Placental Immunology, Viviparity, Evolution
The immunological defences of vertebrates are based on two systems, the innate [non specific] and the adaptive [specific ]. The innate uses largely monocytes, macrophages, granulocytes, NK [natural killer] and mast cells, with complement and acute phase p
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Placental Immunology, Viviparity, Evolution
9.1 9.1.1
Immunology Introduction
The immunological defences of vertebrates are based on two systems, the innate [non specific] and the adaptive [specific]. The innate uses largely monocytes, macrophages, granulocytes, NK [natural killer] and mast cells, with complement and acute phase proteins as humoral effectors. This is integrated with the adaptive system which employs the MHC controlled T and B lymphocytes and humoral antibodies. The innate is more primitive but both had evolved at least 220 million years ago [mya]. Vertebrate matrotrophic viviparity, based on internal fertilisation, retention of an allogeneic conceptus, and the development of a placenta in the uterus evolved 150–100 mya and was therefore always constrained by immunological considerations (Sacks et al. 1999; Parham 2004). These include the need for local tolerance in the uterus but maintenance of a normal level of systemic immunological surveillance. Initially the brief toleration of the spermatozoa and seminal fluid components is based on the uterine epithelium providing a significant cellular barrier together with the local immunomodulation by the progesterone secretion from the corpus luteum developed from the ovulated ovarian follicle. After fertilisation the laying down of a shell around the zygote will provide a solely maternally derived immunological camouflage. Retention of the shelled yolky egg for ovoviviparity could probably be ignored immunologically, as in the monotremes. However in order to dispense with the eggshell and yolk, the outer cellular layer of the conceptus [the eutherian trophoblast] needs to be apposed closely to the uterine epithelium to facilitate nutrient transfer. Such simple epithelial apposition is seen in most brief marsupial gestations, with insufficient time to develop any significant immune recognition. Longer pregnancy always depends on the maintenance of progesterone secretion with its vital endocrine and immunological effects on the growth and maintenance of the placenta (Arck et al. 2007).
P. Wooding, G. Burton, Comparative Placentation, © Springer-Verlag Berlin Heidelberg 2008
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9 Placental Immunology, Viviparity, Evolution
Eutherian placentas range from epitheliochorial through endotheliochorial to hemochorial and are defined by the depth of trophoblast invasion into the uterine tissues. The trophoblast which interacts with the uterine epithelium or stroma is always the outermost layer of the conceptus. The immune reaction will depend on the antigens presented by the trophoblast surface (not the fetus) and their recognition as allogeneic by the innate and adaptive systems. In the few examples investigated so far (pigs, horses and ruminants [epitheliochorial], rodents and humans [hemochorial]) the trophoblast of the bulk of the placental surface does not express MHC I or II antigens (see references in Bainbridge et al. 2001; Moffett-King and Loke 2006) and therefore would not trigger an immune response directly. However in most cases, there is
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