Viviparity is a widespread reproductive mode among vertebrates, involving retention of the embryo within the female reproductive tract and giving birth to living young. Viviparity may involve the formation of a placenta, a structure formed by the apposition of extra-embryonic membranes and maternal tissues 1. Among vertebrates, placental viviparity is present in mammals (except monotremes), squamate reptiles and cartilaginous fishes.

Although viviparity is advantageous for the nourishment and growth of offspring, it involves an immunological risk for the semi-allogenic fetus. Prolonged exposure of embryonic tissues bearing paternal antigens to maternal uterine tissues can cause rejection of the embryo 23. Among immunological mechanisms to protect the fetus and its growth in the maternal tissues, the local release of immunoregulatory peptides (cytokines) seems to play a primary role 45. Cytokines and cytokine receptors are expressed in mammalian placentae independently of the animal species or the type of placenta 6. Cytokines such as Interleukin-1 (IL-1) and Transforming Growth Factor beta (TGF β) are also present in the placenta of a species of squamate reptile, the three-toed skink Chalcides chalcides 78.

IL-1 is one of the most pleiotropic cytokines and a central regulator of both the immune and inflammatory responses 9. Biological activity in response to the two isoforms of IL-1, IL-1 α and IL-1 β appears to be mediated by the receptor type I IL-1R tI, the functional receptor mediating signalling 10. A second receptor IL-1 R tII is thought to act as a decoy non-signaling receptor 11. In reproduction, IL-1 is expressed by the murine endometrium during the peri-implantation period and also by blastocysts 1213. Interestingly, animal experiments have shown that implantation in mice is prevented by blockage of the IL-1R tI 14. Recent evidence indicates that the IL-1 system exerts a direct action on the endometrium 15. In humans the cytokine and the functional IL-1R tI are expressed by endometrial tissue during the late secretory phase whereas the IL-1 R tII is down-regulated in the mid-secretory phase, especially during the implantation window 1617. IL-1 also promotes the success of early pregnancy in in vitro fertilized embryos 18, and contributes to the mechanism inducing labor 19.

The presence of IL-1 in the placenta of reptiles, as well as in mammals, suggests that it could be an important mediator in maternal-fetal immunotolerance throughout vertebrates 7. Therefore, we analyzed the expression of this cytokine and its specific membrane receptor, IL-1R tI, in the maternal and fetal tissues of a placental cartilaginous fish, the only class of placental vertebrates not yet investigated. In this class of vertebrates the phenomenon of feto-maternal immunotolerance is even more striking than in others because of the occurrence of multiple paternity 20. The model we used was the smoothhound shark, Mustelus canis, a placental viviparous shark of the Family Triakidae.

Chondrichthyan fishes include the sharks, skates, stingrays and chimaeras. Among these, placental viviparity occurs only in sharks, accounting for about 10% of extant species. Yolk sac placentation is limited to a few members of the Families Triakidae, the houndsharks; Family Carcharinidae, the requiem sharks and Family Sphyrinidae, the hammerhead sharks 21. The pioneering studies of Ranzi 2223 mark the first serious investigations of maternal-fetal relations in cartilaginous fishes. Ultrastructural descriptions of the uteroplacental complex were then reported by Hamlett et al. 2425262728, Hamlett 29, Otake and Mizue 30 and more recently, by Fishelson and Baranes 31.

Unlike amniotes in which four basic membranes develop, the yolk sac is the only extra-embryonic membrane present in anamniotes. In all species of chondrichthyans, regardless of reproductive mode, embryos are initially nourished by yolk stores in the yolk sac 32333435. Initially yolk is solubilized and transported across the endoderm to the yolk sac vessels 34. Additionally, yolk is transported to the fetal gut by ciliary action of the ductus vitellointestinalis in the yolk stalk. In most placental sharks, as yolk stores are depleted, the distal aspect of the modified yolk sac directly contacts the tertiary egg envelope that intervenes between the fetal and maternal tissues throughout gestation 33363738. At midgestation and in concert with diminution of yolk stores, yolk sac tissues differentiate in to a definitive yolk sac placenta. In all cases, the placenta is epitheliochorial, non-invasive and persists until term. During the ontogenetic modification of the yolk sac, a concomitant change occurs in the yolk stalk. It elongates to become an umbilical cord 2939.

Despite the diversity in placentation mechanisms, morphological and functional similarities have been demonstrated among vertebrates 4041. In particular, the interaction of paternal antigen-bearing fetal cells and maternal tissues is a common aspect across species 2. This means that molecular mechanisms act to prevent rejection in each placenta regardless of its morphology or the animal species 3. Of additional interest in sharks is the fact that recently multiple paternity has been demonstrated 2042. In the placental lemon shark, Negaprion brevirostris it was determined that at least three males had sired the litter of 13 pups 20. In the aplacental nurse shark, Ginglymostoma cirratum at least four males fathered 32 pups 42. The implications for the immune system of the female is considerable. Not only must she mount a surveillance regimen for the pups generally but in these studies there were at least four fathers with different genetic constitution. It is believed that many other shark species mate polygamously 20.

The present study demonstrates, for the first time, that the cytokine IL-1 in the α and β isoforms, and IL-1 R tI are expressed in the uteroplacental complex and in the umbilical cord of the smoothhound shark, M. canis. IL-1 is one of the immunoregulatory factors thought to mediate embryo-maternal interaction 4. Though the local release and action of these factors have been largely demonstrated in different species of mammals, their specific role at the materno-fetal interface still remains unclear. The finding of IL-1 in the yolk sac placenta of a chondrichthyan fish, in the chorioallantoic placentae of squamate reptiles, as well as in various types of mammalian placentae, suggests that this molecule played a critical role in the interaction between maternal and fetal tissues in placental viviparity. Moreover, the finding suggests that the role of interlukins in vertebrate placental evolution is independent of the germ layer derivative of the cells involved in placentation.

The recent finding of the IL-1 β gene in a cartilaginous fish, the small spotted cat shark Scyliorhinus canicula, provides evidence that immune responses in elasmobranchs could be mediated by the IL-1 system 43.

The IL-1 signalling system is also evolutionarily conserved in different species. IL-1 receptors show significant sequence similarities in their cytoplasmic domain to Toll-like receptors that mediate dorsoventral polarity in Drosophila and immune responses to microbial products in both insects and mammals 4445. Cartilaginous fishes are the oldest extant jawed vertebrates and are thus considered a critical model for an understanding of the origin of the vertebrate immune system 46. In this context, studies by Marchalonis and colleagues have shown unequivocal homologies between shark and mammalian immunoglobulins 46. Since the cartilaginous fishes are also the oldest extant placental vertebrates, it can be argued that they should be the preferred model for studies of molecules determining fetal acceptance and growth in the maternal tissues.