- Email: [email protected]
Comparative Aspects of Placental Evolution: A Workshop Report
Placenta 28, Supplement A, Trophoblast Research, Vol. 21 (2007) S129eS132
Comparative Aspects of Placental Evolution: A Workshop Report A.M. Carter a,*, B.A. Croy b,c, V. Dantzer d, A.C. Enders e, S. Hayakawa f, A. Mess g, H. Soma h a
Physiology and Pharmacology, University of Southern Denmark, Odense, Denmark Department of Anatomy and Cell Biology, Queen’s University, Kingston, Ontario, Canada c Department of Biomedical Sciences, University of Guelph, Guelph, Ontario, Canada d Department of Basic Animal and Veterinary Sciences, The Royal Veterinary and Agricultural University, Copenhagen, Denmark e Department of Cell Biology and Human Anatomy, University of California, Davis, CA, USA f Department of Infectious Disease Control, Nihon University Advanced Medical Research Center, Tokyo, Japan g Museum of Natural History, Humboldt-Universita¨t Berlin, Berlin, Germany h Department of Obstetrics and Gynecology, Saitama Medical School, Tokyo, Japan b
Accepted 24 January 2007
Keywords: Carcharhinidae; Cetartiodactyla; Haemophagous organ; Hyaenidae; Luteinising hormone; Placentation; Sphyrnidae; Tenrecoidea; Trophoblast giant cells; Uterine natural killer cells; Vascular endothelial growth factor
1. Introduction
2. Evolution of placentation in Cetartiodactyla
Recent advances in phylogenetics have cast new light on the variation in placental form that is the focus of comparative studies. As an example, ideas about the evolution of placentation in mammals can be tested against current hypotheses on the phylogenetic relationships of mammalian orders [1]. Three contributions to this workshop concern epitheliochorial placentation in Cetartiodactyla, an order comprising whales, dolphins and even-toed ungulates. Carnivora mostly have endotheliochorial placentation, but the hyena is presented as an interesting exception that may indicate how an endotheliochorial placenta could evolve into a haemochorial one. In contrast to orders where one type of placentation predominates, there is variation at the family level between tenrecs. Uterine natural killer cells play a key role in the immunological response to pregnancy and in vascular remodelling of uterine arteries. These functions are reviewed here across a broad spectrum of mammals. Finally, it is recalled that placentation occurs in nearly all classes of vertebrate, as exemplified here by the hammerhead and spottail sharks.
It is apparent from fossil and molecular evidence that whales and dolphins nest within the even-toed ungulates. Relationships within the combined order, Cetartiodactyla, are well understood [2]. Analysis of molecular data sets has given fresh insight into higher-level relationships of living Eutheria, and, using the outcomes of molecular data sets, one can now gain a better understanding of an organ system such as the placenta. According to a recent analysis, the stem species pattern of Eutheria includes a medium-invasive, endotheliochorial placenta [1]. Non-invasive forms have evolved from this condition, but the functional significance of the evolution of epitheliochorial placentae from more invasive ones is unresolved. Since all taxa within Cetartiodactyla have epitheliochorial placentation, they are a suitable choice to address this question. Against this background, Andrea Mess presented a reconstruction of the stem species pattern of Cetartiodactyla and major evolutionary transformations within this clade. The collaborative study [3] was done by application of the computer programme MacClade on a given phylogeny [2]. The main result was that a diffuse epitheliochorial placenta, without placentomes, is part of the stem species pattern of Cetartiodactyla. The polycotyledonary placenta with more than 50 discrete cotyledons is an apomorphic character
* Corresponding author. Tel.: þ45 6550 3716; fax: þ45 6613 3479. E-mail address: [email protected] (A.M. Carter). 0143-4004/$ - see front matter Ó 2007 Published by IFPA and Elsevier Ltd. doi:10.1016/j.placenta.2007.01.014
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for Pecora (higher ruminants) and the oligocotyledonary placenta, which usually has only 5e8 cotyledons, developed as a further step on the lineage of cervids. Trophoblast giant cells (TGCs) evolved independently in camelids and ruminants. They are characterised by distinct morphologies, being multinucleate in camelids and binucleate in ruminants. These findings were discussed in light of the ‘‘viviparitydriven conflict hypothesis’’ [4], which states that divergent interests of mother and offspring lead to a rapid, antagonistic co-evolution, which might have contributed to placental diversity. For instance, the evolution of TGCs can be interpreted as a means to increase fetal endocrine influence on maternal metabolism, either by releasing steroids in camelids or by delivering trophoblast-derived proteins into the maternal system in ruminants.
