Amniotic plaques
Keywords: bovine, artiodactyls, amnion, amnionic, amniotic plaques, pregnancy| Amnionic or amnionic? Both words have the same meaning and are widely used in the scientific literature. However, a search on Google Scholar revealed 440,000 hits for amniotic and about one tenth of that number (only 4,770 hits) for amnionic. Therefore the term amniotic is more widely used than amnionic and accordingly, has been adopted throughout this LORI entry. Incidentally the etymology of the term amnion is not clear. Some references define it simply as having originated from the same word in Latin meaning "the membrane over a lamb's head" |
The image below shows the amnion and fetus from a 95 to 100 day old pregnancy. The allantois and chorion have been removed for clarity. Click on image for larger version.
The whitish-grey plaques distributed towards the center of the amnion are normal findings yet their presence is an enigma; they are amniotic plaques. Because of their histologic resemblance to skin, they have also been referred to as "callusoids" (as in a callous) or "verrucoids" ( < L. verrucosus; resembling a wart).
Amniotic plaques are most obvious in artiodactyls (cloven hoofed animals) especially cattle but they are in fact found in the majority of mammals, including cetaceans, humans and even sloths (a fact of major importance to sloths alone). In some cases, they are barely visible to the naked eye. Indeed, among domestic animals they also occur the horse; our only perissodactyl. Yet there again, they are very small and often overlooked. Although amniotic plaques are more common in abnormal pregnancies in humans, they occur in normal pregnancies as well. Interestingly, in abnormal bovine pregnancies arising from cloned embryos, they are better developed than otherwise and are clearly visible on ultrasonography; something not usually possible in normal bovine pregnancies.
Amniotic plaques are found on the inside of the amnion and this should be obvious on second thought because close observation shows them to be present on the outside of the umbilical cord, which is of course, inside the amnion. Nevertheless, the amnion can be so flaccid as to give the impression that the plaques are on the outside of the membrane. That phenomenon is seen below; the fetus still within the amnion.
Image size: 1200x793px
Histology shows amniotic plaques to have a remarkable resemblance to skin with perhaps 30 layers of tissue, considerably thicker than the surrounding amnion which may be only four or five cells thick. Like skin, they also have layers such as stratum basale, stratum spinosum, stratum granulosum and stratum corneum.
Light microscopy as well as scanning and transmission EM have been used to study amniotic plaques in cattle, humans and some animals (deer and sheep). It has also been shown that the amnion produces low molecular weight protein throughout gestation; yet the function of amniotic plaques is still unknown. Even more intriguing is the fact that they are temporary structures, first visible in cattle at about three months of gestation becoming well developed by five months or so, then gradually disappearing and being completely absent at term. In other animals they are temporary structures as well.
Note: The General Assembly of the World Association of Veterinary Anatomists (2006) contrived the term “bractreolae amnioticae” as an internationally acceptable synonym for the English term amniotic plaques. The general response to this term amoung English speaking colleagues is either a quizzical look or a vacant stare.
Selected references:
Ginther, O.J. 1992 Reproductive biology of the mare and applied
aspects, 2nd ed. Cross Plains, WI: Equiservices. p.379
Kohan-Ghadr, H.R. et al. 2008. Ultrasonographic and histological characterization of the placenta of somatic nuclear transfer-derived pregnancies in dairy cattle. Theriogenology: 69:218-230
Lui, K.H. et al. 1994. Ultrastructure and protein synthesis of bovine amnion. Animal Reprod Sci. 35: 41-55
Mossmann, H.W. 1987. Vertebrate fetal membranes. Rutgers University Press. ISBN: 0-8135-1132-1 pp158-159.
Roberts, S.J. 1971 Veterinary Obstetrics and Genital Diseases. Ithaca, NY, Published by the author, p 43
Schlafer, D.H. et al, 2000. The bovine placenta before and after birth: placental development and function in health and disease. Anim. Reprod, Sci. 60-61:145-160.
Sinha, A. A. et al. 1970. Ultrastructure of the amnion and amniotic plaques of the white-tailed deer. Am.J.Anatomy. 127: 369–395
Wislocki, G.B. 1941. The placentation of an antelope (Rhynchotragus kirkii Nyikae Heller).
The Anat. Record. 81: 221–241
