322:) have seen close to an equal number of independent origins of placentation. This means that the occurrence of placentation in Squamata is more frequent than in all other vertebrates combined, making them ideal for research on the evolution of placentation and viviparity itself. In most squamates, two separate placentae form, utilising separate embryonic tissue (the chorioallantoic and yolk-sac placentae). In species with more complex placentation, we see regional specialisation for gas, amino acid, and lipid transport. Placentae form following implantation into uterine tissue (as seen in mammals) and formation is likely facilitated by a plasma membrane transformation.
381:, the uterus becomes regionally specialised to support placental functions, within each of these regions being a new specialised uterine structure. In the southern grass skink three distinct regions of the placenta form which likely perform different functions; the placentome supports nutrient transfer via membrane bound transport proteins, the paraplacentome supports the exchange of respiratory gasses, and the yolk sac placenta supports lipid transport via apocrine secretion.
282:(or chorionic laevae), a smooth surface. The endometrium (from the mother) over the chorion frondosum (this part of the endometrium is called the decidua basalis) forms the decidual plate. The decidual plate is tightly attached to the chorion frondosum and goes on to form the actual placenta. Endometrium on the opposite side to the decidua basalis is the decidua parietalis. This fuses with the chorion laevae, thus filling up the uterine cavity.
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vertebrates has shown us that many of these signalling molecules are expressed widely in vertebrate species and were probably expressed in the ancestral amniote vertebrate. This suggests that maternal fetal communication has evolved by utilising the existing signalling molecules and their receptors, from which placental tissues are derived.
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Placental functions include nutrient transport, gas exchange, maternal-fetal communication, and waste removal from the embryo. These functions have evolved by a series of general processes such as re-purposing processes found in the ancestral tissues from which a placenta is derived, recruiting the
141:
The placenta can be defined as an organ formed by the sustained apposition or fusion of fetal membranes and parental tissue for physiological exchange. This definition is modified from the original
Mossman (1937) definition, which constrained placentation in animals to only those instances where it
376:
Despite the placenta forming from pre-existing tissues, in many instances new structures can evolve within these pre-existing tissues. For example, in male seahorses the underbelly skin has become highly modified to form a pouch in which embryos can develop. In mammals and some reptiles, including
372:
In all placental animals, placentas have evolved through the utilisation of existing tissues. In viviparous mammals and reptiles placentas form from the intimate interaction of the uterus and a series of embryonic membranes including the chorioallantoic and yolk sac membranes. In guppies placental
359:
that has evolved multiple times independently, evolved relatively recently in some lineages, and exists in intermediate forms in living species; for these reasons it is an outstanding model to study the evolution of complex organs in animals. Research into the genetic mechanisms that underpin the
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In mammals, maternal-fetal communication occurs via the production of a range of signalling molecules and their receptors in the chorioallantoic membrane of the embryo and the endometrium of the mother. Examination of these tissues in egg-laying and other independently evolved live bearing
970:
Griffith, OW; Blackburn, DG; Brandley, MC; Van Dyke, JU; Whittington, CW; Thompson, MB (2015). "Ancestral state reconstructions require biological evidence to test evolutionary hypotheses: A case study examining the evolution of reproductive mode in squamate reptiles".
363:
The genetic processes that support the evolution of the placenta can be best understood by separating those that result in the evolution of new structures within the animal and those that result in the evolution of new functions within the placenta.
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Axile : it is found in bicarpellary to multicarpellary syncarpous ovary. The carpels fuse to form septa forming a central axis and ovules are arranged on the axis. E.g.: Hibiscus, lemon, tomato, lilly.
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expression of genes expressed elsewhere in the organism to perform new functions in placental tissues, and the evolution of new molecular processes following the formation of new placenta specific genes.
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occurs when monozygotic twins develop with only one placenta and bears a higher risk of complications during pregnancy. Abnormal placentation can lead to an early termination of pregnancy, for example in
468:
Free central : It is found in bicarpellary to multicarpellary syncarpous ovary. Due to degradation of false septum unilocular condition is formed and ovules are arranged on the central axis. E.g.:
1095:
Griffith, OW; Brandley, MC; Whittington, CM; Belov, K; Thompson, MB (2016). "Comparative genomics of hormonal signaling in the chorioallantoic membrane of oviparous and viviparous amniotes".
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Adams, S. M., Biazik, J. M., Thompson, M. B., & Murphy, C. R. (2005). Cyto‐epitheliochorial placenta of the viviparous lizard
Pseudemoia entrecasteauxii: A new placental morphotype.
878:
Griffith, O. W., Ujvari, B., Belov, K., & Thompson, M. B. (2013). Placental lipoprotein lipase (LPL) gene expression in a placentotrophic lizard, Pseudemoia entrecasteauxii.
