File Name: placental development and function ppt to .zip
- Human placenta and trophoblast development: key molecular mechanisms and model systems
- The Effects of Glucocorticoids on Fetal and Placental Development
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- The placenta in preeclampsia
Human placenta and trophoblast development: key molecular mechanisms and model systems
The very apt definition of a placenta is coined by Mossman, namely apposition or fusion of the fetal membranes to the uterine mucosa for physiological exchange. As such, it is a specialized organ whose purpose is to provide continuing support to the developing young. By this definition, placentas have evolved within every vertebrate class other than birds.
They have evolved on multiple occasions, often within quite narrow taxonomic groups. As the placenta and the maternal system associate more intimately, such that the conceptus relies extensively on maternal support, the relationship leads to increased conflict that drives adaptive changes on both sides. The story of vertebrate placentation, therefore, is one of convergent evolution at both the macromolecular and molecular levels.
In this short review, we first describe the emergence of placental-like structures in nonmammalian vertebrates and then transition to mammals themselves. We close the review by discussing the mechanisms that might have favored diversity and hence evolution of the morphology and physiology of the placentas of eutherian mammals.
Viviparity has evolved independently and seemingly on multiple occasions across a diverse array of animal groups, including invertebrates Kalinka It is a phenomenon whereby developing embryos are retained within the reproductive tract, leading to release of live offspring as an alternative to the more fecund egg laying or spawning. One consequence of viviparity is that the retained, fertilized egg must either survive off its own reserves, usually yolk, or obtain some or part of these resources from the mother.
The latter situation, of necessity, is expected to lead to increased conflict over how provisions are partitioned between the supplier and the recipient, thus potentially sparking a genetic arms race. In turn, such conflict is expected to drive adaptive changes that lead to a more intimate and possibly more felicitous relationship between offspring and the reproductive tract of the mother that favors the transmission of both maternal and paternal genes to the next generation, despite a loss of overall fecundity.
The transition from oviparity to viviparity and the subsequent emergence of placentation within some vertebrate taxa clearly required major changes in the morphology and physiology of the reproductive tract and has its origins well before the advent of mammals Van Dyke et al. It is a specialized organ whose purpose is to provide continuing support to the developing young through the provision of water, nutrients and gases, and to regulate maternal—fetal interactions often through hormone production.
Although placentation arose once in the common ancestor of mammals, it has arisen independently multiple times within other classes, and even families.
In this short review of placental evolution, we first describe the emergence of placental-like structures in nonmammalian vertebrates and then transition to mammals themselves.
We close the review by discussing mechanisms that might have favored diversity and hence evolution of the morphology and physiology of the placentas of eutherian mammals. Of necessity, many important references cannot be cited in a short review of this kind. Instead, we have attempted to direct the reader to scholarly articles that do list the primary source material.
Viviparity is the most common mode of reproduction in elasmobranchs. In the elasmobranchs, two placental types are observed. Among stingrays, finger-like projections of the uterine wall, termed trophonemata, provide histotrophic nutrition to the developing embryo Hamlett et al. The mature embryo of the cownose ray, for example, weighs times as much as the egg as a result of reliance on the mother rather than the egg yolk Hamlett et al.
After the yolk has been consumed, the yolk sac becomes modified into an umbilical cord region and a placental region Hamlett et al. This epithelial layer is lined with microvilli, while the surrounding tissue is highly vascularized, presumably to facilitate maternal—fetal exchange Kwan et al. This type of placental structure has arisen several times during Poeciliopsis evolution Kwan et al. The degree of contact and exchange between developing embryos and the father range from minimal to situations that display all the defining features of placentation Wilson et al.
Viviparity has arisen multiple times in amphibians, accompanied in some cases with the evolution of placentas. In the marsupial frogs, development occurs inside a specialized maternal pouch on the back of the animal.
Following ovulation, secretory cells of the vascularized pouch enlarge. The pouch epithelium is closely apposed to specialized fetal gills, allowing exchange between maternal and fetal circulations Savage This, as A. Arguably, however, a structure found outside the reproductive tract, especially following external fertilization that occurs in these frogs and the sea horses described earlier, does not fit the typical definition of a placenta. Viviparity occurs in more than 25 families of lizards and snakes squamate reptiles.