gestation. VEGF, Flt-1 and KDR exhibited intense staining in the uterine epithelium, uterine glandular epithelium and trophoblast. The endothelial cells and smooth muscle cells of the vessels in the maternal and fetal compartments were also immunoreactive. In porcine placenta [8] the VEGF ligand-receptor system participates in placental regulation. These factors occur in peccary placenta in a similar pattern and likely with comparable functions. 5. An unusual haemochorial placenta: the hyena at midpregnancy
All mammals exhibit pituitary-specific expression of luteinising hormone (LH) and follicle-stimulating hormone, but placental expression of gonadotrophins has been reported only in primates and equids. Satoshi Hayakawa and collaborators presented work on expression of LH genes in dolphin placenta [5]. They had recently cloned bottlenose dolphin (Tursiops truncatus) LH genes from placental and pituitary cDNAs. Localization of LHb in dolphin placentae was established with immunohistochemical methods. The cDNA sequence of the a-subunit coded for 120 amino acids and showed greatest homology to that of pig pituitary LHa (96.7%). The b-subunit coded for 141 amino acids and showed greatest homology to b-subunits of pig (94.3%) and white rhinoceros (93.3%). Of great interest, dolphin LH genes lack carboxyl terminal peptide (CTP) sequences. Evolutionary conservation of CTP was recently reported in various mammalian LH/chorionic gonadotrophin genes and they are considered to serve as an effective linker to enhance hormonal secretion [6]. It was suggested that the primordial mammalian LHb gene existed in the common ancestor of Perissodactyla, Cetartiodactyla and Primates but that CTP sequences evolved, by different genetic mechanisms, when needed to ensure endocrine efficiency.
The hyena is unique among carnivores in having a definitive placenta that is haemomonochorial. Allen Enders and collaborators had examined the mid-term placenta of the spotted hyena [9]. Although the looping structure of the blood channels of the 30-day placenta suggests that it may have originated as endotheliochorial, by midterm (60e65 days), an elaborate system of anastomotic processes extends throughout the labyrinthine maternal blood space. The arrangement within the labyrinth indicates a crosscurrent blood flow rather than the countercurrent blood flow previously suggested [10]. Primary fetal villi extend from the allantochorionic plate to the basal junctional zone. Processes extend laterally from the primary villi at all levels, but at the junctional zone the tips of the primary villi are expanded and have a different structure. Although the labyrinth has syncytial trophoblast with intrasyncytial lacunae facing the maternal blood space, regions of simple columnar trophoblast are consistently found at the tips of the primary villi and in the paraplacenta adjacent to the zonary placenta. In addition, occasional regions with columnar trophoblast are found at the allantochorionic plate and even within the labyrinth. These regions of columnar trophoblast can ingest uterine products, including cell debris, serum proteins, and especially maternal erythrocytes, but do not accumulate pigments. The paraplacental region in particular shows extensive haemophagous activity, often with greater activity at the tips of the blunt villi and less erythrocyte ingestion in regions at the bases between villi. The abundance of haemophagous activity suggests a nutritive function as well as a function in iron transfer to the fetus.
