Naming of the Trophoblast

Ambrosius Arnold Willem Hubrecht (Wikimedia Commons)

Robert Pijnenborg and Lisbeth Vercruysse have written an excellent paper (here) on the Dutch embryologist A. A. W. Hubrecht. He it was who coined the term trophoblast to describe cells of fetal origin with a nutritive function. 

One of Hubrecht's sections of the hedgehog placenta

Labels as in the original paper

Hubrecht applied the term to the cells in the labyrinth of the hedgehog placenta. The layer beneath that was called the trophospongium. Hubrecht mistakenly thought it was maternal in origin, but we now know it is spongiotrophoblast. There was in addition a layer of cells he called deciduofracts; these are trophoblast giant cells.

Hubrecht's collection is now curated at Museum für Naturkunde in Berlin.

Marsupial placentation

Embryo and fetal membranes of a marsupial

© 2011 Menzies et al; licensee BioMed Central Ltd.

This stunning photo of a tammar wallaby at 18 days gestation shows the vascular (TYS) and non-vascular (BYS) regions of the yolk sac. It will attach to the uterus and, as a yolk sac placenta, support fetal growth until delivery at 26-27 days. All marsupials have a yolk sac placenta. A few develop an additional chorioallantoic placenta (including the wombats).

Common wombat (Vombatus ursinus) Wikimedia Commons

Marsupials and their kin (Metatheria) share a common ancestor with “placental” mammals and their kin (Eutheria). Freyer et al. (2003) suggested that ancestor may have had both a yolk sac placenta and a chorioallantoic placenta.

As in placental mammals, the placenta of marsupials is more than an organ of maternal-fetal exchange. It is known to produce several hormones and recent studies extend the list to include growth hormone (GH), prolactin (PRL) and luteinizing hormone (LH). These hormones are expressed in the placentas of other mammals and in some elaborated as placental lactogens and chorionic gonadotropins. As argued by Menzies et al. (2011) this is further evidence of the affinity between placentas of “placental” mammals and marsupials.

Between Scylla and Charybdis

There are sound reasons for distinguishing between Placentalia and Eutheria. The former is the clade including living mammals (minus marsupials and monotremes) and their common ancestor. The latter includes extinct lineages that share a number of characters with Placentalia. Similar reasoning discriminates between Marsupialia and Metatheria.

The problem arises with the vernacular terms “placentals” or “placental mammals.” Understandably this is anathema to students of marsupial placentation. However, if “eutherians” is used to represent this group, phylogeneticists are quick to take offence.

Learn about exosomes and punt on the Cam

Booking is now open for this year’s CTR Annual Trophoblast Meeting to be held at Clare College Cambridge 8-9 July 2013.

Each year there is a themed session in which invited experts give overview lectures intended to stimulate new thinking and approaches. The topic this year will be "Exosomes and placental-maternal signalling." Exosomes are nanovesicles released from a variety of cells including trophoblast. There is evidence that exosomes mediate important aspects of pregnancy.

In addition to the themed session, there will be a full afternoon of presentations selected from delegate submissions.

Punting on the Cam (50 years ago)

To view the provisional program or to book a place and optional accommodation at Clare go to this link. The number of delegates is limited to 140.

Placentation in anteaters

Giant anteater (Myrmecophaga tridactyla) at Lisbon Zoo

The armadillos, sloths and anteaters of South America (Xenarthra) are of interest because they occupy a basal position in the tree of placental mammals. Studies on anteater placenta are few. Therefore it is exciting to find a paper that applies modern techniques to specimens from two species of anteater.

Villi of a giant anteater placenta

© 2012 Mess et al.; licensee BioMed Central Ltd.

In common with armadillos, anteaters have a villous placenta with a true intervillous space. Thus there is a superficial resemblance to the villous placentas of Old World monkeys, great apes and humans. No other mammal is known to have a definitive placenta of this type.   

The origin of the intervillous space in these mammals is different from in primates. Even before pregnancy the uterine wall contains an extensive network of venous sinuses. In armadillos the placental villi grow into the blood sinuses and the sinuses supply the intervillous space – a process described in detail by Enders and others.

Almost forgotten is the work of Becher on anteater placentas in which he showed an intervillous space first appeared between the villi and only later established contact with the maternal blood sinuses (Gegenbauers Morphol. Jahrb. 67:381-458, 1931). Thus there are subtle differences between placentation in armadillos and anteaters.

Trophoblast Research

Trophoblast Research reports the proceedings of the Annual Meeting of the International Federation of Placenta Associations (IFPA). It is published as a supplement to the journal Placenta.

The current issue contains workshop reports and reviews based on the plenary presentation at IFPA Meeting 2012, which was held in Hiroshima last September. All content can be accessed by following this link and selecting Trophoblast Research in the View for Free side bar.

Disclosure: I am Editor of Trophoblast Research.

What genes are needed to build a placenta?