In many species, the embryo relies on egg contents for nutrition, but in others a range of adaptations of the female reproductive tract provides a means to exchange gases and nutrients with the conceptus Stewart In squamates the placenta is chorioallantoic, but unlike mammals it does not develop from an early arising, extraembryonic trophoblast layer. Most squamate placentas demonstrate a simple interdigitation of the chorioallantois with the uterine epithelium Stewart , but in some skinks the interface is more intimate Fig.
Viviparity in reptiles: A Drawing from a section of the mature chorioallantoic placenta of the Australian skink Egernia cunninghami carrying developing young, showing the maternal capillaries closely adjoining the allantoic capillaries on the fetal side. B Drawing from a section of the immature chorioallantoic placenta of the Australian skink Egernia entrecasteaux.
The section illustrates the folded placental face likely involved in releasing histotrophic material that can be taken up by the chorionic ectoderm. Also, note the close apposition of maternal blood capillaries with the epithelium of the reproductive tract.
The diagrams are based on Fig. Amoroso's review on placentation Amoroso , which were redrawn from Weekes Citation: Reproduction , 5; The chorioallantoic placenta of viviparous lizards can be highly efficient in facilitating exchange of nutrients and gases.
There is also evidence for the expression of active transporters at the sites of maternal—fetal apposition in some species Murphy et al. Overall, where placentation occurs in squamates, it bears a superficial resemblance to the epitheliochorial placentation encountered in some eutherian mammals and carries out many of the same functions, but its formation does not involve the formation of a trophoblast lineage, a feature unique to placental mammals.
All mammals except the egg-laying platypus and the five species of echidnas, the only surviving monotremes, rely on a placenta for their reproduction. Indeed, the first lineage decision made during embryonic development of Mammalia is the segregation of cells that are destined to become the external tissue layer of the placenta.
This lineage, usually called trophoblast or, because it forms the exterior of the conceptus, trophectoderm , diverges early from the pluripotent lineage that advances to form the inner cell mass. The cells that give rise to the hypoblast and epiblast are unique to mammals.
The term trophoblast was first coined by Hubrecht When and how trophoblast emerged in the presumed metatherian ancestors of the Class Mammalia is mysterious and unlikely ever to be revealed from the fossil record. However, the placentas are short-lived and marsupials give birth to relatively underdeveloped young.
Both the embryonic and extraembryonic lineages emerge from this single cell layer, with trophoblast materializing from the pole of the blastocyst opposite the position of the future embryo Frankenberg et al. There are also indications that early lineage specification may be controlled by a similar set of transcription factors as in eutherians Frankenberg et al.
The trophoblast cells originating from the marsupial blastocyst combine with the yolk sac endoderm and extraembryonic mesoderm to form a yolk sac placenta, which can be bi- or trilaminar Renfree However, there are a species of koalas, wombats and bandicoots, in which a chorioallantoic placenta forms, but it appears to be supplementary to the main yolk sac placenta Freyer et al.
Although the placentas of marsupials are generally regarded as noninvasive and of the epitheliochorial type see next section , an area of syncytium forms in the yolk sac placenta of the gray short-tailed opossum and possibly related species. Additionally, trophoblast syncytialization syncytial trophoblast; STB , where adjacent cells fuse to produce cells with more than a single nucleus, is accompanied by the expression of an endogenous, retrovirus-derived protein, analogous to what occurs in many eutherians Cornelis et al.
Another similarity to the eutherians is that the placenta of the tammar wallaby secretes a complement of hormones that include IGF2 and relaxin, which, in that species, may be responsible for the phenomenon of maternal recognition of pregnancy Renfree Origin of trophoblast: Eutherians consist over species in roughly 20 phylogenetic orders. Most of our knowledge about their early development and the events that lead to the initial specification of trophoblast has been obtained only in a handful of these species, especially the mouse.
In mouse Mus musculus , the first visible differentiation event, called compaction, occurs between the 8 and 16 cell stage of embryonic development, when the blastomeres polarize to form extended contact zones with their neighbors and an outward-facing apical surface Posfai et al.
Further symmetrical and asymmetrical divisions follow, so that by the 16 cell morula stage a population of polarized cells exist on the cell surface, which becomes trophectoderm, and another population of cells are positioned inside, which is the precursor of the inner cell mass ICM. By the time of blastocyst formation, when the mouse conceptus comprises about 32 cells, about two-thirds of which are trophectoderm, the precursors of extraembryonic endoderm hypoblast have begun to segregate toward the base of the ICM.