4. VEGF in peccary and porcine placentae
6. Evolution of placentation in tenrecs
Vascular endothelial growth factor (VEGF) is an angiogenic growth factor that has previously been localized in epitheliochorial, endotheliochorial and haemochorial placental types. Vibeke Dantzer and collaborators had used immunohistochemistry to study the placental localization of VEGF and its receptors in the collared peccary (Pecari tajacu) from the Family Tayassuidae within Cetartiodactyla. Reproduction in this species occurs throughout the year with a gestation period of 144e148 days. The peccary placenta [7] is non-deciduate, diffuse and folded with areola-gland subunits for absorption of the uterine gland secretions. The interhaemal barrier is epitheliochorial. In the present study, the immunohistochemical localization of VEGF and its receptors Flt-1 and KDR (VEGFR-1 and VEGFR-2, respectively) was examined at 60, 90 and 120 days
Within the tenrecs there are striking differences in placental development and structure, although all have a substantial allantois that displaces the yolk sac early in gestation. Anthony Carter and collaborators reported on placentation in shrew tenrecs [11]. Fifteen specimens of Microgale were available, ranging from an early yolk sac stage to near term. The placenta was found to be different from that of other tenrecs in that there was an early, simple and lateral haemophagous region rather than a large central one. In addition, a more villous portion of the placental disk appeared before the formation of a more compact labyrinth. Although the definitive placenta was cellular haemomonochorial, as in the hedgehog tenrec [12], it lacked the spongy zone found in the latter. Earlier it had been shown that the otter shrew has endotheliochorial
3. Gonadotropin expression in dolphin placenta
A.M. Carter et al. / Placenta 28, Supplement A, Trophoblast Research, Vol. 21 (2007) S129eS132
placentation [13]. The central region of the placenta of the giant otter shrew was particularly interesting, since the juxtafetal portion was a haemophagous region whereas the labyrinth feeding it was endotheliochorial. It was concluded that there is more variation in placentation within the family Tenrecidae than would ordinarily be expected, with three distinct types differing in their exchange areas and in the extent of their haemophagous regions. 7. Comparative aspects of lymphocyte recruitment to implantation sites Anne Croy and collaborators reviewed the occurrence of uterine natural killer (uNK) cells across orders. Adult female reproductive organs experience repeated hormone-regulated cycles of change in structure, immune cell recruitment and synthesis of products. With conception, specialized expression of additional molecules involved in lymphocyte recruitment occurs on trophoblast and endometrial endothelial cells [14,15]. Lymphocyte recruitment to the maternalefetal interface has been observed in three of the four major clades of mammals. However, for most species, lineage identification of the infiltrating cells is not available [16]. Where cell identification has been made, the most abundantly recruited lymphocytes are uterine natural killer (uNK) cells [17]. Early after their appearance, uNK cells are highly proliferative and, in humans, soon represent w70% of the leucocytes found in decidua basalis. They are also very abundant in early mouse and rat decidua. In these species, decidua provides the recruitment signal. In pigs, uNK cells are enriched three-fold and their recruitment is trophoblast dependent. In humans, mice and pigs, uNK cells transcribe and translate numerous angiogenic molecules, including VEGF [18]. This molecule is more abundant in uterine lineage NK cells than in blood NK cells, suggesting a specialized, evolutionarily/ancient role for these cells that is conserved between species. Because lymphocytes are highly mobile cells, it has been postulated [17] that the role of uNK cells in implantation sites is to find trophoblast cells and, through endocrinologically and antigen driven mechanisms, provide angiogenic signals to endometrial endothelial cells. This unique, lymphocyte-based pathway for induction of endometrial angiogenesis would provide maximum flexibility over a ‘‘hardwired’’ vascular pattern that could be adjusted to numbers of offspring conceived, position of conceptus attachment within the uterus and changes associated with in utero death. 8. Placentation in hammerhead and spottail sharks Although most sharks are oviparous, some species are viviparous, including members of Sphyrnidae and Carcharhinidae. Viviparous placental sharks utilize villogenesis, histotroph and/ or placentation [19]. Hiroaki Soma and collaborators reported on studies of placentation in hammerhead sharks, where the estimated length of gestation is about a year and females usually have 12e30 pups. A pregnant hammerhead shark was captured at Okinawa Churaumi Aquarium and a placenta was removed
