EQUINE VETERINARY EDUCATION / AE / march 2009 145

Clinical Commentary Allantoic vesicles: Only a problem when they are a problem? P. R. Morresey Rood and Riddle Equine Hospital, PO Box 12070, Lexington, Kentucky 40580, USA.

The Case Report in this issue by Singh et al. (2009) details the Little is known about the origin of the allantoic vesicles occurrence of markedly enlarged allantoic vesicles found and they have been classically considered to be an upon routine examination of a 12-year-old pregnant inconsistent incidental finding during placental Palomino mare. This is an unusual presentation as allantoic examination (Schlafer 2004). Structurally, they are multiple vesicles are generally considered to be much smaller in size clear-walled structures attached to the allantoic surface and therefore of no significance to the fetus in utero or along large blood vessels (Fig 1). They are covered with during delivery. However, as pointed out in the article, the allantoic epithelium and filled with gelatinous content. They physical presence of such large fluid accumulations could are usually small but may extend up to several centimetres. have led to dystocia or serious fetal compromise. No pathological significance is assigned to their presence, The allantochorion forms as a fusion of fetal allantois and although in the current report allantochorionic oedema chorion, but is not a homogenous structure throughout its and separation were noted (Singh et al. 2009). entire depth. A basement membrane develops under the Histology of the mega-allantoic vesicle walls themselves allantoic endoderm and is isolated from the vascular layer was not detailed in the current Case Report but would help of the allantois by both a mesothelial layer and an further elucidate the origin of allantoic vesicles. This may exocoelom-like space (Enders and Liu 2000). While the enable the determination of whether abnormalities of basement membrane is seen to support the allantoic allantochorionic histology in response to hypoxia and endoderm, the underlying exocoelom-like space is ischaemia as detailed above are the genesis of allantoic considered to allow movement between the endoderm vesicles. and the allantoic vascular layer during allantoic growth and Other allantoic lesions have been defined. uterine contractions. This exocoelom-like space persists until Adenomatous dysplasia (nodules and plaques) has been near the stage of villus formation (around Day 45), after reported as an infrequent allantoic lesion in the horse, which time it is decreased by collagen deposition (Ginther being often associated with fetal disease (McEntee et al. 1992). The allantoic vascular layer becomes more closely 1988). Larger nodules contain cellular and proteinaceous associated with the endometrial and exocoelomic surfaces as pregnancy progresses (Enders and Liu 2000). Remnants of the space are found to remain over larger allantoic vessels (Ginther 1992). The allantochorion is also not uniform throughout its entire area. As an example, the tip of the pregnant horn has been found to differ histologically from the remainder of the pregnant horn. Growth retardation is evidenced by widespread extended spaces sourced from remnants of the extraembryonic coelom and hypoplastic villi caused by abnormal microcotyledon formation. Resulting placental hypoxia and ischaemia lead to trophoblast necrosis, squamous metaplasia, thickened basement membrane and fibrosis of the villi (Oikawa et al. 1990). Histological deviations from the normal allantochorionic architecture therefore have the potential to allow abnormal fluid shifts and accumulations, as evidenced by the oedematous nature of the pregnant horn tip when compared to the rest Fig 1: Allantoic vesicles are usually small fluid-filled structures of the allantochorion (Whitwell and Jeffcott 1975). associated with large placental blood vessels. 146 EQUINE VETERINARY EDUCATION / AE / march 2009

