Implantation and extravillous trophoblast invasion: From rare archival specimens to modern biobanking
Introduction
Although there is a huge variety of highly sophisticated omics technologies available today, one can still learn a lot from histology and morphological assessment of tissues and organs. In placental research, histology in combination with immunohistochemistry and electron microscopy still plays a major role in identifying new players in the interaction between mother and fetus. Especially, histology of rare archival specimens from early human implantation sites or placenta in utero can be of great value.
At the same time, science has evolved and has led to the development of structured and systematic collections of samples to be used in research, biobanks. Today, biobanks have taken over a major role in biomedical research, offering the hope to enable higher sample quality, better linkage to clinical data and less garbage in – garbage out when analyzing human-derived samples.
Looking at placental development, extravillous trophoblasts originate from the distal side of trophoblast cell columns derived from cytotrophoblasts of anchoring villi. Within these cell columns proliferative but non-invasive cytotrophoblasts switch to an invasive but non-proliferative extravillous phenotype and start to enter maternal tissues (interstitial trophoblast) [1]. From a histological point of view one of the most important characteristics of extravillous trophoblasts is their expression of major histocompatibility complex, class I, G (HLA-G) [2], [3], [4]. Only HLA-G is a specific marker for extravillous trophoblasts, rather than the still commonly used cytokeratin 7. The latter also reacts with maternal glandular epithelial cells [5]; this may lead to the misidentification of maternal uterine glands and maternal vessels where in the latter the endothelium is replaced by extravillous trophoblasts. Researchers should take care when choosing antibodies against HLA-G, since not all antibodies recognize HLA-G1, which is the only isoform expressed at the cell surface [6]. The anti-HLA-G antibody we recommend to use is the clone 4H84; it recognizes all isoforms of HLA-G, including HLA-G1 (which is the full length, membrane bound isoform, expressed on the cell surface) [5].
This review is divided in three parts, the first part summarizes the latest developments in the field of extravillous trophoblast invasion towards uterine glands and vessels. The second part gives an overview about fundamental archival specimens in placental research for “placental rookies”. For well-established placental researchers some new images of archival specimens are discussed in the context of the latest literature. In the third part, this review aims at extending this view to the latest development in the field of biobanking and provides insight into the current situation on sample usage in the absence of biobanks.
Section snippets
Extended view on extravillous trophoblast invasion into uterine vessels and glands
It is commonly accepted, that extravillous trophoblasts start their invasive pathway by invading the decidual stroma (interstitial trophoblast) [7] and thereby anchor the placenta to the uterus. It is also long accepted that extravillous trophoblasts invade and transform uterine spiral arteries (endovascular trophoblast) [8], [9]. This leads to plugging of such vessels during the first trimester of pregnancy and after release of the plugs enables flow of maternal blood towards the placenta
Publicly available archival human implantation sites
As can be seen from the achievements above, revisiting archival material with current knowledge may well enable detection of new, so far undetected features. Unfortunately, today only a few images from a handful of human early implantation sites are available for the scientific (and public) community. As an example we describe the image collections of the well-known collections that are publicly available at the Centre for Trophoblast Research (http://www.trophoblast.cam.ac.uk/Resources). These
Aspects of modern biobanking
As can be seen from the list above, the number of options and chances for a suboptimal treatment of samples is extremely high. Typical post-sampling variables during the pre-analytical phase include but are not limited to the time between collection and fixation, the temperature during transport, the storage temperature and duration, the number of freeze-thaw cycles and the extraction method for biomolecules. This list needs to be extended to the variables at the time of analysis, since taking
Conclusion
Archival material of human implantation sites and placentas in situ from hysterectomy specimens are still an invaluable treasure for placental research, especially when re-assessing it with the background of the recent state of knowledge. The hypothesis that there is a direct connection between the implanting blastocyst and uterine glands was stated more than hundred years ago and only now there is proof and evidence for this. Modern techniques allow expanding our hitherto believed concept of
Funding
This work was supported by the Austrian Science Fund (grant P24739-B23, granted to G.M.). G.M. was funded by the Austrian Science Fund (grant P24739-B23). No competing financial interests exist.
Conflict of interest
There is no conflict of interest of any of the authors.
Acknowledgements
Thanks to Monika Siwetz, Monika Sundl and Rudolf Schmied for their valuable help and expertise.
References (36)
- et al.
A critical look at HLA-G
Trends Immunol.
(2008) - et al.
The art of identification of extravillous trophoblast
Placenta
(2011) - et al.
A caution on the use of HLA-G isoforms as markers of extravillous trophoblasts
Placenta
(2008) - et al.
The uterine spiral arteries in human pregnancy: facts and controversies
Placenta
(2006) - et al.
Deep placentation
Best. Pract. Res. Clin. Obstet. Gynaecol.
(2011) - et al.
Human early placental development: potential roles of the endometrial glands
Placenta
(2007) - et al.
Placental basal plate formation is associated with fibrin deposition in decidual veins at sites of trophoblast cell invasion
Am. J. Obstet. Gynecol.
(2002) - et al.
Lateral placental growth occurs by trophoblast cell invasion of decidual veins
Placenta
(2000) - et al.
The human placenta is encircled by a ring of smooth muscle cells
Placenta
(2000) - et al.
The impact of biosampling procedures on molecular data interpretation
Mol. Cell. Proteom. MCP
(2013)
Nonvillous parts and trophoblast invasion
Human placental HLA-G expression is restricted to differentiated cytotrophoblasts
J. Immunol. Balt. Md
The immunology of pregnancy
Thyroid. Off. J. Am. Thyroid. Assoc.
Early developement of the human placenta
The trophoblast plug during early pregnancy: a deeper insight
Histochem. Cell Biol.
Endoglandular trophoblast, an alternative route of trophoblast invasion? Analysis with novel confrontation co-culture models
Hum. Reprod.
Evidence from the very beginning: endoglandular trophoblasts penetrate and replace uterine glands in situ and in vitro
Hum. Reprod. Oxf. Engl.
Uterine glands provide histiotrophic nutrition for the human fetus during the first trimester of pregnancy
J. Clin. Endocrinol. Metab.
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