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from the uterus. On immunohistochemical examination of the placental tissue, columnar epithelial cells stained strongly with PAS, hCG, hPL and SP1. In addition, these cells were stained for AFP. It was suggested that these epithelial cells may have activities similar to those of human trophoblast. The spottail or Horai shark has 3e6 pups. A pregnant spottail shark was captured at Okinawa Churaumi Aquarium and two fetuses were found with placentas weighing 27.5 and 23 g. On immunohistochemical examination of the placental tissue, columnar epithelial structures were well stained with PAS, PLAP, hCG, hPL and AFP. In particular, goblet-shaped cells interspersed between arrangements of epithelial cells stained strongly for hPL. These cells may serve in a pinocytotic capacity, absorbing substances from the uterine lumen [20]. In summary, these placental cells of the shark yolk sac may produce placental proteins like human trophoblast, by which they may influence materno-fetal exchange. 9. Conclusions Recently it was postulated that the last common ancestor of living eutherian mammals had an endotheliochorial placenta [1]. Work on the spotted hyena certainly indicates that haemochorial placentation can arise in an order of mammals better known for the endotheliochorial type. The idea that epitheliochorial placentation is a derived state may seem more radical. Nevertheless, analysis of this placental type in the Cetartiodactyla does provide a rationale based on conflict theory. In stark contrast to the rather constant pattern of placentation in Cetartiodactyla is the great plasticity demonstrated at the family level by the tenrecs. The impetus to research provided by molecular phylogenetics will increase with the advent of comparative genomics. The hope is that genomics will offer insights into disease processes but, for this to work, we need to learn more about the associated phenotypes. Therefore, we need to intensify research in comparative morphology, physiology and immunology. References [1] Mess A, Carter AM. Evolutionary transformations of fetal membrane characters in Eutheria with special reference to Afrotheria. J Exp Zool (Mol Dev Evol) 2006;306B:140e63. [2] Price SA, Bininda-Emonds ORP, Gittleman JL. A complete phylogeny of the whales, dolphins and even-toed hoofed mammals (Cetartiodactyla). Biol Rev 2005;80:445e73. [3] Klisch K, Mess A. Evolutionary differentiation of cetartiodactyl placentae in the light of the viviparity-driven conflict hypothesis. Placenta 2006, May 17; Epub ahead of print. [4] Zeh DW, Zeh JA. Reproductive mode and speciation: the viviparitydriven conflict hypothesis. Bioessays 2000;22:938e46. [5] Watanabe N, Hatano J, Asahina K, Iwasaki T, Hayakawa S. Molecular cloning and histological localization of LH-like substances in a bottlenose dolphin (Tursiops truncatus) placenta. Comp Biochem Physiol A Mol Integr Physiol 2007;146:105e18. [6] Nakav S, Dantes A, Pen S, Chadna-Mohanty P, Braw-Tal R, Amsterdam A, et al. Homologous and heterologous carboxyl terminal peptide (CTP) linker sequences enhance the secretion of bioactive single-chain bovine LH analogs. Exp Clin Endocrinol Diabetes 2006;114:95e104.
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[7] Santos TC, Dantzer V, Jones CP, Oliveira MC, Miglino MA. Macroscopic and microscopic aspects of the collared peccary and white-lipped peccary placenta. Placenta 2006;27:244e57. [8] Winther H, Ahmed A, Dantzer V. Immunohistochemical localization of vascular endothelial growth factor (VEGF) and its two specific receptors, Flt-1 and KDR, in the porcine placenta and non-pregnant uterus. Placenta 1999;20:35e43. [9] Enders AC, Blankenship TN, Conley AJ, Jones CJP. Structure of the midterm placenta of the spotted hyena, Crocuta crocuta, with emphasis on the diverse hemophagous regions. Cells Tissues Organs 2006;183: 141e55. [10] Wynn RM, Amoroso EC. Placentation in the spotted hyaena (Crocuta crocuta Erxleben) with particular reference to the circulation. Am J Anat 1964;115:327e62. [11] Enders AC, Goodman SM, Soarimalala V, Carter AM. Placental diversity in Malagasy tenrecs: placentation in shrew tenrecs (Microgale spp.), the mole-like rice tenrec (Oryzorictes hova) and the web-footed tenrec (Limnogale mergulus). Placenta, in press. doi:10.1016/j.placenta.2006. 10.003). [12] Carter AM, Blankenship TN, Kunzle H, Enders AC. Development of the haemophagous region and labyrinth of the placenta of the tenrec, Echinops telfairi. Placenta 2005;26:251e61.
[13] Carter AM, Blankenship TN, Enders AC, Vogel P. The fetal membranes of the otter shrews and a synapomorphy for Afrotheria. Placenta 2006;27: 258e68. [14] Kruse A, Martens N, Fernekorn U, Hallmann R, Butcher EC. Alterations in the expression of homing-associated molecules at the maternal/fetal interface during the course of pregnancy. Biol Reprod 2002;66: 333e45. [15] Hanna J, Wald O, Goldman-Wohl D, Prus D, Markel G, Gazit R, et al. CXCL12 expression by invasive trophoblasts induces the specific migration of CD16- human natural killer cells. Blood 2003;102:1569e77. [16] Stewart IJ. Granulated metrial gland cells in ‘‘minor’’ species. J Reprod Immunol 1998;40:129e46. [17] Croy BA, van den Heuvel MJ, Borzychowski AM, Tayade C. Uterine natural killer cells-a specialized differentiation regulated by ovarian hormones. Immunol Rev, in press. [18] Tayade C, Black GP, Fang Y, Croy BA. Differential gene expression in endometrium, endometrial lymphocytes, and trophoblasts during successful and abortive embryo implantation. J Immunol 2006;176:148e56. [19] Hamlett WC. Evolution and morphogenesis of placenta in sharks. J Exp Zool 1989;Suppl. 2:35e52. [20] Gilbert PW, Schlernitzauer DA. The placenta and gravid uterus of Carcharhinus falcifornis. Copeia 1966;3:451e7.
