THE
PLACENTA
A
Multicategorica
I,
Interd
isciphnary
Phenomenon
(Borden
Award
Address)
Joseph Dancis, M.D.
Department of Pediatrics, New York University School of Medicine and Medical Center, New York
Presented at the meeting of the American Academy of Pediatrics, Chicago, October 25, 1965.
J.D. is Career Investigator, The National Institute of Child Health and Human Development,
K6-GM-16, 710-03.
ADDRESS: 550 First Avenue, New York, New York 10016.
PEDIATRICS, Vol. 38, No. 2, Part I, August 1966
ARTICLES
167
T
AM greatly indebted to the BordenAward Selection Committee for
invit-ing me here today. I am also grateful for
the opportunity to acknowledge another
debt, the one that I owe to Dr. L. Emmett
Holt. Last night was passed pleasantly in the company of several old friends. As has
become common on such evenings, our talk
tumed to Dr. Holt, because all of us there
felt that he had exerted a strong influence
on the course of our lives. It was Dr. Holt
that made it possible for me to enter the
field of research, and then provided me
with an example of a teacher who is not
above being taught, of an inquirer who
doesn’t resent being questioned, of a tire-less worker capable of concentrated effort
but with broad interests.
Pleasant as these reminiscences are, I re-alize that I have been invited here to talk about the placenta, and that is what I in-tend to do. An occasion such as this offers the speaker the opportunity of summarizing experiences and concepts that have
de-veloped over an extended period of
preoc-cupation with a subject. This is particularly
pleasurable if one can discem a continuity
that spans the years and provides a unify-ing theme for the countless details and ap-parently isolated observations involved in
research. Now that teleology0 is becoming
#{176}Althoughthe word still remains disreputable,
the process is again achieving propriety. (See B. D.
Davis in Cold Spring Harbor Symposia on
Quanti-tative Biology 26:1, 1961.)
more respectable it is no longer quite as
improper to seek to interpret experimental
results in terms of a harmonious design
obeying a logic of selection for survival. It
is in that context that I would like to
re-view the results of some of our studies of
placental function.
First, I feel that I must explain, and
apol-ogize for, the long and awkward title. It is
derived from a phrase that is all too
famil-iar to those who deal with granting
agen-cies. It is most useful-some claim,
indis-pensable-when requesting large sums of
money, conjuring up visions of a corps of
investigators preferably with assorted
de-grees, all merging mystically into an entity
capable of unravelling complex problems.
In contrast, the solution to many of the complicated problems presented by the
evolution of intrauterine pregnancy have
been entrusted to a single organ, the pla-centa, which has been endowed with a wide diversity of functions.
Today I would like to tell you about a few of our experiences in studying placen-tal function, selected to demonstrate the
necessity of maintaining a broad perspective
and a flexible approach to this versatile organ.
To make intrauterine pregnancy possible,
the placenta had to undertake two major
and apparently incompatible roles-that of a barrier and that of a channel of communi-cation between mother and child. The need
MOThER PLACENTA
FIG. 1. Schematic representation of the in situ
perfusion of the placenta. A clamp is applied close
to the fetus and an umbilical artery and the vein
are cannulated and perfused.
fetus is genetically different from the moth-er, having inherited half its chromosomes from the father. Immunological defenses in mammalian organisms are designed to rec-ognize and reject foreign cells. One of the
mysteries intriguing immunologists is the
manner in which a mother can avoid ‘re-jecting’ her infant during the 9 months of
gestation. Regardless of the details of the
mechanism, it is evident that a major
fea-ture is the prevention by the placenta of the intimate mingling of maternal and fetal cells.
How perfect is this barrier? To study this
aspect of the hemodynamics of the placen-ta, we resorted to an in situ perfusion tech-nique of the placenta of the guinea pig. The development of the technique was ar-duous and would have been impossible without the imaginative innovations of Dr. Money.’ The fetus is delivered by hysterot-omy into a warm saline bath, and the um-bilical vessels are cannulated and perfused (Fig. 1). When the pressure in the umbilical vein was increased, there was a transfer of water into the matemal circulation. If radi-ochromated red cells were added to the “fetal circulation,” a large flux of red blood cells was easily detected in the maternal circulation by the time the pressure reached
20 cm H,O.’ That such transfers of red cells do indeed occur clinically has been
shown by demonstrating fetal hemoglobin
in the maternal circulation with consider-able regularity during pregnancy.3 There is no certainty that the mechanism for trans-fer of cells is the one described, but it is a reasonable suggestion.
FETUS How effective is the barrier in the reverse
direction, from mother to fetus? To
evalu-ate this, we resorted to the use of a bacte-riophage. This virus is pathogenic only for
E.
coli
and, therefore, unable to infect theplacenta and complicate the problem of placental transfer. Following the injection of phage into pregnant guinea pigs, phage could be regularly demonstrated in the fetus; but the concentration was several logs10 lower than in the mother. The
bar-rier is very effective though somewhat less
than perfect. Clinically, the possibility of
infecting the placenta, and thus breaching
the barrier, and the extreme susceptibility of the fetus to infections, greatly increase the hazards to the fetus of matemal bac-teremia and viremia. The major protection of the fetus against infection lies in the de-fenses of the mother against sepsis.
