A.M. Carter

Comparative aspects of trophoblast development and placentation

Based on the number of tissues separating maternal from fetal blood, placentas are classified as epitheliochorial, endotheliochorial or hemochorial. We review the occurrence of these placental types in the various orders of eutherian mammals within the framework of the four superorders identified by the techniques of molecular phylogenetics. The superorder Afrotheria diversified in ancient Africa and its living representatives include elephants, sea cows, hyraxes, aardvark, elephant shrews and tenrecs. Xenarthra, comprising armadillos, anteaters and sloths, diversified in South America. All placentas examined from members of these two oldest superorders are either endotheliochorial or hemochorial. The superorder Euarchontoglires includes two sister groups, Glires and Euarchonta. The former comprises rodents and lagomorphs, which typically have hemochorial placentas. The most primitive members of Euarchonta, the tree shrews, have endotheliochorial placentation. Flying lemurs and all higher primates have hemochorial placentas. However, the lemurs and lorises are exceptional among primates in having epitheliochorial placentation. Laurasiatheria, the last superorder to arise, includes several orders with epitheliochorial placentation. These comprise whales, camels, pigs, ruminants, horses and pangolins. In contrast, nearly all carnivores have endotheliochorial placentation, whilst bats have endotheliochorial or hemochorial placentas. Also included in Laurasiatheria are a number of insectivores that have many conserved morphological characters; none of these has epitheliochorial placentation. Consideration of placental type in relation to the findings of molecular phylogenetics suggests that the likely path of evolution in Afrotheria was from endotheliochorial to hemochorial placentation. This is also a likely scenario for Xenarthra and the bats. We argue that a definitive epitheliochorial placenta is a secondary specialization and that it evolved twice, once in the Laurasiatheria and once in the lemurs and lorises.

Evolution of placental function in mammals: the molecular basis of gas and nutrient transfer, hormone secretion, and immune responses

Placenta has a wide range of functions. Some are supported by novel genes that have evolved following gene duplication events while others require acquisition of gene expression by the trophoblast. Although not expressed in the placenta, high-affinity fetal hemoglobins play a key role in placental gas exchange. They evolved following duplications within the beta-globin gene family with convergent evolution occurring in ruminants and primates. In primates there was also an interesting rearrangement of a cassette of genes in relation to an upstream locus control region. Substrate transfer from mother to fetus is maintained by expression of classic sugar and amino acid transporters at the trophoblast microvillous and basal membranes. In contrast, placental peptide hormones have arisen largely by gene duplication, yielding for example chorionic gonadotropins from the luteinizing hormone gene and placental lactogens from the growth hormone and prolactin genes. There has been a remarkable degree of convergent evolution with placental lactogens emerging separately in the ruminant, rodent, and primate lineages and chorionic gonadotropins evolving separately in equids and higher primates. Finally, coevolution in the primate lineage of killer immunoglobulin-like receptors and human leukocyte antigens can be linked to the deep invasion of the uterus by trophoblast that is a characteristic feature of human placentation.

Evolution of Factors Affecting Placental Oxygen Transfer

A review is given of the factors determining placental oxygen transfer and the oxygen supply to the fetus. In the case of continuous variables, such as the rate of placental blood flow, it is not possible to trace evolutionary trends. Discontinuous variables, for which we can define character states, are more amenable to analysis. This is exemplified by factors contributing, respectively, to blood oxygen affinity and placental diffusing capacity. Comparative genomics has given fresh insight into the evolution of the beta-globin gene complex. In higher primates, duplication of an embryonic gene yielded HBG-T2, a gene that is expressed in the fetus and confers high oxygen affinity on its haemoglobin. A separate event in ruminants involved duplication of an adult gene, again resulting in a fetally expressed variant (HBB-T3) that conveys high oxygen affinity. In rodents and lagomorphs, where fetal and adult haemoglobin are not different, developmental regulation of 2, 3-diphosphoglycerate ensures the high oxygen affinity of fetal blood. Oxygen diffusing capacity is dependent on diffusion distance, which may vary with the type of interhaemal barrier. It has been shown that epitheliochorial placentation is a derived state and that the common ancestor of placental mammals probably had a placenta of the endotheliochorial type. Where evolutionary trends are implied for mammals as a whole or within orders such as primates they often accompany a switch in reproductive strategy that is manifested in a change of newborn state from poorly developed (altricial) to well developed (precocial).

The role of invasive trophoblast in implantation and placentation of primates

We here review the evolution of invasive placentation in primates towards the deep penetration of the endometrium and its arteries in hominoids. The strepsirrhine primates (lemurs and lorises) have non-invasive, epitheliochorial placentation, although this is thought to be derived from a more invasive type. In haplorhine primates, there is differentiation of trophoblast at the blastocyst stage into syncytial and cellular trophoblast. Implantation involves syncytiotrophoblast that first removes the uterine epithelium then consolidates at the basal lamina before continuing into the stroma. In later stages of pregnancy, especially in Old World monkeys and apes, cytotrophoblast plays a greater role in the invasive process. Columns of trophoblast cells advance to the base of the implantation site where they spread out to form a cytotrophoblastic shell. In addition, cytotrophoblasts advance into the lumen of the spiral arteries. They are responsible for remodelling these vessels to form wide, low-resistance conduits. In human and great apes, there is additional invasion of the endometrium and its vessels by trophoblasts originating from the base of the anchoring villi. Deep trophoblast invasion that extends remodelling of the spiral arteries to segments in the inner myometrium evolved in the common ancestor of gorilla, chimp and human.