Comparative Aspects of Placental Evolution: A Workshop Report A.M. Carter a,*, B.A. Croy b,c, V. Dantzer d, A.C. Enders e, S. Hayakawa f, A. Mess g, H. Soma h a
Physiology and Pharmacology, University of Southern Denmark, Odense, Denmark Department of Anatomy and Cell Biology, Queen’s University, Kingston, Ontario, Canada c Department of Biomedical Sciences, University of Guelph, Guelph, Ontario, Canada d Department of Basic Animal and Veterinary Sciences, The Royal Veterinary and Agricultural University, Copenhagen, Denmark e Department of Cell Biology and Human Anatomy, University of California, Davis, CA, USA f Department of Infectious Disease Control, Nihon University Advanced Medical Research Center, Tokyo, Japan g Museum of Natural History, Humboldt-Universita¨t Berlin, Berlin, Germany h Department of Obstetrics and Gynecology, Saitama Medical School, Tokyo, Japan b
Accepted 24 January 2007
Keywords: Carcharhinidae; Cetartiodactyla; Haemophagous organ; Hyaenidae; Luteinising hormone; Placentation; Sphyrnidae; Tenrecoidea; Trophoblast giant cells; Uterine natural killer cells; Vascular endothelial growth factor
1. Introduction
2. Evolution of placentation in Cetartiodactyla
Recent advances in phylogenetics have cast new light on the variation in placental form that is the focus of comparative studies. As an example, ideas about the evolution of placentation in mammals can be tested against current hypotheses on the phylogenetic relationships of mammalian orders [1]. Three contributions to this workshop concern epitheliochorial placentation in Cetartiodactyla, an order comprising whales, dolphins and even-toed ungulates. Carnivora mostly have endotheliochorial placentation, but the hyena is presented as an interesting exception that may indicate how an endotheliochorial placenta could evolve into a haemochorial one. In contrast to orders where one type of placentation predominates, there is variation at the family level between tenrecs. Uterine natural killer cells play a key role in the immunological response to pregnancy and in vascular remodelling of uterine arteries. These functions are reviewed here across a broad spectrum of mammals. Finally, it is recalled that placentation occurs in nearly all classes of vertebrate, as exemplified here by the hammerhead and spottail sharks.
It is apparent from fossil and molecular evidence that whales and dolphins nest within the even-toed ungulates. Relationships within the combined order, Cetartiodactyla, are well understood [2]. Analysis of molecular data sets has given fresh insight into higher-level relationships of living Eutheria, and, using the outcomes of molecular data sets, one can now gain a better understanding of an organ system such as the placenta. According to a recent analysis, the stem species pattern of Eutheria includes a medium-invasive, endotheliochorial placenta [1]. Non-invasive forms have evolved from this condition, but the functional significance of the evolution of epitheliochorial placentae from more invasive ones is unresolved. Since all taxa within Cetartiodactyla have epitheliochorial placentation, they are a suitable choice to address this question. Against this background, Andrea Mess presented a reconstruction of the stem species pattern of Cetartiodactyla and major evolutionary transformations within this clade. The collaborative study [3] was done by application of the computer programme MacClade on a given phylogeny [2]. The main result was that a diffuse epitheliochorial placenta, without placentomes, is part of the stem species pattern of Cetartiodactyla. The polycotyledonary placenta with more than 50 discrete cotyledons is an apomorphic character
* Corresponding author. Tel.: þ45 6550 3716; fax: þ45 6613 3479. E-mail address: [email protected] (A.M. Carter). 0143-4004/$ - see front matter Ó 2007 Published by IFPA and Elsevier Ltd. doi:10.1016/j.placenta.2007.01.014
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A.M. Carter et al. / Placenta 28, Supplement A, Trophoblast Research, Vol. 21 (2007) S129eS132
for Pecora (higher ruminants) and the oligocotyledonary placenta, which usually has only 5e8 cotyledons, developed as a further step on the lineage of cervids. Trophoblast giant cells (TGCs) evolved independently in camelids and ruminants. They are characterised by distinct morphologies, being multinucleate in camelids and binucleate in ruminants. These findings were discussed in light of the ‘‘viviparitydriven conflict hypothesis’’ [4], which states that divergent interests of mother and offspring lead to a rapid, antagonistic co-evolution, which might have contributed to placental diversity. For instance, the evolution of TGCs can be interpreted as a means to increase fetal endocrine influence on maternal metabolism, either by releasing steroids in camelids or by delivering trophoblast-derived proteins into the maternal system in ruminants.