debris within their lumina. Adenomatous hyperplasia has References been associated with other placental lesions including chronic or chronic-active placentitis, placental oedema, Demir, R., Kosanke, G., Kohnen, G., Kertschanska, S. and Kaufmann, P. evidence of fetal diarrhoea and hyperplasia of the (1997) Classification of human placental stem villi: review of chorionic epithelium. In severely affected cases, cystic structural and functional aspects. Microsc. Res. Tech. 38, 29-41. masses of varying sizes are observed near the insertion site Enders, A.C. and Liu, I.K. (2000) A unique exocelom-like space during of umbilical blood vessels. They are lined with cuboidal or early pregnancy in the horse. Placenta 21, 575-583. low to medium columnar epithelium and contain a Ginther, O.J. (1992) Embryology and placentation. In: Reproductive mixture of neutrophils and cellular secretions (Hong et al. Biology of the Mare: Basic and Applied Aspects, 2nd edn., Ed: O.J. 1993). Chorionic lesions, manifest as chronic active Ginther, Equiservices, Cross Plains. pp. 345-418. inflammation and a lack of chorionic villi, were often found H'mida, D., Gribaa, M., Yacoubi, T., Chaieb, A., Adala, L., Elghezal, H. fused to the sites of cystic allantoic hyperplasia, which and Saad, A. (2008) Placental mesenchymal dysplasia with beckwith-wiedemann syndrome fetus in the context of biparental themselves had no signs of inflammation (Shivaprasad and androgenic cell lines. Placenta 29, 454-460. et al. 1994). Hong, C.B., Donahue, J.M., Giles, R.C., Jr., Petrites-Murphy, M.B., Mesenchymal dysplasia is reported in human Poonacha, K.B., Tramontin, R.R., Tuttle, P.A. and Swerczek, T.W. placentas (deciduate, haemochorial and discoid) and is (1993) Adenomatous hyperplasia of equine allantoic epithelium. considered as a differential diagnosis when there is Vet. Pathol. 30, 171-175. ultrasonographic evidence of a cystic placenta or dilated McEntee, M., Brown, T. and McEntee, K. (1988) Adenomatous dysplasia vasculature. The placenta may be grossly enlarged and of the equine allantois. Vet. Pathol. 25, 387-389. ‘grape-like’ vesicles are present (Parveen et al. 2007). Oikawa, M., Yoshihara, T., Kaneko, M., and Yoshikawa, T. (1990) Normal stem villi have developed but others are Morphology of equine allantochorion at the tip of the pregnant associated with the grape-like fluid-filled vesicles where horn. J. Comp. Pathol. 103, 343-349. trophoblastic proliferation is absent (H’mida et al. 2008). Parveen, Z., Tongson-Ignacio, J.E., Fraser, C.R., Killeen, J.L. and Stem villi are branching mechanical structural supports for Thompson, K.S. (2007) Placental mesenchymal dysplasia. Arch. the placenta and consist of a condensed fibrous stroma in Pathol. Lab. Med. 131, 131-137. which the fetal arteries and veins as well as the arterioles Schlafer, D.H. (2004) Post mortem examination of the equine placenta, and venules are embedded. Fetal blood flow to the fetus and neonate: methods and interpretations of findings. Proc. fetomaternal exchange area in the peripheral villi is Am. Ass. equine Practnrs. 50, 144-161. thought to be controlled from this area (Demir et al. 1997). Shivaprasad, H.L., Sundberg, J.P., McEntee, K., Gordon, L., Johnstone, The equine placenta (adeciduate, epitheliochorial, A.C., Lombardo de Barros, C.S. and Hoffman, R.L. (1994) Cystic adenomatous hyperplasia of the equine allantois: a report of eight diffuse and villus) is morphologically distinct from the cases. J. vet. Diagn. Invest. 6, 107-110. human and it is difficult to draw strong parallels between Singh, K., Holyoake, G., Snider, T., DeFrancisco, A. and Schlafer, D. human mesenchymal dysplasia and the equine allantoic (2009) Mega allantoic vesicles of the equine placenta. Equine vet. vesicle. Therefore, increased knowledge of the allantoic Educ. 21, 143-144. vesicle and determination of its significance will only be Whitwell, K.E. and Jeffcott, L.B. (1975) Morphological studies on the gained by continued reporting and histological fetal membranes of the normal singleton foal at term. Res. vet. Sci. evaluation. 19, 44-55.