267:, placentation is the formation and growth of the placenta inside the uterus. It occurs after the implantation of the embryo into the uterine wall and involves the remodeling of
836:
Itonaga, K.; Wapstra, E.; Jones, S. M. (2012). "A novel pattern of placental leucine transfer during mid to late gestation in a highly placentotrophic viviparous lizard".
1045:"Reptile pregnancy is underpinned by complex changes in uterine gene expression: a comparative analysis of the uterine transcriptome in viviparous and oviparous lizards"
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Griffith, OW; Ujvari, B; Belov, K; Thompson, MB (2013). "Placental lipoprotein lipase (LPL) gene expression in a placentotrophic lizard, Pseudemoia entrecasteauxii".
925:
Blackburn, D. G., & Flemming, A. F. (2010). Reproductive specializations in a viviparous
African skink: Implications for evolution and biological conservation.
347:). Unlike eutherian mammals, epitheliochorial placentation is not maintained by maternal tissue as embryos do not readily invade tissues outside of the uterus.
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Marginal : It is found in monocarpellary unilocular ovary, placenta forms a rigid along ventral side and ovules are arranged in two vertical rows. E.g.:
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Parietal: It is found in bicarpellary to multicarpellary syncarpous ovary. Unilocular ovary becomes bilocular due to formation of false septum.E.g.: Cucumber
509:
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231:, which it does not in the other two types. It may avail for more efficient transfer of nutrients etc., but is also more challenging for the systems of
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Mossman, H. Comparative
Morphogenesis of the Fetal Membranes and Accessory Uterine Structures Vol. 26 (Carnegie Institution of Washington, 1937).
521:
891:
Murphy, C. R.; Hosie, M. J.; Thompson, M. B. (2000). "The plasma membrane transformation facilitates pregnancy in both reptiles and mammals".
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tissues form between the ovarian tissue and the egg membrane. In pipefish placentas form following the interaction with the egg and the skin.
447:
Basal: The placenta is found in mono to multi carpellary, syncarpous ovary. Usually a single ovule is attached at the base (bottom). E.g.:
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1183:"Ancient transposable elements transformed the uterine regulatory landscape and transcriptome during the evolution of mammalian pregnancy"
1301:
718:"The steady state concentration gradients of an electron-dense marker (ferritin) in the three-layered hemochorial placenta of the rabbit"
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Griffith, O. W.; Van Dyke, J. U.; Thompson, M. B. (2013). "No implantation in an extra-uterine pregnancy of a placentotrophic reptile".
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172:. Mammalian placentas can be classified based on the number of tissues separating the maternal from the fetal blood. These include:
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As placentation often results during the evolution of live birth, the more than 100 origins of live birth in lizards and snakes (
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In botany, the term placentation most commonly refers to the arrangement of ovules inside an ovary. Placentation types include:
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In humans, the placenta develops in the following manner. Chorionic villi (from the embryo) on the embryonic pole grow, forming
1015:
134:. In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in
1293:
596:
Griffith, OW; Wagner, GP (2017). "The placenta as a model for understanding the origin and evolution of vertebrate organs".
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436:, the plant part equivalent to an umbilical cord. The part of the ovary where the funiculus attaches is referred to as the
232:
115:. The function of placentation is to transfer nutrients, respiratory gases, and water from maternal tissue to a growing
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evolution of the placenta have been conducted in a diversity of animals including reptiles, seahorses, and mammals.
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in order to supply the needed amount of blood. In humans, placentation takes place 7–8 days after fertilization.
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Lynch, V. J.; Nnamani, M. C.; Kapusta, A.; Brayer, K.; Plaza, S. L.; Mazur, E. C.; Graf, A. (2015).
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776:"Morphology, development, and evolution of fetal membranes and placentation in squamate reptiles"
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1132:"Seahorse brood pouch transcriptome reveals common genes associated with vertebrate pregnancy"
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672:(2006). "Placental invasiveness mediates the evolution of hybrid inviability in mammals".
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however at least two examples of endotheliochorial placentation have been identified (
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Chorionic villi, growing into the apertures of uterine glands ( epithelium). (e.g. in
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Citing: Dorland's
Medical Dictionary for Health Consumers. Copyright 2007 by Saunders
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In hemochorial placentation maternal blood comes in direct contact with the fetal
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T.W. Sadler, Langman's
Medical Embryology, 11th edition, Lippincott & Wilkins
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Journal of
Experimental Zoology Part B: Molecular and Developmental Evolution.
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are in contact with the endothelium of maternal blood vessels. (e.g. in most
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Journal of
Experimental Zoology Part B: Molecular and Developmental Evolution
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Most reptiles exhibit strict epitheliochorial placentation (e.g.