gestation. VEGF, Flt-1 and KDR exhibited intense staining in the uterine epithelium, uterine glandular epithelium and trophoblast. The endothelial cells and smooth muscle cells of the vessels in the maternal and fetal compartments were also immunoreactive. In porcine placenta [8] the VEGF ligand-receptor system participates in placental regulation. These factors occur in peccary placenta in a similar pattern and likely with comparable functions. 5. An unusual haemochorial placenta: the hyena at midpregnancy
All mammals exhibit pituitary-specific expression of luteinising hormone (LH) and follicle-stimulating hormone, but placental expression of gonadotrophins has been reported only in primates and equids. Satoshi Hayakawa and collaborators presented work on expression of LH genes in dolphin placenta [5]. They had recently cloned bottlenose dolphin (Tursiops truncatus) LH genes from placental and pituitary cDNAs. Localization of LHb in dolphin placentae was established with immunohistochemical methods. The cDNA sequence of the a-subunit coded for 120 amino acids and showed greatest homology to that of pig pituitary LHa (96.7%). The b-subunit coded for 141 amino acids and showed greatest homology to b-subunits of pig (94.3%) and white rhinoceros (93.3%). Of great interest, dolphin LH genes lack carboxyl terminal peptide (CTP) sequences. Evolutionary conservation of CTP was recently reported in various mammalian LH/chorionic gonadotrophin genes and they are considered to serve as an effective linker to enhance hormonal secretion [6]. It was suggested that the primordial mammalian LHb gene existed in the common ancestor of Perissodactyla, Cetartiodactyla and Primates but that CTP sequences evolved, by different genetic mechanisms, when needed to ensure endocrine efficiency.
The hyena is unique among carnivores in having a definitive placenta that is haemomonochorial. Allen Enders and collaborators had examined the mid-term placenta of the spotted hyena [9]. Although the looping structure of the blood channels of the 30-day placenta suggests that it may have originated as endotheliochorial, by midterm (60e65 days), an elaborate system of anastomotic processes extends throughout the labyrinthine maternal blood space. The arrangement within the labyrinth indicates a crosscurrent blood flow rather than the countercurrent blood flow previously suggested [10]. Primary fetal villi extend from the allantochorionic plate to the basal junctional zone. Processes extend laterally from the primary villi at all levels, but at the junctional zone the tips of the primary villi are expanded and have a different structure. Although the labyrinth has syncytial trophoblast with intrasyncytial lacunae facing the maternal blood space, regions of simple columnar trophoblast are consistently found at the tips of the primary villi and in the paraplacenta adjacent to the zonary placenta. In addition, occasional regions with columnar trophoblast are found at the allantochorionic plate and even within the labyrinth. These regions of columnar trophoblast can ingest uterine products, including cell debris, serum proteins, and especially maternal erythrocytes, but do not accumulate pigments. The paraplacental region in particular shows extensive haemophagous activity, often with greater activity at the tips of the blunt villi and less erythrocyte ingestion in regions at the bases between villi. The abundance of haemophagous activity suggests a nutritive function as well as a function in iron transfer to the fetus.
4. VEGF in peccary and porcine placentae
6. Evolution of placentation in tenrecs
Vascular endothelial growth factor (VEGF) is an angiogenic growth factor that has previously been localized in epitheliochorial, endotheliochorial and haemochorial placental types. Vibeke Dantzer and collaborators had used immunohistochemistry to study the placental localization of VEGF and its receptors in the collared peccary (Pecari tajacu) from the Family Tayassuidae within Cetartiodactyla. Reproduction in this species occurs throughout the year with a gestation period of 144e148 days. The peccary placenta [7] is non-deciduate, diffuse and folded with areola-gland subunits for absorption of the uterine gland secretions. The interhaemal barrier is epitheliochorial. In the present study, the immunohistochemical localization of VEGF and its receptors Flt-1 and KDR (VEGFR-1 and VEGFR-2, respectively) was examined at 60, 90 and 120 days