However, the greater risk to the fetus lies, not in infection, but in the transfer
of immunologically competent cells from mother to fetus. This could be disastrous to
the relatively unprotected fetus. The essen-tials of the experimental situation of “runt disease” would be reproduced. Transfer in the reverse direction is not as hazardous be-cause the maternal immunological system is
capable of destroying fetal cells. Increases in
intravascular pressure within the fetal cir-culation within the villus may permit the
transfer of fetal cells to the mother as
dem-onstrated in the previously described pla-cental perfusion experiments. However, in-creases in pressure in the matemal sinus-oids into which the villi dip will compress the villus and thus limit the transfer of ma-ternal cells to the fetus. The structure of the
placenta permits it to act, in a way, like a
one-way valve. The chorionic villus pro-truding into the cerebrospinal fluid provides a parallel situation.6
The leakage of maternal cells into the
f “Runt disease” is produced experimentally in
mice by injecting lymphocytes from a mature
ani-mal of one strain into an immunologically
defense-less newborn mouse of another strain. The
recipi-ent mouse fails to grow, loses weight, and often
TABLE I
RECIPIENT MICE AFTER INJECTIONS ARTICLES
fetus is potentially so dangerous that it
would be of advantage if immunologically competent cells of fetal origin were present in the placenta to provide an immunologi-cal defense against such transfer. To inves-tigate this possibility, we resorted to an ex-perimental model used by the
immunolog-ist. If immunologically competent cells
from one strain of mouse are injected into a genetically different mouse that has been previously irradiated so that it cannot de-stroy the injected cells, rapid death of the recipient ensues. The injection of placental cells from C57B1/6J mice into irradiated
CBA mice sharply increased the mortality
rate in the recipient mice indicating the presence of immunologically competent cells (Table I). Appropriate control experi-ments indicated that the immunologically competent cells were of fetal origin.7 This clever protective device has so far been demonstrated only in the mouse.
Let us now proceed to examine the pla-centa in its other major role-that of an organ of transport. It had been realized ever since the elaboration of techniques to detect antibodies, that some proteins could find their way through the placenta. This surprised physiologists because of the large size of the protein molecule, and it was
generally assumed that antibodies repre-sented an exception, trickling across the placenta in minute quantities detectable only with delicate immunological tech-niques.
With the advent of radioisotopes it be-came relatively simple to trace the behavior of plasma proteins, in general. The injec-tion of radioiodinated plasma proteins in the pregnant guinea pig revealed the trans-fer of relatively large amounts of proteins
(Fig. 2).8 Interpretation of these results in
terms of their effects on the fetal economy
requires some knowledge of the amounts of
proteins needed-in other words, of the turnover rates in the fetus. One might sus-pect that those proteins with prolonged half-lives might be supplied completely, or almost completely, from the mother. This is, of course, true of the gamma-globulins
Days after irradi-ation flanks -‘p C57B1/6J (4,9 mice) flanks -CBA (44 mice) Co7B1/6J placenta ,, C57Bl/6J (if mice) C57B1/6J placenia ,,., CBA (15 mice) 5 6 7 8 9 10 11 1 13 14 15 16 17 4 38 54 .. 58 64 7 86 90 9 96 7 16 36 4 51 69 8 91 93 .. 96
..
98 5 S .. 8650
59 63 73 77 .. 8..
91 7 47 73 87 100Recipient mice were irradiated with 800 r and then
injected as indicated. The injection of placental cells into recipient mice of the same strain does not increase the mortality rate; however, when injected into another strain, there is a sharp increase in mortality rate (re-produced from reference 7).
and may also be true of albumin. On the other hand, if the turnover rates of a and globulins are as short in the fetus as they are, in general, in the adult, it is not likely that fetal needs can be met by placental transfer. These generalities may have to be modified to some extent, because the indi-vidual proteins vary in their transfer rates.
Let us jump now to a consideration of the transfer of carbohydrates, molecules of
much smaller size. To study this, we again
TRANSFER
OF
I
PLASMA
PROTEINS
ACROSS
GUINEA
PIG
PLACENTA
6000
5250
4500
#{149} #{149}MATERNAL
* * FETAL
P00,000
z
0
I-0
80,000
60,000
40.000
8
-J
4
z
20,000
FRACTIONS
Ftc. 2. Transfer of 1131 plasma proteins across the guinea pig placenta (Reprinted
M. Shafran, J. Clin. Invest. 37:1093, 1958).
15 10 5 0
z
Q
I-0
4
CE
LL
z
U)
I.-z
3
0LU
from J. Dancis and
be needed. A facilitated transport
mecha-nism may be of definite advantage.
