Placentation

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Placentation
Placentation resulting from cleavage at various gestational ages
Details
Identifiers
Latinplacentatio
MeSHD010929
Anatomical terminology

In biology, placentation refers to the formation, type and structure, or arrangement of placentas. The function of placentation is to transfer nutrients, respiratory gasses, water from maternal tissue to a growing embryo and in some instances to remove waste from the embryo. Placentation is best known in live bearing mammals (theria), but also occur in some fish, reptiles, amphibians, a diversity of invertebrates, and flowering plants. In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in squamate reptiles.

The placenta can be defined as an organ formed by the sustained apposition or fusion of fetal membranes and parental tissue for physiological exchange[1]. This definition is modified from the original Mossman (1937)[2] definition, which constrained placentation in animals to only those instances where it occurred in the uterus.

Placentation in mammals

Further information: Placenta

In placental mammals, the placenta forms after the embryo implants into the wall of the uterus. The developing fetus is connected to it via an umbilical cord. Animal placentas are classified based on the number of tissues separating the maternal from the fetal blood. The placentation types found in animals are:

  • endotheliochorial placentation

In this type of placentation, the chorionic villi are in contact with the endothelium of maternal blood vessels. (e.g. in most carnivores like cats and dogs)

  • epitheliochorial placentation

Chorionic villi, growing into the apertures of uterine glands ( epithelium). (e.g. in ruminants, horses, whales, lower primates)

In hemochorial placentation maternal blood comes in direct contact with the fetal chorion, which it does not in the other two types.[4] It may avail for more efficient transfer of nutrients etc., but is also more challenging for the systems of gestational immune tolerance to avoid rejection of the fetus.[5]

During pregnancy, 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 blood vessels in order to supply the needed amount of blood. In humans, placentation takes place 7–8 days after fertilization.

In humans, the placenta develops in the following manner. Chorionic villi (from the embryo) on the embryonic pole grow, forming chorion frondosum. Villi on the opposite side (abembryonic pole) degenerate and form the chorion laeve (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.[6]

In the case of twins, dichorionic placentation refers to the presence of two placentas (in all dizygotic and some monozygotic twins). Monochorionic placentation 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 pre-eclampsia.

Placentation in lizards and snakes

As placentation often results during the evolution of live birth, the more than 100 origins of live birth in lizards and snakes (Squamata) 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,[7] 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,[8] amino acid,[9] and lipid transport.[10] Placentae form following implantation into uterine tissue (as seen in mammals) and formation is likely facilitated by a plasma membrane transformation.[11]

Most reptiles exhibit strict epitheliochorial placentation (e.g. Pseudemoia entrecasteauxii) however at least two examples of endotheliochorial placentation have been identified (Mabuya sp. and Trachylepis ivensi).[12] Unlike eutherian mammals, epitheliochorial placentation is not maintained by maternal tissue as embryos do not readily invade tissues outside of the uterus.[13]

Placenta as a model for understanding the evolution of new organs

The placenta is an organ that has evolved multiple times independently[14], 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[1][15]. Research into the genetic mechanisms that underpin the evolution of the placenta have been conducted in a diversity of animals including reptiles[16][17], seahorses[18], and mammals[19].

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[1].

Evolution of placental structures

In all placental animals, placentas have evolved through the utilisation of existing tissues[1]. In viviparous mammals and reptiles placenta's form from the intimate interaction of the uterus and a series of embryonic membranes including the chorioallantoic and yolk sac membranes. In guppies placental tissues form between the ovarian tissue and the egg membrane. In pipefish placentas form following the interaction with the egg and the skin.

Despite the placenta forming from pre-existing tissues, in many instances new structures can evolve within this 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 the viviparous Southern grass skink, 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[20][21].

Placentation in plants

Main article: Ovule

In flowering plants, placentation occurs where the ovules are attached inside the ovary.[22] The ovules inside a flower's ovary (which later become the seeds inside a fruit) are attached via funiculi, the plant part equivalent to an umbilical cord. The part of the ovary where the funiculus attaches is referred to as the placenta.

In botany, the term placentation most commonly refers to the arrangement of placentas inside a flower or fruit. Plant placentation types include:

  • Basal placentation: The placenta is at the base (bottom) of the ovary. Simple or compound carpel.
  • Apical placentation: The placenta is at the apex (top) of the ovary. Simple or compound carpel.
  • Parietal placentation: The placentas are in the ovary wall within a non-sectioned ovary. Compound carpel.
  • Axile placentation: The ovary is sectioned by radial spokes with placentas in separate locules. Compound carpel.
  • Free or central placentation: The placentas are in a central column within a non-sectioned ovary. Compound carpel.
  • Marginal placentation: There is only one elongated placenta on one side of the ovary, as ovules are attached at the fusion line of the carpel's margins . This is conspicuous in legumes. Simple carpel.