Within the tenrecs there are striking differences in placental development and structure, although all have a substantial allantois that displaces the yolk sac early in gestation. Anthony Carter and collaborators reported on placentation in shrew tenrecs [11]. Fifteen specimens of Microgale were available, ranging from an early yolk sac stage to near term. The placenta was found to be different from that of other tenrecs in that there was an early, simple and lateral haemophagous region rather than a large central one. In addition, a more villous portion of the placental disk appeared before the formation of a more compact labyrinth. Although the definitive placenta was cellular haemomonochorial, as in the hedgehog tenrec [12], it lacked the spongy zone found in the latter. Earlier it had been shown that the otter shrew has endotheliochorial
3. Gonadotropin expression in dolphin placenta
A.M. Carter et al. / Placenta 28, Supplement A, Trophoblast Research, Vol. 21 (2007) S129eS132
placentation [13]. The central region of the placenta of the giant otter shrew was particularly interesting, since the juxtafetal portion was a haemophagous region whereas the labyrinth feeding it was endotheliochorial. It was concluded that there is more variation in placentation within the family Tenrecidae than would ordinarily be expected, with three distinct types differing in their exchange areas and in the extent of their haemophagous regions. 7. Comparative aspects of lymphocyte recruitment to implantation sites Anne Croy and collaborators reviewed the occurrence of uterine natural killer (uNK) cells across orders. Adult female reproductive organs experience repeated hormone-regulated cycles of change in structure, immune cell recruitment and synthesis of products. With conception, specialized expression of additional molecules involved in lymphocyte recruitment occurs on trophoblast and endometrial endothelial cells [14,15]. Lymphocyte recruitment to the maternalefetal interface has been observed in three of the four major clades of mammals. However, for most species, lineage identification of the infiltrating cells is not available [16]. Where cell identification has been made, the most abundantly recruited lymphocytes are uterine natural killer (uNK) cells [17]. Early after their appearance, uNK cells are highly proliferative and, in humans, soon represent w70% of the leucocytes found in decidua basalis. They are also very abundant in early mouse and rat decidua. In these species, decidua provides the recruitment signal. In pigs, uNK cells are enriched three-fold and their recruitment is trophoblast dependent. In humans, mice and pigs, uNK cells transcribe and translate numerous angiogenic molecules, including VEGF [18]. This molecule is more abundant in uterine lineage NK cells than in blood NK cells, suggesting a specialized, evolutionarily/ancient role for these cells that is conserved between species. Because lymphocytes are highly mobile cells, it has been postulated [17] that the role of uNK cells in implantation sites is to find trophoblast cells and, through endocrinologically and antigen driven mechanisms, provide angiogenic signals to endometrial endothelial cells. This unique, lymphocyte-based pathway for induction of endometrial angiogenesis would provide maximum flexibility over a ‘‘hardwired’’ vascular pattern that could be adjusted to numbers of offspring conceived, position of conceptus attachment within the uterus and changes associated with in utero death. 8. Placentation in hammerhead and spottail sharks Although most sharks are oviparous, some species are viviparous, including members of Sphyrnidae and Carcharhinidae. Viviparous placental sharks utilize villogenesis, histotroph and/ or placentation [19]. Hiroaki Soma and collaborators reported on studies of placentation in hammerhead sharks, where the estimated length of gestation is about a year and females usually have 12e30 pups. A pregnant hammerhead shark was captured at Okinawa Churaumi Aquarium and a placenta was removed
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from the uterus. On immunohistochemical examination of the placental tissue, columnar epithelial cells stained strongly with PAS, hCG, hPL and SP1. In addition, these cells were stained for AFP. It was suggested that these epithelial cells may have activities similar to those of human trophoblast. The spottail or Horai shark has 3e6 pups. A pregnant spottail shark was captured at Okinawa Churaumi Aquarium and two fetuses were found with placentas weighing 27.5 and 23 g. On immunohistochemical examination of the placental tissue, columnar epithelial structures were well stained with PAS, PLAP, hCG, hPL and AFP. In particular, goblet-shaped cells interspersed between arrangements of epithelial cells stained strongly for hPL. These cells may serve in a pinocytotic capacity, absorbing substances from the uterine lumen [20]. In summary, these placental cells of the shark yolk sac may produce placental proteins like human trophoblast, by which they may influence materno-fetal exchange. 