LORI.canine

Canine fetus at 58 days

Keywords: Fetus, canine, anatomy, placenta

The image below shows a canine fetal-placental unit approximately 58 days into gestation.  The fetus is still within the placenta. Note the zonary placentation, a wide ring-like structure surrounding the chorion. Zonary placentation is typical of canids, felids, hyenas, seals, bears and elephants.

Image size: 1203 x 674 px

Dogs and cats have endotheliochorial placentation i.e. the endothelium of endometrial blood vessels is in direct contact with the chorion. Although this is the most intimate form of placentation in domestic animals, they are not able absorb transferrin-bound iron directly from the maternal compartment like humans. Apart from horses which absorb iron from endometrial secretions, most domestic animals phagocytose red blood cells that have been extravasated by endometrial capillaries. This occurs across the areas of placental attachment. It is only in canids and felids that phagocytosis of red blood cells occurs predominantly at the margins of their zonary placentas.

In dogs and cats, red blood cells are phagocytosed by specialized chorionic epithelium that does not interdigitate with the maternal epithelium as occurs in the central, intimate area of the zonary placenta. The special villi in the marginal area have intricate surfaces that lie within the marginal hematomas on the endometrium forming the so-called hematophagous zones. The nature of the free standing fern-like villi in the hematophagous zone become obvious if the reader enlarges the image below.

Image size: 1203 x 794 px

Heme from maternal blood on the endometrium is immediately broke down by hemoxidase into biliverdin. Biliverdin is bright green in color, accounting for the appearance of the hematophagous zone (marginal hematoma). Heme is broken down in the same manner in felids, yet the hemotophagous zone is not green. In cats, biliverdin may be catabolized to bilirubin very rapidly in this area, so that its presence never becomes obvious but that is only supposition on the part of the author. Suffice to say, the postpartum discharge in a queen is brownish red and that in a bitch is green in color. Indeed, the image below shows the typical appearance of that discharge during second stage parturition in a bitch. The membrane at the vulvar lips is an amnion.

Image size: 720 x 1103 px

In the image below, the 58 day old fetus has been removed from the chorion but still lies within the amnion, attached to the inner surface of the chorion by the exocelom and extra-amnionic umbilical cord 

Image size: 2000 x 1122 px

References:

de Oliveira, C. M. et  al Iron transportation across the placenta. Anais da Academia Brasileira de Ciências Epub Aug 30, 2012 http://dx.doi.org/10.1590/S0001-37652012005000055

Enders, A.C. and Carter, A.M. 2012 The evolving placenta: Convergent evolution of variations in the endotheliochorial relationship. Placenta 33:319-326