Ovine placentation: placentome variation
Key words: multiplex, placenta, sheep, ovine
Sheep usually have about 80 to 90 placentomes with little variation. Caruncles are already present in the non-pregnant uterus (even before birth) and form future attachment points for cotyledons. The combination of caruncles on the maternal side and cotyledons on the fetal side form joint structures known as placentomes. Caruncles determine the sites for cotyledons on the surface of the chorion. Caruncles are not usually concave at the time of conception but develop into concave structures in most cases as pregnancy progresses. In some cases however, flat or even convex caruncles develop. As discussed below, this may be influenced by stress, nutrition and multiple fetuses.
Figure 1. An image of a normal fetal-placental unit from a singlet pregnancy towards the end of gestation. Image size:1000 x 750px
Figure 2. A cotyledon being withdrawn from a caruncle in this pregnancy. Although the majority of caruncles were concave in this pregnancy, a few caruncles (not shown) were indeed convex as occurs in cattle. Viewing the image at its highest resolution (right click on image) reveals the fine structure of the fetal villi. Image size: 4764 x 3456px
Figure 3. An area of caruncles with the cotyledons removed in most cases. As demonstrated by the small caruncle close to the center of the image, there can be considerable variation in their size. Image size: 4800 x 3456px
In times of nutritional stress, fewer placentomes are formed than otherwise. This has been shown in controlled studies on stressed and energy-deprived ewes. Interestingly, this not only affects the number of placentomes but their shape as well. In such states, some placentomes became flatter and even become bovine-like as alluded to above. Interestingly, these changes also occur in apparently normal pregnancies. Therefore one should exercise caution in studies of this nature. It not clear if caruncles established as concave can change their shape as pregnancy progresses but some data do suggest this may occur. i.e.
Alexander G. (1963) reported that "....there was a marked tendency towards cotyledonary eversion; in nine out of ten placentae at both the 95th and 114th day the maternal tissue almost completely enveloped the foetal cotyledon, but at 135 days, the foetal tissue tended to surround the maternal in eight out of fourteen placentae, much as in the normal bovine cotyledon illustrated by Andersen (1926)."
From these data and those reviewed by Vatnick et al, 1991, the author has a drawn a representation of this situation:
Figure 4. The marked tendency towards cotyledonary eversion seen in some ovine pregnancies. These placentomes have been classified as types A to D. Type D is the most bovine-like of these. The green arrow questions whether this transformation occurs as pregnancy advances or if placentomes develop this architecture from the outset in some pregnancies. Interestingly, using a mathematical model, type D placentomes appear to the most efficient of all these forms of placentation. Using similar modeling, it has also been determined that ruminant placentation is highly efficient; 5 to 10 times more so than the diffuse, micro-cotyledonary placentation found in mares. Image size: 800 x 780px
| This author has often wondered how essential the inter-placentomal areas are in maternal-fetal exchange in ruminants. As reviewed by Wooding and Burton (an excellent resource on placentation) there are small areolae over the mouths of the uterine glands in these areas but other than that, very little fetal-maternal exchange occurs in those areas. In fact, the inter-placentomal areas account for less than 5% of the total exchange at the end of ruminant gestation. |
Figure 5: An ovine pregnancy approximately 60 days duration showing some caruncles with a bovine (type D) appearance while others are clearly concave developing into the most common form of placentomes (type A). Right click to read labels and see in full size. Although the dark black areas on the endometrium are labeled as "Degenerate caruncles", this author is not at all certain if that is correct. Image size: 2599 x 1572px
Selected references:
Alexander G. 1963 Studies on the placenta of sheep J. Reprod Fert. 7:289-305
Vatnick I. et al. 1991. Growth and metabolism of the placenta after fetectomy in twin pregnant ewes. J. Development Physiology 15: 351-356
Vonnahme, K. A. et al. 2006 Placentomal differentiation may compensate for maternal nutrient restriction in ewes adapted to harsh range conditions J. Anim Sci 84:3451-3459
Wooding, P. and Burton, G. 2008 in Comparative Placentation: Structures, Functions and Evolution. Pub. Springer eISBN: 9783540787976