9. Conclusions Recently it was postulated that the last common ancestor of living eutherian mammals had an endotheliochorial placenta [1]. Work on the spotted hyena certainly indicates that haemochorial placentation can arise in an order of mammals better known for the endotheliochorial type. The idea that epitheliochorial placentation is a derived state may seem more radical. Nevertheless, analysis of this placental type in the Cetartiodactyla does provide a rationale based on conflict theory. In stark contrast to the rather constant pattern of placentation in Cetartiodactyla is the great plasticity demonstrated at the family level by the tenrecs. The impetus to research provided by molecular phylogenetics will increase with the advent of comparative genomics. The hope is that genomics will offer insights into disease processes but, for this to work, we need to learn more about the associated phenotypes. Therefore, we need to intensify research in comparative morphology, physiology and immunology. References [1] Mess A, Carter AM. Evolutionary transformations of fetal membrane characters in Eutheria with special reference to Afrotheria. J Exp Zool (Mol Dev Evol) 2006;306B:140e63. [2] Price SA, Bininda-Emonds ORP, Gittleman JL. A complete phylogeny of the whales, dolphins and even-toed hoofed mammals (Cetartiodactyla). Biol Rev 2005;80:445e73. [3] Klisch K, Mess A. Evolutionary differentiation of cetartiodactyl placentae in the light of the viviparity-driven conflict hypothesis. Placenta 2006, May 17; Epub ahead of print. [4] Zeh DW, Zeh JA. Reproductive mode and speciation: the viviparitydriven conflict hypothesis. Bioessays 2000;22:938e46. [5] Watanabe N, Hatano J, Asahina K, Iwasaki T, Hayakawa S. Molecular cloning and histological localization of LH-like substances in a bottlenose dolphin (Tursiops truncatus) placenta. Comp Biochem Physiol A Mol Integr Physiol 2007;146:105e18. [6] Nakav S, Dantes A, Pen S, Chadna-Mohanty P, Braw-Tal R, Amsterdam A, et al. Homologous and heterologous carboxyl terminal peptide (CTP) linker sequences enhance the secretion of bioactive single-chain bovine LH analogs. Exp Clin Endocrinol Diabetes 2006;114:95e104.
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[7] Santos TC, Dantzer V, Jones CP, Oliveira MC, Miglino MA. Macroscopic and microscopic aspects of the collared peccary and white-lipped peccary placenta. Placenta 2006;27:244e57. [8] Winther H, Ahmed A, Dantzer V. Immunohistochemical localization of vascular endothelial growth factor (VEGF) and its two specific receptors, Flt-1 and KDR, in the porcine placenta and non-pregnant uterus. Placenta 1999;20:35e43. [9] Enders AC, Blankenship TN, Conley AJ, Jones CJP. Structure of the midterm placenta of the spotted hyena, Crocuta crocuta, with emphasis on the diverse hemophagous regions. Cells Tissues Organs 2006;183: 141e55. [10] Wynn RM, Amoroso EC. Placentation in the spotted hyaena (Crocuta crocuta Erxleben) with particular reference to the circulation. Am J Anat 1964;115:327e62. [11] Enders AC, Goodman SM, Soarimalala V, Carter AM. Placental diversity in Malagasy tenrecs: placentation in shrew tenrecs (Microgale spp.), the mole-like rice tenrec (Oryzorictes hova) and the web-footed tenrec (Limnogale mergulus). Placenta, in press. doi:10.1016/j.placenta.2006. 10.003). [12] Carter AM, Blankenship TN, Kunzle H, Enders AC. Development of the haemophagous region and labyrinth of the placenta of the tenrec, Echinops telfairi. Placenta 2005;26:251e61.
[13] Carter AM, Blankenship TN, Enders AC, Vogel P. The fetal membranes of the otter shrews and a synapomorphy for Afrotheria. Placenta 2006;27: 258e68. [14] Kruse A, Martens N, Fernekorn U, Hallmann R, Butcher EC. Alterations in the expression of homing-associated molecules at the maternal/fetal interface during the course of pregnancy. Biol Reprod 2002;66: 333e45. [15] Hanna J, Wald O, Goldman-Wohl D, Prus D, Markel G, Gazit R, et al. CXCL12 expression by invasive trophoblasts induces the specific migration of CD16- human natural killer cells. Blood 2003;102:1569e77. [16] Stewart IJ. Granulated metrial gland cells in ‘‘minor’’ species. J Reprod Immunol 1998;40:129e46. [17] Croy BA, van den Heuvel MJ, Borzychowski AM, Tayade C. Uterine natural killer cells-a specialized differentiation regulated by ovarian hormones. Immunol Rev, in press. [18] Tayade C, Black GP, Fang Y, Croy BA. Differential gene expression in endometrium, endometrial lymphocytes, and trophoblasts during successful and abortive embryo implantation. J Immunol 2006;176:148e56. [19] Hamlett WC. Evolution and morphogenesis of placenta in sharks. J Exp Zool 1989;Suppl. 2:35e52. [20] Gilbert PW, Schlernitzauer DA. The placenta and gravid uterus of Carcharhinus falcifornis. Copeia 1966;3:451e7.