Pregnancy; 25 to 27 days

Keywords: canine, pregnancy, 25, 27, diagnosis, surgery, polyovular, fetus,embryo,placenta

The image shows a canine pregnancy estimated to be 25 to 27 days of gestation. This estimation was based on the crown rump length of the embryos and the size of the embryo-placental bulges. There is little information on actual crown-rump length in postmortem specimens of known gestation, therefore estimates were extrapolated from ultrasonographic data. The size of the bitch was unknown and could have belonged to a small or giant breed, another factor that could contribute to inaccuracy of this estimation.

Image size: 1962 x 1198px

The author repeatedly refers to embryos in this entry, not fetuses. That is because organogenesis is not yet complete and the embryo is not yet recognizable as the species of interest; two factors occasionally used to define the transition between an embryo and a fetus. In fact, there is no gestational age when organogenesis is indeed complete (CNS development continues even after birth) and recognition of the species of the embryo-fetus lies in the eye of the beholder. The reader is left with that dilemma. Certainly, refereed data do not suggest firm opinions on this matter.

Several items are of note in this specimen. 

1. The ovaries were carefully dissected and only five corpora lutea were present; yet there were six embryos. This is to be expected in bitches, where a single Graafian follicle can contain several oocytes i.e. Canis familiaris is a polyovular species.  

2. A reminder that bitches (and queens) do not have major (middle) uterine arteries such as those present in ruminants and horses. Therefore in young, non pregnant bitches that do not have excessive abdominal fat, little potential exists for severe hemorrhage when the mesometrium is transected during ovariohysterectomy.

3. Ultrasonography has largely rendered transabdominal palpation for pregnancy redundant. However, palpation of  embryonic-placental bulges are potentially important when ultrasonography is not available. In this regard, the author developed a mantra for students on how to recall when palpation was valuable and for what reason. It also included the advent of mammary development and was known as the "25, 35, 45, 55, 65" guide. At its best it is only approximate, but still potentially valuable. Timing was based on the occurrence of the LH surge and it read as follows:

25 to 35 days: embryonic-fetal bulges are palpable except in obese bitches or those that are extremely muscular. 

Note: After 35 days, the embryonic-fetal bulges lose tone and become more difficult to palpate.

45 days: mammary development is first noted in maiden bitches (Colostrum can only be expressed during the last 2 to 3 days of gestation).

55 days: the fetuses themselves become palpable.

65 days: parturition.

4. Carnivora have intimate (endotheliochorial) placentation, arranged in a band around the embryo/fetus. This is called zonary placentation. The margins of zones of placentation in bitches develop hematomas where heme breakdown products facilitate iron transport into the conceptus. One of these products is biliverdin,  the bright green pigment known as uteroverdin in this context. That pigment is not yet visible in a 25 to 27 day pregnancy but becomes a remarkable feature of late gestation, signaling the onset of whelping and in some cases, retention of the placenta after whelping is complete.

Selected references:

Yeager, A.E. and Concannon, P.W. 1990. Association between the preovulatory luteinizing hormone surge and the early ultrasonographic detection of pregnancy and fetal heartbeats in Beagle dogs

Theriogenology 34:655-665

Michelle A. Kutzler, M.A. 2003. Accuracy of canine parturition date prediction using fetal measurements obtained by ultrasonography. Theriogenology. 60:1309-1317

Lopate, C. 2008 Estimation of gestational age and assessment of canine fetal maturation using radiology and ultrasonography: A review. Theriogenology. 70:397-402

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 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.

Canine placentation.

Keywords: canine, placenta, zonary, fetus, dog, bitch

An image of two canine pregnancies; one at about 39 days and the other very close to term. In the image at top, the chorion has been ruptured but, because it does not become as intimately attached to the allantois (All) as in the farm animals, the allantois remained intact here. Interestingly, even at term (see the lower image) the allantois and chorion are not firmly fused. That image shows the "halo" of the chorion, just peripheral to the allantois.

In the upper image at about 39 days, the reddish-colored yolk sac (YS) is still quite large and interestingly, is strongly echogenic on ultrasonography (more so than in horses). The amnion (Am) is intact in both of these pregnancies and in the image at top, the amnion can be seen floating, closely opposed to the fetus, adjacent to the yolk sac and allantois. The canine YS is very interesting because, unlike the situation in the mare where it is almost completely utilized by 65 days and is completely subsumed (incorporated into) by the intra-amnionic cord, the yolk sac in the canine species diminishes in size and remains in its own little cavity (an of extra-celome) up until birth. This cavity can easily go un-noticed unless one inspects the placenta closely.

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 to 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 occur 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 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