From this location, they migrate outward to line the trophectoderm, forming the yolk sac cavity. Later, extraembryonic mesoderm joins with the trophectoderm to form what at this stage is a bilayered chorion. Although there are clear morphological similarities in the steps leading to blastocyst formation in rodents, humans and domestic mammalian species, there are differences in how these events are timed and subsequently played out.
In many species, for example, mesoderm differentiates to vascularize the yolk sac, forming a yolk sac placenta that is eventually replaced by the chorioallantoic placenta.
Such a placenta does not form in humans, but persists to term in rodents and lagomorphs. Similarly, compared with the mouse, conceptuses from many species undergo at least one further round of cell division before the blastocoel cavity becomes visible.
For details on how placentas subsequently develop and begin to diverge in morphology, the reader is directed to the still germane reviews by Amoroso and Renfree For example, placentas come in a range of shapes and sizes, and have been placed in groups based on the general gross morphologies of the sites where the chorion attaches to the endometrium Fig. They have also been classified according to how many cell layers separate the maternal and fetal circulations Fig.
A simplified version of this system recognizes three main placental types Renfree : epitheliochorial, in which there is no erosion of the uterine epithelium; endotheliochorial, where the invading trophoblast reaches but does not penetrate maternal capillaries within the endometrium; and hemochorial, where the trophoblast surface is in direct contact with maternal blood Fig.
A fourth type, synepitheliochorial, has been recently used to describe the placentas of ruminants Wooding Here, specialized trophoblast binucleated cells fuse with uterine epithelial cells to form trinucleated cells, a syncytium, or both.
The placentas of ruminants, with the exception of the phylogenetically more primitive tragulids, are also characterized as cotyledonary Fig. Cotyledons are the primary sites of attachment of the chorion to the endometrium and comprise tufts of villous trophoblast that interdigitate and interlock with complementary structures on the endometrium, called caruncles. It is within these sites, often known as placentomes, that binucleated cells, placental and maternal blood capillaries, and placental interchange are most concentrated.
Therefore, this kind of placenta is a derived form, secondary to the more superficial, diffuse epitheliochorial type typical of most other artiodactyls. Cartoon indicating how placentas can be classified according to the numbers and kinds of cell layers that separate the bloodstreams of the mother and conceptus.
Superficial vs invasive placentation: Intuitively, it might be inferred that the least invasive types of placenta, the kind where multiple cell layers separate the two blood supplies Fig. Also, such placentas have a resemblance to placentas described earlier for nonmammalian species, such as skinks Fig. However, this is clearly not the case Wildman et al. Compare, for example, the relative maturity and independence of a newborn horse, pig or whale, supported during their gestations by diffuse epitheliochorial placentas, to a hapless pup born to a mouse or a baby born to a human, species where the placenta is hemochorial and discoid.
Nor is the more superficial form of placenta evolutionarily more ancient than the kinds that are invasive and make direct contact with maternal blood.
On the contrary, careful analyses, in which placental features were mapped to mammalian phylogenies, have clearly dispelled this myth and demonstrated that the epitheliochorial placenta is not only a derived form that has evolved from a more invasive placental type, but has arisen independently in three distinct mammalian lineages, including primates Wildman et al.
The evolution of the placental interface in terms of degree of invasiveness of the placental tissue into maternal tissue, with epitheliochorial being the least invasive and hemochorial being most invasive. From Fig. It should be stressed that the perceived disadvantages of extended diffusion distances for dissolved gases and solutes encountered with a superficial placenta, like that of the pig, are largely illusory.
As pregnancy proceeds, the surfaces of the uterine and trophoblast epithelial layers become interlocked Fig. Capillaries on the maternal and fetal sides come within a few microns of each other Fig. Movement of macromolecules that carry essential components such as metals and vitamins is more problematic, but an alternative strategy has been employed, namely the delivery of these components not in the blood but in uterine secretions — a process called histotrophic nutrition Fig.
The uterine glands of the pig, for example, release uteroferrin, a bi-iron containing acid phosphatase to supply iron. Uteroferrin and other proteins with a similar provisioning function are taken up by specialized regions of endocytic trophoblast cells congregated in cup-like structures called areolae, which develop opposite the mouths of each uterine gland Fig.
From there, the maternal factors are transported via the fetal bloodstream to the liver and other organs for processes such as hematopoiesis Renegar et al.
