588 Nature Vol. 271 9 February 1978

Seeber calls our analogy between (in both systems), and even these are blast is strictly found only in placental antibodies and enzymes "not very ap­ controversial. ," thus misquoting his auth­ propriate" because "enzymes are homo­ With reference to the specific points onties, Davies and Hesseldahl, who genous and so multis.pecificity in that Cameron and Erlanger raise say that trophoblast is a peculiar substrate binding could not enhance above. Binding of antigen is only one mammalian structure. Lillegraven (page their specificity ... ". Perhaps we were aspect of the biological significance of 720, ref. 2) states further that "The unclear in our paper. We were not a cross-reaction. The ability to trigger 'invention' of trophoblastic tissues by trying to draw a functional analogy lymphocytes into division and antibody primaeval eutherians was probably the but were trying to indicate that all production must also be considered, single most important evolutionary protein-ligand interactions re.!y on and also the extent to which such event in the history of the infraclass". the same play of short range forces and cross-reactions are representative of This has led to the mistaken conclusion geometry. Hence, if enzymes are mul­ the total antibody population or arc that it is the evolution of trophoblast tispecific, it should not be surprising rare exceptions. which enables only to obviate to find that antibodies are also multi­ I agree that if "enzymes are multi­ immunological crisis during preg­ specific. specific then it should not be surprising nancy'·'. We should like to comment also on to find that antibodies are also multi­ Lillegraven says (page 101, ref. 1) Seeber's suggestion that the best anti­ specific". To what extent these that " ... true implantation by erosion bodies (with binding constants of 10"- generalities have been demonstrated is of the maternal epithelium never 10' I mol-') may be highly specific still not clear (see above for definition occurs in ". It has long been (that is, not multispecific). From our of multispecificity). known that in the chorioal­ point of view, this is not a logical con­ The final sentence of my original lantoic trophoblast disappears clusion. Whether a binding constant article was intended to be taken as pure as a layer late in '-', probably for a ligand is 10" I mol-' or 10" I speculation and not as a logical conclu­ through fusion with the uterine luminal mol-', the character of the binding site sion epithelium•. Comparable fusion may of the antibody is no different. It still Cross-reactions with binding (asso­ also occur during early implantation of should be able to accommodate other ciation) constant higher than that of certain Eutheria". Similar, less exten­ ligands and interact with them. Pre­ the immunogen do indeed exist and sive invasion of uterine epithelium by sumably the binding constant of a cross have been named 'heteroclitic''. trophoblastic cells occurs at the yolk reaction could be even higher than that (An error in my original article has sac placenta in several marsupials (such of the reaction with the immunogen. been corrected'). as in Dasyurus viverrinus and Sminth­ We should like to thank Secher for DAVID S. SECHER opsis crassicaudata), the uterine epi­ reminding us of the early suggestions M RC Laboratory of thelial layer becoming modified at the of Talmage'. We are embarrassed by Molecular Biology, placental site by interaction of foetal our failure to cite his paper. Cambridge, UK and maternal tissues during implanta­ tion. D. J. CAMERON I. Imanishi, T. & Makela, 0. J. cxp. Mcd. 140, 1498 Thus, evidence concerning tropho­ B. F. ERLANGER (1974). 2. Correction Nature 269, 197 (1~77). blast and uterine erosion tends to Department of Microbiology, unite rather than separate evolutionary Columbia University, aspects of eutherian and New York, New York 10032 Marsupial trophoblast placentation. Although it is not known whether marsupial trophoblast con­ I. Scchcr, D. S. Nuturc 268, 6~1 (1977). and mammalian evolution 2. Cameron, D.J. & Erlanger, B. F. Naturt: 268~ 763 tains immunological or endocrine pro­ (IY77). 3. Hornick. C. L. & Karush, F.lmmunoclwmistry 9, 352 WE present here a view of marsupial­ perties comparable with those proposed (1972). eutherian ongtns and evolutionary for eutherian trophoblast", attention is 4. Gopal<.~krjshmm, P. V. & Karush, F. J. lmmu.n. 113, 76Y ( 1974). trends, which constitutes an alternative being focused on this important 5. Talmage. n. W. Science 129, 1643 ( 1959). to the view of Lillegraven'·' recently problem'. Because trophoblast is discussed by Cox in a Nature News and certainly not a eutherian innovation, Views article". the argument for the "competitive The trophoblast constitutes the outer inferiority" of marsupials is consider­ SECHER REPLIES Enzymes (and anti­ foetal boundary of the eutherian ably weakened, as Kirsch', on other bodies) can bind ligands (antigens) chorioallantoic placenta where it func­ grounds, has also concluded. other than the 'primary ligand'. tions as the major foetal component of J. MARY TAYLOR Ligands with very similar chemical the placental barrier as well as a site Department of Zoology, structure may bind almost as tightly as of endocrine activity. The trophoblast The University of British Columbia, the main ligand and one might expect is also a foetal component of the Vancouver, British Columbia, to find a series of compounds of de­ eutherian yolk sac placenta. In most V6T IW5, Canada creasing association constant and de­ marsupials, maternal foetal transfers HELEN A. PADYKULA creasing chemical similarity to the occur solely through a yolk sac placenta primary ligand. The concept of multi­ where trophoblast associates with both Laboratory of Electron Microscopy, specificity (as used by Inman for ex­ vascular and avascular regions. An Wellesley College, ample) i~ different. It implies that two important exception occurs in the Wellesley, Massachusetts 02181 or more antigens of unrelated structure marsupial where hoth yolk 1. Lillcgr<.tven, J. A. Unh. Kansas Palcant. Contribs can bind specifically to a single anti­ sac and chorioallanotoic placentae (Art. 50 (Vemhruta 12)) (196Y). body molecule. This could take place develop; trophoblast differs in form 2. Lillegravcn, J. A. £1'0/ution 29,707-722 (1975). 3. Cox, B. Nuturc 265, 14-15 (1977). at distinct sites, overlapping sites, or regionally within the boundaries of 4. Hill, J.P. Q. Jl Microsc. .'ici. 40, 385-44h (1897). 5. Flynn, T. T. Q J/ Micrasc. Sci. 67, 123 182 (1'!23). the same site, but presumably will use each type of placenta'-'. 6. Padykula, H. A. & Taylor. J. M. Anat. Rec. 186, different contacts between the antibody The trophoclastic layer persists until 357-3R6 (1976); in J

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