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Placenta xxx (2012) 1e3

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Placenta

journal homepage: www.elsevier .com/locate/placenta

Review: Epithelial aspects of human placental trophoblast

C.A.R. Boyd*

Brasenose College, Oxford OX1 4DJ, UK

a r t i c l e i n f o

Article history:Accepted 15 November 2012

Keywords:TrophoblastEpitheliumSmall intestineTransportImmunologyCellular turnover

* Tel.: þ44 (0)1865 515085.E-mail address: richard.boyd@dpag.ox.ac.uk.

0143-4004/$ e see front matter � 2012 Published byhttp://dx.doi.org/10.1016/j.placenta.2012.11.013

Please cite this article in press as: Boyd CA10.1016/j.placenta.2012.11.013

a b s t r a c t

The placenta must act as a surrogate lung, gastrointestinal tract and kidney for the fetus as well as actingas an endocrine gland necessary for the maintenance of a successful pregnancy: to achieve this, to whatextent does the trophoblast necessarily share a similar epithelial phenotype? Here I review froma historical and a contemporary perspective some relevant studies with an emphasis on the similaritiesand differences between small intestinal and trophoblast biology. Certain physiological, structural andcell biological similarities are striking.

� 2012 Published by Elsevier Ltd.

1. Introduction

It is instructive from time to time to consider the way theplacenta has been viewed historically. Some 50 years ago a medicalor scientific text book might have written that with respect to itsfunction little was known but the placenta must act as a surrogatelung, gastrointestinal tract and kidney for the fetus as well as actingas an endocrine gland necessary for the maintenance of a success-ful pregnancy. Additionally, regarding metabolism the placentamight well have been described as behaving as “the fetal liver”during the conceptus’s nine months of intrauterine life. Thirtyyears ago probably more caution would have been used in makingsuch analogies, and moreover there would have been a verysubstantial interest, which has continued to grow, in immunolog-ical aspects of pregnancy and of the central role of the placenta inthis. Before the onset of nucleotide sequencing (generated bySanger’s pioneering methodology, gaining him his second NobelPrize [24], “genomics” was not viewed as a discipline with thepotential of giving insight into function; furthermore at that timethere was little interest in epigenetics (even though this conceptoriginally was proposed by an embryologist, C.H. Waddington, [27].Nor of course was there knowledge of the complexities of thegenetic control systems embodied in, for example, regulatory RNAmolecules. All these exceptionally exciting insights have onlycontributed to an appreciation of the subtleties of placental biologyin the last few years.

Elsevier Ltd.

R, Review: Epithelial aspects

At the time that I entered the field of placental physiology I wasstrongly influenced by the structural similarities at a cellular levelbetween trophoblast and the small intestinal epithelium. Some ofthis structural work was at the level of cell populations (somethingI will return to later); but there was also similarity apparent at thelevel of the newly-opened world of “fine structure” (that is whatwas revealed by electron microscopy). For example in an importantbook of such micrographs, assembled by D.W. Fawcett, [11] thebrave new world of cell biology suddenly emerged at a structurallevel; it was immediately apparent that the chemistry andbiochemistry needed to understand these images was going to bea very substantial challenge. From the perspective of somebodyinterested in membrane transport, one of the fascinating questionswas how in the placenta would transporters be distributed andhow would they work. Fascinatingly, for the second question thefield has reached a spectacular conclusion with the publication ofatomic level structures of many ion channels, solute transporterand ion pumps (and associated regulatory molecules). Throughsuch recent work a natural bridge has been forged linking theconceptual ideas of W.F. Widdas, [28] on ‘carrier mediated trans-port’ (these ideas emerged from attempts to understand data ontrans-placental sugar transport) to a physical reality seen inmultiple conformations of individual transport proteins; sixty yearsof reductive science can be viewed as highly productive withplacental research playing at least some contributory role. Howeverthese studies on individual transporters were solely dealing withthe workings of isolated components of the whole and thus couldnot by themselves give insight into the working of the intactplacental epithelium. For this it was necessary to use the insightsfrom another major physiological figure, H.H. Ussing, who realised[19] that it was how such pumps, channels and carriers were

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distributed in the distinct ‘apical’ and ‘basal’ faces of an epithelium,and of how they interacted functionally, that would determineepithelial function.

