelom-like space created within the mesoderm, not the exocelom itself. Many readers will not be familiar with this anatomical nuance. The following text box explains the difference between the two spaces.

Using figure 3 as a guide, note that a mesodermal layer begins to cover the allantois as it grows upward. The image shown here is one of a 28 day embryo with the yolk sac only partially surrounded by the allantois at its base. The allantois will surround the yolk sac almost completely at 55 to 60 days of gestation. Note that the yolk sac lies on the inward side of the allantois and the chorion on its outside. Figure 3 shows how the mesoderm grows into the exocelom, a space between the yolk sac and chorion, bringing a blood supply to the chorion. Within the advancing mesoderm itself however, numerous cavities develop and form an exocelom-like space, separate from the exocelom itself. This space persists until foaling. It was first described by Enders, A.C. and Liu, I.K. in 2000 and is an important part in the genesis of allantoic cysts.

Figure 3. The genesis of an exocelom-like space within the mesoderm. On occasion, this is manifested as an allantoic cyst as shown on the right hand side of the image. Image size: 2632 X1717 px

The anatomy of a partially collapse allantoic cyst is shown in figure 4. This is a very large image, allowing close examination of its sub sections.

Figure 4. The anatomy of a partially collapsed allantoic cyst. Subsections of the image can be examined in detail after the image is expanded. They include: 1. The allantoic membrane, 2. A microcotyledon, 3. A cross section of an artery, 4. A vas vasorum (subsidiary vessel) within the artery wall and the collapsed lumen of the artery itself. Note the abbreviations: allantois (All) and chorion (Ch). Image size: 6050 x 4800 px

In summary, the allantoic membrane is only loosely connected to the chorion. This allows for separation of the toxic waste product in the allantois from the nutritional and respiratory functions of the chorion. As discussed in another LORI entry, the exocelom-like space may also form part of a de facto lymphatic system in the placenta. Furthermore, it has been suggested that the exocelom-like space may allow for a degree of motility for the fetus against the chorion which must remain attached to the endometrium at all costs.  However, it is likely that the allantoic membrane can be torn away from the chorion by movement of the foal's torso or limbs against the allantochorion, especially in late gestation when uterine space is limited. This would create an enlarged exocelom-like interstitial space in which fluid could accumulate, forming an allantoic cyst.

Selected references:

Bellini, C. et al. 2012. Are there lymphatics in the placenta? Lymphology 45:34-36

Castro, E. et al. 2011. Neither normal nor diseased placentas contain lymphatic vessels. Placenta 32:310-316

Ebrahim EI-Nefiawy, N. 2017. Development of Human Umbilical Vessels in The Second Trimester of regnancy: Histological, Immunohistochemical and Morphometric study. DOI:10.21608/EJH.2017.4079

Enders, A.C. and Liu, I.K. 2000. A unique exocelom-like space during early pregnancy in the horse. Placenta 21:575-583.

Ginther, O.J. 1992. Embryology and placentation. In: Reproductive Biology of the Mare: Basic and Applied Aspects, 2nd edn., Ed: O.J. Ginther, Equiservices, Cross Plains. page 462

Morresey P.R. 2009. Allantoic vesicles: Only a problem when they are a problem? Equine Vet Edu. 21:145-146

Schlafer, D.H. 2004. Post mortem examination of the equine placenta, fetus and neonate: methods and interpretations. Proc. Am. Ass. Equine Pract. 50:144-161.

Singh, K. et al 2009. Mega allantoic vesicles of the equine placenta. Equine Vet.Ed. 21: 143-144.