The Effects of Glucocorticoids on Fetal and Placental Development
The very apt definition of a placenta is coined by Mossman, namely apposition or fusion of the fetal membranes to the uterine mucosa for physiological exchange. As such, it is a specialized organ whose purpose is to provide continuing support to the developing young. By this definition, placentas have evolved within every vertebrate class other than birds. They have evolved on multiple occasions, often within quite narrow taxonomic groups. As the placenta and the maternal system associate more intimately, such that the conceptus relies extensively on maternal support, the relationship leads to increased conflict that drives adaptive changes on both sides. The story of vertebrate placentation, therefore, is one of convergent evolution at both the macromolecular and molecular levels.
Looking for other ways to read this?
The placenta is a temporary fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the baby via the umbilical cord, and on the opposite aspect to the maternal uterus in a species dependent manner. In humans, a thin layer of maternal decidual endometrial tissue comes away with the placenta when it is expelled from the uterus following birth sometimes incorrectly referred to as the 'maternal part' of the placenta. Placentas are a defining characteristic of placental mammals , but are also found in marsupials and some non-mammals with varying levels of development.
The root cause of preeclampsia is the placenta. Preeclampsia begins to abate with the delivery of the placenta and can occur in the absence of a fetus but with the presence of trophoblast tissue with hydatidiform moles. In view of this, study of the placenta should provide insight into the pathophysiology of preeclampsia. In this presentation we examine placental pathological and pathophysiological changes with preeclampsia and fetal growth restriction FGR. It would seem that this comparison should be illuminating as both conditions are associated with similarly abnormal placentation yet only in preeclampsia is there a maternal pathophysiological syndrome.
Glucocorticoids GCs , steroid hormones produced predominantly by the adrenal gland, are key mediators of stress responses. Whilst the acute and chronic effects of pharmacological glucocorticoid excess are well-recognized including induction of hyperglycemia, insulin resistance, hyperlipidemia, hypertension and dysphoria, with suppression of immune, inflammatory and cognitive processes , their role in the biology of the response to stress is more nuanced, with balanced homeostatic effects to facilitate short-term survival and recovery from challenge [ 1 , 2 ]. In addition, glucocorticoids play an essential role in normal fetal development and are important for the development and maturation of various fetal tissues including the liver, lungs, gut, skeletal muscle and adipose tissue in preparation for extrauterine life. Glucocorticoids most notably act during late gestation to stimulate surfactant production by the lung.
The placenta in preeclampsia
Abnormal placentation is considered as an underlying cause of various pregnancy complications such as miscarriage, preeclampsia and intrauterine growth restriction, the latter increasing the risk for the development of severe disorders in later life such as cardiovascular disease and type 2 diabetes. Despite their importance, the molecular mechanisms governing human placental formation and trophoblast cell lineage specification and differentiation have been poorly unravelled, mostly due to the lack of appropriate cellular model systems. However, over the past few years major progress has been made by establishing self-renewing human trophoblast stem cells and 3-dimensional organoids from human blastocysts and early placental tissues opening the path for detailed molecular investigations.
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. The mechanisms by which this takes place involve a complex interplay of systems, including the microbiome, metabolome, placenta, and epigenetic changes. The seventh session of the workshop, moderated by Emily Oken of the Harvard Medical School and the Harvard Pilgrim Health Care Institute, explored the evidence on these systems and the implications of maternal nutrition on early-life programming. Highlights from the session presentation and panel discussion are presented in Box
Development of the maternal blood supply to the placenta is complete by the end of the first trimester of FUNCTIONS OF PLACENTA;
If your institution subscribes to this resource, and you don't have a MyAccess Profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus. Please consult the latest official manual style if you have any questions regarding the format accuracy. Almost immediately after the implantation of the ovum, its trophoblast begins to proliferate and invade the surrounding decidual tissue.
It lies implanted on the uterine wall. It is connected with fetus through umbilical cord in the amniotic cavity. It maintains pregnancy and carries vital fetal functions.
ГЛАВА 23 Сьюзан, сидя в одиночестве в уютном помещении Третьего узла, пила травяной чай с лимоном и ждала результатов запуска Следопыта. Как старшему криптографу ей полагался терминал с самым лучшим обзором. Он был установлен на задней стороне компьютерного кольца и обращен в сторону шифровалки. Со своего места Сьюзан могла видеть всю комнату, а также сквозь стекло одностороннего обзора ТРАНСТЕКСТ, возвышавшийся в самом центре шифровалки. Сьюзан посмотрела на часы.