See also

References

  1. ^ a b c d Griffith, OW & Wagner, GP (2017) The placenta as a model for understanding the origin and evolution of vertebrate organs, Nature Ecol Evol, 1 (4) 0072. http://dx.doi.org/10.1038/s41559-017-0072
  2. ^ Mossman, H. Comparative Morphogenesis of the Fetal Membranes and Accessory Uterine Structures Vol. 26 (Carnegie Institution of Washington, 1937).
  3. ^ Claim for guinea pigs, rabbits, mice, and rats taken from: Thornburg KL, Faber JJ (October 1976). "The steady state concentration gradients of an electron-dense marker (ferritin in the three-layered hemochorial placenta of the rabbit". J. Clin. Invest. 58 (4): 912–25. doi:10.1172/JCI108544. PMC 333254. PMID 965495.
  4. ^ thefreedictionary.com > hemochorial placenta Citing: Dorland's Medical Dictionary for Health Consumers. Copyright 2007 by Saunders
  5. ^ Elliot, M.; Crespi, B. (2006). "Placental invasiveness mediates the evolution of hybrid inviability in mammals". The American Naturalist. 168 (1): 114–120. doi:10.1086/505162. PMID 16874618.
  6. ^ T.W. Sadler, Langman's Medical Embryology, 11th edition, Lippincott & Wilkins
  7. ^ Blackburn, DG; Flemming, AF (2009). "Morphology, development, and evolution of fetal membranes and placentation in squamate reptiles". J. Exp. Zool. (Mol. Dev. Evol.). 312B: 579–589. doi:10.1002/jez.b.21234.
  8. ^ 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. Journal of morphology, 264(3), 264-276.Chicago
  9. ^ 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. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, 318(4), 308-315.
  10. ^ Griffith, O. W., Ujvari, B., Belov, K., & Thompson, M. B. (2013). Placental lipoprotein lipase (LPL) gene expression in a placentotrophic lizard, Pseudemoia entrecasteauxii. Journal of Experimental Zoology Part B: Molecular and Developmental Evolution.
  11. ^ Murphy, C. R.; Hosie, M. J.; Thompson, M. B. (2000). "The plasma membrane transformation facilitates pregnancy in both reptiles and mammals". Comparative Biochemistry and Physiology A. 127 (4): 433–439. doi:10.1016/s1095-6433(00)00274-9.
  12. ^ Blackburn, D. G., & Flemming, A. F. (2010). Reproductive specializations in a viviparous African skink: Implications for evolution and biological conservation.
  13. ^ Griffith, O. W.; Van Dyke, J. U.; Thompson, M. B. (2013). "No implantation in an extra-uterine pregnancy of a placentotrophic reptile". Placenta.
  14. ^ Griffith, OW, Blackburn, DG, Brandley, MC, Van Dyke, JU, Whittington, CW, & Thompson, M.B. (2015) Ancestral state reconstructions require biological evidence to test evolutionary hypotheses: A case study examining the evolution of reproductive mode in squamate reptiles, J Exp Zool Part B, 493 (6) 493-503.  http://dx.doi.org/10.1002/jez.b.22614
  15. ^ Griffith, Oliver (23 February 2017). "Using the placenta to understand how complex organs evolve". The Conversation. Retrieved 24 February 2017. {{cite news}}: Cite has empty unknown parameter: |dead-url= (help)
  16. ^ Griffith, OW, Brandley, MC, Belov, K, & Thompson, MB (2016) Reptile pregnancy is underpinned by complex changes in uterine gene expression: a comparative analysis of the uterine transcriptome in viviparous and oviparous lizards, Genome Biol Evol, 8 (10) 3226-3239. http://dx.doi.org/10.1093/gbe/evw229
  17. ^ 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, Gen Comp Endocrinol. http://dx.doi.org/10.1016/j.ygcen.2016.04.017
  18. ^ Whittington, CW, Griffith, OW, Qi, W, Thompson, MB, & Wilson, AB (2015) Seahorse brood pouch transcriptome reveals common genes associated with vertebrate pregnancy, Mol Biol Evol, 32 (12) 3114-3131. http://dx.doi.org/10.1093/molbev/msv177
  19. ^ Lynch, V. J., Nnamani, M. C., Kapusta, A., Brayer, K., Plaza, S. L., Mazur, E. C., ... & Graf, A. (2015). Ancient transposable elements transformed the uterine regulatory landscape and transcriptome during the evolution of mammalian pregnancy. Cell reports, 10(4), 551-561.
  20. ^ Griffith, OW, Brandley, MC, Belov, K, & Thompson, MB (2016) Reptile pregnancy is underpinned by complex changes in uterine gene expression: a comparative analysis of the uterine transcriptome in viviparous and oviparous lizards, Genome Biol Evol, 8 (10) 3226-3239. http://dx.doi.org/10.1093/gbe/evw229
  21. ^ Griffith, OW, Ujvari, B, Belov, K, & Thompson, MB (2013) Placental lipoprotein lipase (LPL) gene expression in a placentotrophic lizard, Pseudemoia entrecasteauxii. J Exp Zool Part B, 320 (7) 465-470. http://dx.doi.org/10.1002/jez.b.22526
  22. ^ "Flowers" At: Botany Online At: University of Hamburg Department of Biology. (see External links below).

External links

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