One last example should be adequate to document the variety and ingenuity of the solutions to transport problems adopted by the placenta. Dr. Levitz and I were attract-ed to the study of estrogens both because they are present in large amounts during pregnancy and because they represent a
class of compounds, of which bilirubin is
but another example. These compounds are usually very poorly soluble in water unless they are bound to proteins, and it is in this
form that they are transported in plasma. However, to be excreted into urine or bile where there is very little protein, they must be first converted into water-soluble com-pounds. This is largely accomplished by conjugation to glucuronic acid.
Conjugation is the method of disposal after birth, but what is the story before birth? Our initial sorties into this field were completely defeated by the extremely rapid metabolism of estrogens. Within minutes
after the injection of radioactive estrogens into a pregnant guinea pig they had been
so extensively modified that interpretation
of the results were impossible. Once again
we resorted to the “Money technique,” and concentrated on fetal to maternal transfer so that the estrogens were affected only by placental metabolism and placental trans-port. Placental metabolism proved an inter-esting but minor complication, and it be-came quickly evident that the unconjugated estrogens were transferred rapidly through the placenta whereas the glucuronides were not.’#{176}
a
I,
FIG. 5. The Chemist.
FIG. 3. The Cuardian.
ARTICLES
Fic. 4. Mother Provider. Ftc. 6. Department of Sanitation.
glucuronides because of their poor solubili-ty in lipids. Excretion before birth is pri-marily of the unconjugated estrogens, ex-actly the reverse of the situation after birth,
and exactly contrary to our naive
assump-tions before undertaking these studies. This experience demonstrates another
important feature in the study of placental
physiology. Unless the investigator keeps in
mind the close interrelations of mother,
pla-centa, and fetus he may easily overlook the
significance of an isolated observation. It is
now clear that the relative inability to
glu-curonidate manifested by the newborn
child is not an oversight of nature or a
handicap resulting from incomplete
Fic. 7. Lady of Infinite Mystery.
temporary holdover of the normal intra-uterine situation.”
In the remaining few minutes, I would like to present somewhat personalized rep-resentations of this versatile organ, the pla-centa, in some of its varied roles.
In the first illustration (Fig. 3), we see the Guardian, protecting the infant from ma-rauding invaders. Then we see the placenta in a more gentle role, that of Mother Provid-er (Fig. 4) offering amino acids, no doubt, to help the baby grow while glucose, vita-mins, minerals, and other nutrients are available on the shelf. Here is the brilliant chemist (Fig. 5) synthesizing steroids, cho-rionic gonadotrophin, lactogenic hormone, and possibly other important materials as yet undiscovered. The importance of waste disposal (Fig. 6), other than carbon dioxide, is not known because the amount and the nature of metabolites formed during intrau-terine life are not known. However, a route
is available for urea, bilirubin, and proba-bly other excretory products. And finally we
have the placenta in her most appealing role, that of a Lady of Infinite Mystery (Fig. 7)-alluring, provocative-enough to bring life-long contentment to the heart of
any investigator.
REFERENCES
1. Money, W. L., and Dancis, J.: Technique for
in situ study of placental transport using
the pregnant guinea pig. Amer. J. Obstet. Gynec., 80:209, 1960.
2. Dancis, J., Brenner, M., and Money, W. L.:
Some factors affecting the permeability of
guinea pig placenta. Amer. J. Obtet.
Gynec., 48:570, 1962.
3. Zipursky, A., Hull, A., White, F. D., and
Israels, L. G.: Foetal erythrocytes in the
maternal circulation. Lancet, 1:451, 1959.
4. Uhr, J. W., Dancis, J., and Finkelstein, M. S.:
Passage of bacteriophage 0X174 across the
placenta in guinea pigs. Proc. Soc. Exper.
Biol. Med., 113:391, 1963.
5. Billingham, R. E., and Brent, L.:
Quantita-tive studies on tissue transplantation im-munity. IV. Induction of tolerance in new-born mice studies on the phenomenon of runt disease. Phil. Trans. Roy. Soc. London,
B 242:439, 1959.
6. Welch, K., and Freedman, V.: Cerebrospinal
fluid valves. Brain, 83:454, 1960.
7. Dancis, J., Douglas, G. W., and Fierer, J.: Immunologic competence of mouse placen-tal cells in irradiated hosts. Amer. J. Obstet.
Gynec., 94:50, 1966.
8. Dancis, J., and Shafran, M.: The origin of
plasma proteins in the guinea pig fetus. J.
Clin. Invest., 37:1093, 1958.
9. Folkart, G., Money, W. L., and Dancis, J.:
Transfer of carbohydrates across guinea pig
placenta. Amer. J. Obstet. Gynec., 80:221,
1960.
10. Dancis, J., Money, W. L., Condon, G. P., and
Levitz, M.: The relative transfer of
estro-gens and their glucuronides across the
pla-centa in the guinea pig. J. Clin. Invest., 37: 1373, 1958.
11. Dancis, J.: Aspects of bilirubin metabolism
before and after birth. PEDIATRICS, 24:980,
