The exocelom (exocoelom) of the canine feto-placental unit
Keywords: anatomy, canine, placenta, exocelom, cord
Figure 1. A fetus estimated to be approximately 46-48 days of age. Image size: 1772 x 1635 px.
The inset image has been re-used with permission from permission of Rutgers University Press*
When one examines the canine placentation closely (or even fleetingly as fetuses are extracted from the uterus during cesarean section!) it is evident that there is no clearly delineated extra-amnionic cord. However, the extra-amnionic cord is indeed present. It is just obscured within a wide swath of tissue that bridges the gap between the amnion and the chorion. That transparent swath of tissue can be seen here, attached to the ventral aspect of the amnion along its length. Along the lower length of that swath, it is attached to the inner surface of the chorion, within the thick zonary band.
In this image, the intra-amnionic cord is obvious.
As mentioned, the canine fetus has an umbilical cord with both intra-and extra-amnionic segments, like that of an equine fetus, a convenient species for comparison. When an equine fetus is born, the extra-amnionic cord is obvious; a distinct and separate entity. Yet, this is not the case in dogs. Why? This is because there is a large, seemingly empty cavity that surrounds the extra-amnionic cord in the canine fetal-placental unit. In embryological terms, this cavity is known as the exocelom (literally: "out-of-the-body-cavity" ).
The exocelom is colored green in the inset image. In the main image, the tips of the forceps lie in the exocelom. In horses this cavity exits only in early pregnancy but the space has diminished so much by about 65 days of gestation that there is no longer an obvious space around the extra-amnionic cord. This explains why the extra-amnionic cord is a well-defined entity at foaling.
In the following image, another facet of the extra-amnionic cord is illustrated: the fact that the yolk sac (or what remains of the yolk sac) forms a major part of the extra-amnionic cord. A pair of forceps has been placed under the extra-anionic cord, spreading it to show the highly vascularized yolk sac. Once again, the equine placenta affords good comparison; remnants of the yolk sac often being found in the extra-amnionic cord in the newborn foal.
Figure 2. Image size: 1080 x 1436 px.
Other structures in the extra-amnionic cord are the umbilical arteries and veins. The urachus on the other hand, leaves the umbilical cord immediately after it leaves the amnion, filling the allantois which lies between the amnion and chorion.
*Mossman, Harland, W. Vertebrate Fetal Membranes. ISBN: 0-8135-1132-1. Copyright © 1987 by Rutgers, the State University. This inset image can only be used commercially with their permission.
