1. Placentation in mammals: Definitive placenta, yolk sac, and paraplacenta 
A.M. Carter, A.C. Enders 
Theriogenology 
Volume 86, Issue 1, Pages (July 2016)
DOI: /j.theriogenology
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2. Fig. 1
Yolk sac membranes of eutherian mammals. (A) Fetal membranes of the sea otter (Enhydra lutis) during the transition from choriovitelline (ch-v p) to chorioallantoic (ch-a p) placentation; note the paraplacental chorion (ch p). Reprinted with permission from Sinha and Mossman [9]. Copyright 1966 Wiley-Liss, Inc. (B) Trilaminar yolk sac of domestic cat (Felis catus) at early apposition phase, showing yolk sac lumen (YL), endoderm (ED), mesoderm (MD), and trophoblast (T); also shown are the uterine lumen (UL) uterine epithelium (UE) and degenerating trophoblast (DT). Reprinted with permission from Leiser and Koob [10]. Copyright 1993 Wiley-Liss, Inc. (C) Yolk sac of a sucker-footed bat (Myzopoda aurita). The endoderm adjacent to the lumen comprises large, columnar basophilic cells. The connective tissue between the endoderm and the pale cuboidal mesothelial cells is sparse but well vascularized. Reprinted from Carter et al. [11]. Copyright 2008 with permission from Elsevier. (D) Yolk sac of the Hottentot golden mole (Amblysomus hottentotus) at midgestation; a vascular yolk sac is retained to term. Reprinted from Jones et al. [12] with permission from Elsevier. (E) Inverted yolk sac of the lowland paca (Cuniculus paca). The columnar endodermal cells face the uterine lumen (ut lum) and the mesoderm (fm) is well supplied with vitelline vessels (vit bv).
Reprinted from Bonatelli et al. [13]. Copyright Bonatelli et al.; licensee Biomed Central Limited.
Theriogenology  , DOI: ( /j.theriogenology )
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3. Fig. 2
Interhemal barrier of representative eutherian mammals. (A) Epitheliochorial placenta of the bush baby (Otolemur crassicaudatus). (B) Synepitheliochorial placenta of the cow (Bos taurus); note the binucleate trophoblast cells (asterisks) and trinucleate cells (arrows). (C) Endotheliochorial placenta of a bat (Natalus sp.); note the enlarged endothelial cells of the maternal capillary in contact with the trophoblast. (D) Hemotrichorial labyrinthine placenta of the mouse (M. musculus). (E) Hemomonochorial labyrinthine placenta of the guinea pig (Cavia porcellus). (F) Hemomonochorial villous placenta of an armadillo (Dasypus novemcinctus). Reproduced from Carter and Mess [4]. Copyright 2014 with permission from Elsevier. CYTO TR, cytotrophoblast; ENDO, endothelium; FC, fetal capillary; MC, maternal capillary; MBS, maternal blood space; MES, enlarged mesenchymal cell; SYN TR, syncytial trophoblast; T1, trophoblast layer 1 (cellular); T2, trophoblast layer 2 (syncytial); T3, trophoblast layer 3 (syncytial); UT EP, uterine epithelium.
Theriogenology  , DOI: ( /j.theriogenology )
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4. Fig. 3
Smooth chorioallantois (vascular paraplacenta) of eutherian mammals. (A) Electron micrograph of the interplacentomal region of a midpregnant cow (Bos taurus) showing fetal trophoblast (F) above and uterine epithelium (u) below. In the trinucleate cell, the binucleate cell-derived nuclei (b) are clearly different from the one originating in the uterine epithelium. Two lymphocytes (L) are seen, one of which has granules (arrow). Reproduced from Lee et al. [37]. Copyright 1997 with permission from Elsevier. (B) The smooth chorioallantois of the mink (Neovison vison) is highly folded as is common in carnivores. Note the abundance of uterine glands in the underlying endometrium. (C) Smooth chorioallantois of the Egyptian slit-faced bat (Nycteris thebaica) showing abundant allantoic vessels, cytotrophoblast, decidua, and myometrium. Note that the uterine epithelium is absent.
Theriogenology  , DOI: ( /j.theriogenology )
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5. Fig. 4
Hemophagous areas and areolas of eutherian mammals. (A) Paraplacental hemophagous area of the domestic cat (Felix catus). Maternal blood has penetrated between the columnar uterine epithelium and the columnar, hemophagous cytotrophoblast. Erythrocytic material becomes darker and granulated as it progresses between the apex (arrows) and the base of the cells. Lipid droplets (ld) accumulate as digestion proceeds. Reproduced with permission from Leiser and Enders [62]. Copyright 1980 Karger Publishers, Basel, Switzerland. (B) Intraplacental hemophagous region of the spotted hyena (Crocuta crocuta). The trophoblast at the tips of the villi stains strongly for the presence of cytokeratin (brown). This is columnar trophoblast with a heterophagous function. The uterine glands below also stain for cytokeratin. Elsewhere in the placenta, a labyrinth is formed by the complex branching of the trophoblast. (C) Chorionic vesicle of the gray slender loris (Loris lydekkerianus) situated above the opening of a uterine gland. The placenta on either side is epitheliochorial (Harland W. Mossman Collection, University of Wisconsin Zoological Museum). (D) Higher power view of the same chorionic vesicle showing the columnar trophoblast and an underlying layer of smooth muscle. (E) Yolk sac of the Asian house shrew (Suncus murinus). Maternal red cells are ingested by the columnar trophoblast of the parietal yolk sac. The breakdown products cross Reichert's membrane, parietal endoderm (pys endo), and yolk sac cavity and are taken up by the endoderm of the visceral yolk sac (vys endo). Hematopoiesis occurs in the visceral yolk sac (Harland W. Mossman Collection, University of Wisconsin Zoological Museum). Cam, chorioallantoic mesenchyme; fv, fetal vessel.
Theriogenology  , DOI: ( /j.theriogenology )
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