Eutheria placenta shared and lineage-specific expressed genes

From Huo et al. (2012) © The Authors 2012

The chorioallantoic placenta is a defining feature of placental mammals and evolved after the split between Eutheria and Metatheria around 160 million years ago (most marsupials have a yolk sac placenta). What genes were recruited to build it? An answer based on mouse placenta is not fully satisfactory as a new study from Derek Wildman’s group confirms.

They determined which genes were highly expressed in the placenta of the African elephant. A comparison was made with similar information from human, mouse and cow placentas. Together these species cover three of the four major clades of placental mammal with the elephant representing the basal clade Afrotheria.

In an insightful analysis, the authors looked for a subset of 245 genes that when knocked out in the mouse gave abnormal placental morphology. Only 90 of these genes were expressed in all four placental types examined, although an additional 62 were shared by mouse and human.

 

What about me? Nine-banded armadillo (Wikimedia Commons)

The analysis did not include a member of Xenarthra — by some considered basal even to Afrotheria. Perhaps this was because the genome of the nine-banded armadillo is available only as a draft sequence.

Honest Jim

New Edition of The Double Helix

Reading this book again, after 45 years, I was struck by how fresh it still seems. No wonder it made such a strong impression on a graduate student. The new edition is an annotated version packed with documents and supplemented with the reminiscences of the protagonists.

Although Watson’s working title was Honest Jim, others involved in deciphering the structure of DNA found his account disingenuous. Francis Crick tried to dissuade him from going ahead with publication. The controversy is richly documented in an Appendix to the new edition.

Watson was rather dismissive of Rosalind Franklin, whose X-ray diffraction images were the only solid evidence for the helical structure of DNA. The icy relations between Franklin and Maurice Wilkins at King’s College London, mentioned in the first edition, are well documented here. When Franklin decided to move to Birkbeck College, Sir John Randall forbade her to take the DNA work with her. Ultimately Wilkins profited since one of the best images of DNA, obtained by Franklin and Raymond Gosling, was then handed to him. Women have yet to achieve full equality in science as highlighted in the current issue of Nature.

Double Helix Sculpture at Clare College Cambridge

Watson was affiliated with Clare College Cambridge and donated this sculpture by Charles Jencks. Clare is the venue for the Annual Meeting of the Centre for Trophoblast Research in July.

An enigma explained

Binucleate trophoblast cell of bovine placenta from Benirschke

The basic structure of the ruminant placenta is epitheliochorial. Yet the binucleate trophoblast cells of ruminants are able to fuse with uterine epithelial cells to form trinucleate cells (in cattle) or a syncytium (in sheep).To me this is an enigma.

Now we know how it is done. Cornelis and co-authors have found a syncytin in ruminants. It is expressed in the binucleate cells of the placenta. In a previous post it was explained that syncytins are encoded by env genes captured from retroviruses. In a retrovirus, the envelope protein is responsible for fusion with a host cell. In the ruminant placenta, as a syncytin, it enables the binucleate cell (fetal) to fuse with a uterine epithelial cell (maternal).  The envelope protein has an immunosuppressive domain and this is retained in ruminant syncytin.

Binucleate cells make a number of interesting proteins such as placental lactogens  and growth hormone. By fusing with the uterine epithelium they can pass these products to the mother thereby circumventing the epitheliochorial barrier.

A chevrotain from Singapore Zoo Wikimedia Commons

Cornelis and colleagues were able to find the syncytin gene in all families of ruminant save one – the chevrotains or mouse deer. This family (Tragulidae) differs from other ruminants in having a diffuse placenta – all the others have cotyledons (collectively they are the pecoran ruminants). However, it has been clearly shown that chevrotains have binucleate cells that fuse with maternal ones to form syncytial plaques.

It would be nice to think that capture of an env gene was a prerequisite for emergence of the ruminant type of placentation, but it should then be present in chevrotains. Perhaps it is but just could not be found with primers that worked in other ruminants. That might be because of sequence divergence during the 50 million years since tragulids separated from pecorans. That possibility needs to be explored further.

See you at Whistler

Whistler Conference Centre

The annual meeting of the International Federation of Placenta Associations will be held at Whistler, British Columbia, on 11-14 September 2013. Full details are available at this link.

The meeting theme, "Of Mice & Women," has the subtext, "Models for molecular understanding of placental development and associated disorders."

The best laid schemes o' mice an' men

Gang aft agley

—Robert Burns

Followers of this blog should know I take a dim view of the mouse as a model of human pregnancies. For some of my reasons look here and for concurring opinions here and here. Notwithstanding I intend to be at Whistler as the IFPA Meetings are always rewarding. Early registration starts now at this link.

If you are a New Investigator and think Whistler sounds expensive, be advised that you are eligible for a reduced rate and can choose affordable accommodation. Even better, there is a good chance of getting a travel grant not least due to a scheme established by the generosity of YW (Charlie) Loke. To be eligible you need to submit an abstract and tick the right box. The abstract deadline is 12 April 2013.