Placentation in the paca (Agouti paca L)

BackgroundThe paca is a South American rodent with potential as a commercial food animal. We examined paca placenta as part of a wider effort to understand the reproductive biology of this species.MethodsThirteen specimens between midgestation and term of pregnancy were studied by light and transmission electron microscopy.ResultsThe placenta is divided into several lobes separated by interlobular trophoblast. Maternal arterial channels and fetal veins are found at the centre of each lobe. In the labyrinth, maternal blood flows through trophoblast-lined lacunae in close proximity to the fetal capillaries. The interhaemal barrier is of the haemomonochorial type with a single layer of syncytiotrophoblast. Caveolae occur in the apical membrane of the syncytiotrophoblast and recesses in the basal membrane, but there is no evidence of transtrophoblastic channels. The interlobular areas consist of cords of syncytiotrophoblast defining maternal blood channels that drain the labyrinth. Yolk sac endoderm covers much of the fetal surface of the placenta. The subplacenta comprises cytotrophoblast and syncytiotrophoblast. There are dilated intercellular spaces between the cytotrophoblasts and lacunae lined by syncytiotrophoblast. In the junctional zone between subplacenta and decidua, there are nests of multinucleated giant cells with vacuolated cytoplasm. The entire placenta rests on a pedicle of maternal tissue. An inverted yolk sac placenta is also present. The presence of small vesicles and tubules in the apical membrane of the yolk sac endoderm and larger vesicles in the supranuclear region suggest that the yolk sac placenta participates in maternal-fetal transfer of protein.ConclusionThe paca placenta closely resembles that of other hystricomorph rodents. The lobulated structure allows for a larger exchange area and the development of precocial young.

Comparative studies of placentation and immunology in non-human primates suggest a scenario for the evolution of deep trophoblast invasion and an explanation for human pregnancy disorders

Deep trophoblast invasion in the placental bed has been considered the hallmark of human pregnancy. It occurs by two routes, interstitial and endovascular, and results in transformation of the walls of the spiral arteries as they traverse the decidua and the inner third of the myometrium. Disturbances in this process are associated with reproductive disorders such preeclampsia. In contrast, trophoblast invasion in Old World monkeys occurs only by the endovascular route and seldom reaches the myometrium. Recently, it was shown that this pattern is maintained in gibbons, but that the human arrangement also occurs in chimpanzee and gorilla. There is an interesting parallel with results from placental immunology regarding the evolution of the major histocompatability complex class I antigen HLA-C and its cognate receptors. HLA-C is not present in Old World monkeys or gibbons. It emerged in the orangutan and became polymorphic in the lineage leading to gorilla, bonobo, chimpanzee, and human. Interaction between HLA-C1 and HLA-C2 on the surface of trophoblast and killer immunoglobulin-like receptors (KIRs) expressed by uterine natural killer cells are important regulators of trophoblast invasion. Evolution of this system in great apes may have been one prerequisite for deep trophoblast invasion but seems to have come at a price. The evidence now suggests that certain combinations of maternal genotype for KIRs and fetal genotype for HLA-C imply an increased risk of preeclampsia, fetal growth restriction, and recurrent abortion. The fetal genotype is in part derived from the father providing an explanation for the paternal contribution to reproductive disorders.

Glycosylation at the fetomaternal interface in hemomonochorial placentae from five widely separated species of mammal: is there evidence for convergent evolution?

Hemomonochorial placentation occurs in diverse species. We have examined placental glycosylation in five widely separated mammals with this type of placentation – lesser hedgehog tenrec (Echinops telfairi), spotted hyena (Crocuta crocuta), nine-banded armadillo (Dasypus novemcinctus), human (Homo sapiens) and guinea pig (Cavia porcellus) – in order to assess whether evolutionary convergence to the hemomonochorial state is accompanied by a similar convergence of glycan expression. Placentae from 2 E. telfairi, 3 C. crocuta, 1 D. novemcinctus, 4 womenand 1 C. porcellus were fixed and processed into epoxy resin. Binding of twenty-three lectins was assessed using a semiquantitative ranking system. The trophoblast apical/microvillous membrane of all five species showed marked similarities in glycosylation. In the N-linked series, there were abundant bi/tri-antennary complex chains, while the non-bisected variants were much scarcer. All species had plentiful N-acetyl lactosamine sequences; at chain termini, binding to Galβ1,4GlcNAc and Galβ1,3GalNAc sequences was greatly enhanced after neuraminidase treatment. In all species, terminal NeuNAcα2,3 residues were detected. The tenrec had unusually abundant terminal N-acetyl galactosamine. The basal plasma membrane/basal lamina showed glycosylation patterns distinct from the microvillous membrane in each case, indicating chemical diversity of the two opposite faces of trophoblast. Similar classes of glycan at the hemochorial interface suggest conservation of function. The observed lectin binding patterns suggest broad similarities of glycosylation that may have arisen by convergent evolution.