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In live bearing mammals, the placenta forms after the embryo
1016:"Using the placenta to understand how complex organs evolve"
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is the formation, type and structure, or arrangement of the
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Griffith, OW; Brandley, MC; Belov, K; Thompson, MB (2016).
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Claim for guinea pigs, rabbits, mice, and rats taken from:
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At: University of
Hamburg Department of Biology. (see
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293:refers to the presence of two placentas (in all
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44:Placentation in the human resulting from
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27:Formation and structure of the placenta
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412:, placentation is the attachment of
168:is connected to the placenta via an
179:In this type of placentation, the
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424:'s ovary (which later become the
368:Evolution of placental structures
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385:Evolution of placental functions
780:J. Exp. Zool. (Mol. Dev. Evol.)
949:10.1016/j.placenta.2013.03.002
176:endotheliochorial placentation
1:
905:10.1016/s1095-6433(00)00274-9
198:epitheliochorial placentation
1306:- The Internet Hypertextbook
1199:10.1016/j.celrep.2014.12.052
598:Nature Ecology and Evolution
233:gestational immune tolerance
1109:10.1016/j.ygcen.2016.04.017
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328:Pseudemoia entrecasteauxii
303:Monochorionic placentation
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563:Choriovitelline placenta
291:dichorionic placentation
224:hemochorial placentation
142:occurred in the uterus.
674:The American Naturalist
618:10.1038/s41559-017-0072
420:. The ovules inside a
1314:How to Identify Plants
92:Anatomical terminology
1290:Department of Biology
1149:10.1093/molbev/msv177
824:Journal of morphology
314:In lizards and snakes
237:higher order primates
160:into the wall of the
150:Further information:
1286:Fachbereich Biologie
379:southern grass skink
1244:10.1002/jez.b.22526
1159:20.500.11850/110832
1097:Gen Comp Endocrinol
993:10.1002/jez.b.22614
985:2015JEZB..324..493G
858:10.1002/jez.b.22446
850:2012JEZB..318..308I
801:10.1002/jez.b.21234
792:2009JEZB..312..579B
610:2017NatEE...1...72G
432:) are attached via
355:The placenta is an
1335:Plant reproduction
1061:10.1093/gbe/evw229
344:Trachylepis ivensi
1302:Homepage: Botany
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1055:(10): 3226–3239.
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164:. The developing
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109:Placentation
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33:Placentation
1025:24 February
604:(4): 0072.
481:(primroses)
416:inside the
299:monozygotic
249:guinea pigs
73:placentatio
62:Identifiers
48:at various
1340:Embryology
1329:Categories
670:Crespi, B.
574:References
449:Helianthus
185:carnivores
146:In mammals
138:reptiles.
1103:: 19–29.
568:Gynoecium
428:inside a
398:In plants
297:and some
295:dizygotic
265:pregnancy
203:ruminants
18:Placentae
1310:Contents
1298:B-Online
1252:23939756
1217:25640180
1168:26330546
1117:27102939
1079:27635053
1001:25732809
957:23522396
937:Placenta
913:11154940
866:22821866
810:18683170
702:16661549
694:16874618
634:32213223
626:28812655
557:See also
551:Marginal
515:Parietal
472:Dianthus
438:placenta
434:funiculi
351:Research
320:Squamata
301:twins).
158:implants
152:Placenta
136:squamate
113:placenta
46:cleavage
1318:Flowers
1272:below).
1208:4447085
1070:5174741
981:Bibcode
846:Bibcode
788:Bibcode
606:Bibcode
478:Primula
457:Tagetus
263:During
245:rabbits
229:chorion
219:dugongs
124:mammals
85:D010929
57:Details
1304:Online
1250:
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453:Tridex
422:flower
414:ovules
335:Mabuya
255:, and
241:humans
211:whales
207:horses
162:uterus
128:Theria
117:embryo
698:S2CID
630:S2CID
527:Axile
503:Basal
490:(pea)
430:fruit
426:seeds
418:ovary
404:Ovule
357:organ
287:twins
187:like
166:fetus
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68:Latin
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1248:PMID
1213:PMID
1164:PMID
1113:PMID
1075:PMID
1027:2017
997:PMID
953:PMID
909:PMID
862:PMID
806:PMID
784:312B
748:PMID
690:PMID
622:PMID
341:and
257:rats
253:mice
193:dogs
191:and
189:cats
80:MeSH
1240:doi
1236:320
1203:PMC
1195:doi
1154:hdl
1144:doi
1105:doi
1101:244
1065:PMC
1057:doi
989:doi
977:493
945:doi
901:doi
897:127
854:doi
842:318
796:doi
738:PMC
730:doi
682:doi
678:168
614:doi
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339:sp.
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