2. Placental brush border

In this context, the placental structure revealed by electronmicroscopy that fascinated me was the “Burstenbesatz” (in theterminology of the pioneering 19th century Germanhistologists), or“brush border”, the apical membrane of the trophoblast overlyingthe chorionic villi. This is the fetal surface of the placenta, the surfacethat is in direct contact with maternal blood. Electron microscopyshowed this in placenta as in many epithelia to be a discreteorganelle comprising microvilli. During my first excursion intoresearch I had used fractionation methods to isolate ‘brush borders’fromsmall intestine [4,5] andaswell as characterising some featuresof this organelle enzymatically, I took the opportunity with mymentor DS Parsons (in Oxford) and with the assistance of a highlyskilledEMtechnician (K.W. Thurley, inCambridge) to carryout somesimple experiments using negative staining to examine the ultra-structure of these organelles. A resulting publication [3] was thefirst to show that microvilli contained actin filaments (rather thanmicrotubules as previously thought); and in a parallel study theultra-structure of the brush border membrane of human term syn-cytiotrophoblastwas described [5], and the remarkable similarity tothat of the small intestine noted. Thus more than 45 years ago therewere evident similarities between a functionally well-studiedtissue, the small intestinal epithelium, and a structure, the tropho-blast, of which very much less was known functionally.

2.1. Electron microscopy, the brush border and molecular pathology

How similar are microvilli from different epithelial tissues? Insmall intestine and in proximal tubule the regularity of microvillusstructure is extremely striking both as regards individual micro-villus dimensions and the pattern of the layout of microvilli withrespect to each other. Thus, seen in cross section, the hexagonaldisplay of microvilli is indeed striking. The discovery of themicrofilament core as being actin rather than tubulin based led tospeculation and consideration of whether microvilli were motile.Here there has been progress at least as far as the microvilli of smallintestine are concerned. In a recent study proteomics was used tomap the mass spectroscopic signatures detected in a preparation ofmicrovilli from rodent small intestine [21] leading to the identifi-cation of more than 600 proteins contributing to this organelle. Asubstantial number of these proteins are clearly related to thecytoskeleton of the microvillus, and the dominance of myosin 1A isof particular interest. Running alongside these structural andbiochemical studies on normal microvilli has been the recentdiscovery of the molecular basis of a very rare but devastatinggenetic disorder originally described in the 1960s, ‘microvillus bodyinclusion disease’. In Ref. [22] showed that the molecular basis ofthe disorder was a point mutation in one of the myosin sub-typesencoded by the MYOVB gene. With a point mutation in this gene(reviewed by [29]) it is clear that not only is intestinal microvillusassembly abnormal in the affected fetus, explaining the associatedphenotype of gastrointestinal malabsorption, but there is also anassociated deficit in intestinal hydrolytic enzyme function in theenterocyte brush border. Rather intriguingly there is circumstantialevidence to suggest that placental function (and proximal tubularrenal function, [12] may similarly be affected in these rare preg-nancies: a single case report [8] describes, in a prenatally diagnosedfetus, associated raised levels of amniotic fluid alpha fetoprotein; ina fetus that macroscopically is structurally normal, such a finding isknown to reflect abnormal placental permeability [6].

Please cite this article in press as: Boyd CAR, Review: Epithelial aspects10.1016/j.placenta.2012.11.013

3. Transport and hydrolysis

In the small intestinal brush border because of the very closeinteraction between extracellular enzyme active sites (for examplepeptidase or disaccharidase activity) and transporters present inclose proximity, there is a ‘kinetic advantage’ for the absorption ofthe products of surface hydrolysis. In a very recent study from theBroer lab [10] this has been investigated by co-expressing bothhydrolases and specific transporters in Xenopus oocytes and theresulting transport properties of the assembly have been comparedwith those of transporters acting on their own without interactingenzyme partners. The results provide a fascinating molecularcontext for interpretation and confirmation of data obtained some50 years ago by intestinal physiologists. The reason for pointing outthese recent and early studies on intestinal physiology is toemphasise the need for equivalent studies to be undertaken onplacenta.