Review: The evolving placenta: Different developmental paths to a hemochorial relationship

The way in which maternal blood is associated with trophoblast prior to the formation of the different types of hemochorial placenta may be conveniently grouped into four main patterns: a transitory endotheliochorial condition; maternal blood released into a mass of trophoblast; maternal blood confined to lacunae; and fetal villi entering preexisting maternal blood sinuses. Although it might be considered logical that developing placentas would pass through an endotheliochorial stage to become hemochorial, this developmental pattern is seen only as a transient stage in several species of bats and sciuromorph rodents. More commonly a mass of trophoblast at the junction with the endometrium serves as a meshwork through which maternal blood passes, with subsequent organization of a labyrinth when the fetal vascular component is organized. The initial trophoblast meshwork may be cellular or syncytial, often leading to a similar relationship in the spongy zone and labyrinth. Old World monkeys, apes and humans have a lacunar stage prior to establishing a villous hemochorial condition. New World monkeys lack a true lacunar stage, retaining portions of maternal vessels for some time and initially forming a trabecular arrangement similar to though differently arrived at than that in the tarsier. In armadillos, preexisting maternal venous sinuses are converted into an intervillous blood space by intruding fetal villi. Variations from the major patterns of development also occur. The way in which the definitive placental form is achieved developmentally should be considered when using placental structure to extrapolate evolution of placentation.

Immunohistochemical identification of epithelial and mesenchymal cell types in the chorioallantoic and yolk sac placentae of the guinea-pig

To define the epithelial and mesenchymal cell types of the guinea-pig placenta, immunostaining patterns were determined for the intermediate filament proteins cytokeratin and vimentin. Chorionic and yolk sac placentae were studied at 15, 20, 25, 29-30, 44-45, 55 and 65 days of gestation. Immunohistochemistry was performed on 5-microm thick sections of paraffin embedded tissue using specific antibodies against cytokeratin, a marker for epithelial cells, including trophoblast, and vimentin, a marker for mesenchymal cells and stromal decidua. Immunostaining was identified by the avidin-biotin-peroxidase technique with diaminobenzidine as the chromogen. Most of the surface of the placenta is covered by the columnar epithelium of the parietal yolk sac, beneath which is found a layer of chorionic giant cells. In the guinea-pig, a sheet of mesenchymal cells interposed between these cell layers immunostained for vimentin, a protein that is expressed only intracellularly, and had nuclei orientated parallel to the surface of the placenta. This cell layer is quite different from Reicherts membrane in the rat or mouse, which is acellular. Within the main placenta, cytokeratin immunostaining demonstrated that the trophoblasts lining the large maternal blood sinuses are different in character from the surrounding syncytiotrophoblast, confirming earlier ultrastructural observations. In the subplacenta, some trophoblast did not immunostain for cytokeratin and there was non-specific staining of cellular debris, so that immunostaining for vimentin provided the clearest indication of the maternal-fetal interface. In later stages of gestation (30-55 days), trophoblasts invading the walls of maternal arteries immunostained for cytokeratin and were vimentin negative. In early gestation, however, trophoblast invasion of the maternal vessels was indicated by cells that were immunoreactive for both cytokeratin and vimentin.

Restriction of placental and fetal growth in the guinea-pig

Ligation of the uterine artery in mid-pregnancy is a simple procedure that restricts the subsequent growth of the placentae and fetuses. It was introduced for the rat by Wigglesworth (1964) and first applied to the guinea-pig by Lafeber (1981). The guinea-pig fetus weighs > 100 g at term (68 days) and even the growth-restricted fetus is amenable to surgical intervention, permitting study of circulatory adaptations to intrauterine growth restriction (IUGR) and measurements of placental transfer. The limitations of the model are set by fetal size and by the necessity of performing acute studies under anaesthesia. Therefore, it is important to recognize that there are alternative models of IUGR in sheep that allow measurements to be made on the unanaesthetized fetus. Each horn of the guinea-pig uterus is supplied by an arterial arcade formed by the ovarian and uterine arteries (Egund and Carter, 1974). Ligation of one uterine artery results in the loss of all fetuses from the ipsilateral horn in one-third of all animals (Detmer and Carter, 1992). In the remainder, some or all of the fetuses survive, supported by the blood supply from the ovarian artery. The extent to which growth restriction occurs is variable, both within and between litters. Because asymmetrical growth restriction is a significant feature of IUGR, we have chosen to select fetuses with an increased ratio of brain weight to liver weight. With the cut-offset arbitrarily at a ratio of 0.85, 35 per cent of fetuses are growth restricted, with at least one such fetus occurring in 48 per cent of experiments. A different result is obtained if the fetuses are classified by body weight, which some authors use to divide the material into subgroups with different degrees of growth restriction (Lafeber, Rolph and Jones, 1984).