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W. D. BILLINGTON

Summary.

Experiments are described which assess the relative developmental capacities of morulae, blastocysts and ectoplacental cones transplanted to the mouse testis. The pre-implantation stages developed less successfully, yet frequently gave rise to embryonic or extra-embryonic elements as well as trophoblast. The ectoplacental cones almost invariably developed and produced a luxuriant growth of trophoblast; this trophoblast caused a measurable increase in the weight of the testis which was shown histologically to be a direct indication of the degree of trophoblast invasion into the testis tissue. Ectoplacental cones were transplanted within and between inbred strains of mice. Those transplanted to genetically dissimilar hosts produced a greater increase in testis weight; thus trophoblast invasion appears to be more extensive when there is antigenic difference between the trophoblast and the host. These findings are discussed with reference to immunological factors in placentation.

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E. J. JENKINSON and W. D. BILLINGTON

Summary.

The susceptibility of the mouse yolk sac to immune cell lysis has been investigated using an in-vitro microcytotoxicity test. Monolayer cultures derived from preparations of whole yolk sac and of the outer endodermal component were used as target cells. With combinations of mice differing either at major (H-2) or minor (nonH-2) histocompatibility loci, both types of culture showed extensive destruction following incubation with spleen cells from specifically immunized donors. Possible reasons for the survival of the yolk sac in vivo, despite the demonstration of antigenic determinants recognizable in vitro, are discussed, together with the significance of these findings for understanding of human fetal membrane function.

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W. D. BILLINGTON and B. J. WEIR

It is well established that deportation of trophoblast from the placental bed to the maternal system occurs during normal human pregnancy (see review by Bardawil & Toy, 1959). Trophoblastic `sprouts' break away from the syncytium into the intervillous space and pass directly into the maternal venous system (Hamilton & Boyd, 1966). These cells can be found in the venous drainage of the uterus throughout most of pregnancy (Douglas, Thomas, Carr, Cullen & Morris, 1959) and, although many are subsequently destroyed, probably by some lytic action of the blood, considerable numbers reach the maternal lungs (Attwood & Park, 1961). The functional significance of this phenomenon is obscure, but a number of suggestions have been put forward : that it is merely a physiological desquamation, that the trophoblast cells may have an

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M.-T. Badet, S. C. Bell and W. D. Billington

Summary. Supernatants from short-term in-vitro cultures of decidual tissue, obtained from the uteri of pregnant mice from Days 4 to 13 post coitum (Day 1 = day of mating), were assessed for immunoregulatory activity by their addition to a mixed lymphocyte reaction (MLR), an in-vitro analogue of the afferent arm of the immune response. All culture supernatants tested possessed inhibitory activity in the MLR, although the extent of inhibition was affected by seeding density, length of culture, and the day of pregnancy from which decidual tissue was obtained. Inhibitory activity produced by decidual cultures increased from Day 4 to reach a maximum on Day 8, and then declined to Day 11. Two morphologically distinct cell types were present in all decidual cultures; flat dendritic cells, considered to represent decidual cells, and small round cells, but whether immunoregulatory factors are associated with both is uncertain. The results suggest that decidual tissue could fulfil a role in the local partial blockade of the afferent arm of the maternal immune response during pregnancy.