Equine placental gross anatomy
Keywords: equine, mare, placenta, anatomy, yolk sac
The placenta of a normal equine term fetus. A clear plastic tube containing food dye has been inserted into the urachus. Note that the color of the dye is clearly visible through the wall of the umbilical cord. Although the urachus is lined with transitional epithelium (urothelium) that is continuous with the fetal bladder, it is little more than a space between the major vessels that occupy this portion of the cord. Therefore it cannot be isolated as a distinct vessel for surgical ligation when a neonate has a persistent urachus. Only if it is infected, inflamed and thickened will it attain substantial bulk.
The three major vessels adjacent to the fetus are the two umbilical arteries and the single umbilical vein. The more distal to the fetus, the more complex the vasculature. Close to the amnion there may be four or more vessels. In this specimen, four vessels can be seen at the point of emergence of the urachus into the allantois. Adjacent to the chorion, there are many more vessels as the arteries spreads out to perfuse the placenta (and the veins collect from their drainage points).
Although the histology of veins and arteries is usually different, this author finds it virtually impossible to discriminate between them in the umbilical cord adjacent to the fetus. Although the pressure gradients between the two systems may be different from those elsewhere, the reason for the similarity in their histology at this site is not obvious.
It is only the intra-amnionic portion of the cord that contains the urachus; the urine enters the cord from the foal's bladder (in place of the syringe in this image) and is expelled into the allantois where the tube emerges from the amnionic part of the cord. In contrast with other species, equids have long umbilical cords. The extra-amnionic cord is short and indistinct in carnivores and completely absent in ruminants. The presence of a extra-amnionic cord in equids allows the amnion to rotate freely within the allantoic cavity. In turn, this allows the equine fetus to change its presentation (anterior-posterior) in the uterus much later in gestation than in ruminants (~ 9 months vs. ~5 months). In other words, the presentation of an equine fetus is not fixed until about 9 months of gestation.
In this placenta one can see a golf-balled sized structure just above the extra-amnion cord in the image. This a yolk sac remnant, a common finding in equine placentas. A close-up view is provided later in this entry. Sometimes these remnants are quite large and calcified. When opened, they contain dark brown remnants of the yolk sac fluid. Perhaps obviously, these remnants are never found within the amnion.
The image below shows the gross components, labeled.
Image size: 1519 x 698px
Although is is common to refer to the allantoamnion as the amnion, it is incorrect to do so because the "amnion" is actually a double membrane that results from fusion between the outer surface of the amnion and the outer surface of the allantois. The same principle applies to the allantochorion where the outside of the allantois fuses with the chorion. Despite common use of the term allantochorion, the term allantoamnion is never used. This convention may have arisen from anatomy in humans and sub-human primates where there is no functional allantois and therefore, no allantoamnion.
A macroscopic view of the micro-cotyledons in the body of this placenta. that characterize equine placentation. Although interdigitation between the maternal and fetal epithelia starts to become established as early as 40 days of gestation, experience with fresh slaughter plant specimens shows that even at 60 days, the chorion can be separated from the endometrium with very little effort.
Image size: 4198 x 2636px; a large file. This can be viewed at full resolution to appreciate the appearance of the tufts of microcotyledons.
Image size: 1241 x 479px
| Interestingly, the terms allantochorion and chorioallantois appear to be interchangeable. In a general Google search (April 2014) the term allantochorion triggered about four times as many hits as chorioallantois. However, in Google Scholar (where far fewer hits for both terms were found) the opposite was true; the hits for chorioallantois far outnumbered hits for allantochorion. Of additional interest was the fact, that the terms were not exclusively confined to journals published in certain countries. For example, both chorioallantois and allantochorion were found in refereed Australian scientific journals. With regard to the words amnionic and amniotic, a Google search shows that the term amniotic is used in preference to the term amnionic at a ration of about 25:1. However, the perennial question must be asked : "Are those in the minority incorrect or are are those in the majority following blindly?" |
The yellow ring indicates the entry point of urine (from the urachus) into the allantois. The yellow arrow points to the yolk sac remnant mentioned earlier.
A high-lited area in the image shows the point of attachment of the placenta to the allantochorion. This point of attachment is normal ; at the base of one of the uterine horns. Occasionally the cord will be attached in the so-called "non-pregnant" horn or the body. Body attachments have been associated with abortion but abortion is not inevitable in all such cases. The terms "pregnant horn" and "non-pregnant horn" are of course terms of convenience, not accuracy.
A macroscopic view of the micro-cotyledons in the body of this placenta. that characterize equine placentation. Although interdigitation between the maternal and fetal epithelia starts to become established as early as 40 days of gestation, experience with fresh slaughter plant specimens shows that even at 60 days, the chorion can be separated from the endometrium with very little effort.
Although the microcotyledonary tufts form an almost continuous carpet, the areas between each tuft are functionally important too. It is into those spaces that endometrial glands pour their secretions. These are called arcade spaces and they curve (hence the name "arcade" i.e. old English for "arch") towards the fetal side of the placenta, providing area for accumulation and absorption of fetotroph. The arcade spaces are also responsible for iron transport into the fetus via uteroferrin in gland secretions Arcades are also referred to as areola but the term areola is simply derived from the old English word for "area" so there is very little specificity to its use.
Below, see the external view of the chorion on the left, the internal view of the allantois, and the interposing tissue that fuses the two layers.
As one would expect, the allantois is well vascularized because gaseous, nutritional and waste exchange between the fetus and the dam. In some areas (below) it is particularly well developed.
A close-up view of the yolk sac remnant in this placenta. The casing of the remnant was firm and probably partially mineralized (as is common).
Image size: 2000 x 1333px
This amalgum of images shows a large, multiloculated yolk sack remnant. The remnant was radiographed, revealing its highly mineralized character.
A similar case is shown below.
Image size: 1500 x 1324px
In this image, the allantochorion has been turned inside-out, so that the bright red chorionic surface is not visible. In many cases this is how the placenta is found after foaling, perhaps because the foal has moved away from the expelled placenta, inverting it. Otherwise, it is expelled from the mare with the bright red chorionic surface still on the outside.
Acknowledgements: The author wishes to thank Dr Metcalf for her images and Dr Don Schlafer for his editorial comments.
Reference: Wisher, S. 2009. Review Article: Abnormal umbilical cord attachment sites in the mare: a review illustrated by three case reports. Eq. Vet J. 41:930-939
Reference: Wisher, S. 2009. Review Article: Abnormal umbilical cord attachment sites in the mare: a review illustrated by three case reports. Eq. Vet J. 41:930-939