3.1. Transport studied in placental brush border membranes andbasal membranes

Transport studies on placental “brush border membranes” (iso-lated using the original protocol of NC Smith, [26]) have beenfrequent ever since the pioneering studies in theUSA [23]; and somework in the UK [2]. These early studies on placenta contributedimportant insights into ion coupling and electrogenicity of trans-porters for example of the System A family of amino acid trans-porters in the maternal facing membrane of the trophoblast.Subsequent work again led by Carl Smith led to protocols enablingseparation of apical from basal membranes [18]; see also [1]. Suchbiochemical fractionation of theplacenta alloweddirect comparisonof the maternal and fetal facing membranes in terms of theirtransport properties and thus opened the way to rethinking the keyissue in an epithelium namely polarity. Such studies on trophoblastof course ran inparallelwith, and receivedmuch conceptual supportfrom, the earlier work of Murer and colleagues (for example [14]). Itis difficult now to appreciate why these studies were of significancein defining epithelial transport at a time before the notion of para-cellular transport had been introduced rigorously.

3.2. Cell physiology

Thenotionof primaryand secondaryactive transport arose in the1950s and 60s from the work of I.M. Glynn, [13] in Cambridgeregarding the primary active transport system of the sodium pump,and associated studies that linked beautifully with this work fromSkou in Denmark [25]. The identification of the sodium pump’sbiochemical basis as being the NaeK-ATPase was certainly animportant landmark. In parallel with such studies on primary activetransport was on-going work in the USAwhich led to the discoveryof ‘fluxcoupling’, originally byR.K. Crane [9]; andhence to thenotionof ‘secondary’ active transport inwhich sodium ions extruded fromthe cell against an electrochemical gradient by the sodium pumpwere subsequently used as an energy gradient to drive the ‘up-hill’-movement of another solute (such as glucose) in to the cell.Which ofthese processes were present in the trophoblast? Where were theydistributed in trophoblast? Experimental studies (e.g. by Ref. [17]have helped identify the sodium potassium ATPase and to showthat its distribution is less straightforward than had originally beenenvisaged. For example in a symmetrical cell like the erythrocytethere is random distribution of primary active transporters yet ina polarised epithelium one of the hallmarks is their vectorialdistribution specifically within the basolateral membrane. Thus theobservation of non-identically polarised sodium pumps in tropho-blast and the realisation that there aredifferent isoformsof the alpha

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C.A.R. Boyd / Placenta xxx (2012) 1e3 3

and beta subunits underpinning the biology of this protein havebegun to be explored but much remains to be discovered in thedevelopmentof placentalmammals.What is of course clear is that asin all animal cells the low intracellular sodium and the insidenegative membrane potential endow the trophoblast as an epithe-liumwith the ability to couple ion fluxes, specifically sodium, to theflow of solutes such as amino acids given expression in apical orbasal membrane of relevant co-transporters. The genes encodingthese transporters have been rapidly uncovered (e.g. [7]).

4. Conclusion

It is now clear that in reviewing fetal nutrition simply to considerthe placenta as the functional equivalent of the post-natal smallintestine must be naïve. But there are indeed some striking simi-larities, some of which I have touched on here. Other similarities arecontinuing to emerge. For example the turnover of the small intes-tinal epithelium is verywell known, but it has takenmuch longer forplacentologists and for obstetricians to appreciate the significance ofearly workers’ findings (for example those of [15]). Iklé emphasisedboth the normal physiological (rather than pathological) nature oftrophoblast desquamation and export (via the uterine veins andinferior vena cava ultimately for removal in thematernal pulmonarymicrovasculature); and measured its rate quantitatively (approxi-mately 105 trophoblast cells per day). In a rather similar way someparallels in immunobiology between intestine and placenta arebecoming more apparent, as is the critical relevance of epithelialamino acid transport in this function in both tissues (see for examplebrief reviews of topical studies by [16] relating to intestinal immu-nology; and by [20] relating to placental immunobiology). It will befascinating to see what other overlaps in the function of the twoepithelia may emerge from future experimental insights.

Conflict of interest statement

The author confirms that there is no conflict of interest in theabove paper.

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of human placental trophoblast, Placenta (2012), http://dx.doi.org/