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The whereabouts and properties of the posterior end of the primitive streak have not been identified in any species. In the mouse, the streak's posterior terminus is assumed to be confined to the embryonic compartment, and to give rise to the allantois, which links the embryo to its mother during pregnancy. In this study, we have refined our understanding of the biology of the murine posterior primitive streak and its relation to the allantois. Through a combination of immunostaining and morphology, we demonstrate that the primitive streak spans the posterior extraembryonic and embryonic regions at the onset of the neural plate stage (ϳ7.0 days postcoitum, dpc). Several hours later, the allantoic bud emerges from the extraembryonic component of the primitive streak (XPS). Then, possibly in collaboration with overlying allantois-associated extraembryonic visceral endoderm, the XPS establishes a germinal center within the allantois, named here the Allantoic Core Domain (ACD). Microsurgical removal of the ACD beyond headfold (HF) stages resulted in the formation of allantoic regenerates that lacked the ACD and failed to elongate; nevertheless, vasculogenesis and vascular patterning proceeded. In situ and transplantation fate mapping demonstrated that, from HF stages onward, the ACD's progenitor pool contributed to the allantois exclusive of the proximal flanks. By contrast, the posterior intraembryonic primitive streak (IPS) provided the flanks. Grafting the ACD into T C /T C hosts, whose allantoises are significantly foreshortened, restored allantoic elongation. These results revealed that the ACD is essential for allantoic elongation, but the cues required for vascularization lie outside of it. On the basis of these and previous findings, we conclude that the posterior primitive streak of the mouse conceptus is far more complex than was previously believed. Our results provide new directives for addressing the origin and development of the umbilical cord, and establish a novel paradigm for investigating the fetal/placental relationship. Developmental Dynamics 238: 532-553, 2009.

Portions of the labyrinth or villi of placentas from late pregnancy from nine species in four orders of mammals were examined with the electron microscope.Pronounced patterns of layering of the trophoblast were found in these placentas which were all of the hemochorial type. The laboratory rat, laboratory mouse, hamster, and deer mouse have three layers of trophoblast between the maternal blood space and fetal vessels (hemotrichorial), the rabbit two layers (hemodichorial), the guinea pig and chipmunk one layer (labyrinthine hemomonochorial), and the human and armadillo one layer (villozcs hemomonochorial ).

By making use of the known sequence of ovulation and fertilization in Holtzman rats, the Pontamine Blue reaction, and electron microscopy, the first stages in implantation were studied. Implantation is initiated when the blastocyst becomes clasped by the endometrium and hence assumes a fixed position (evening of day 5 ) . At this stage, the trophoblast cells are in close association with the uterine epithelial cells, with interdigitating microvilli in places, but decidualization of the fibroblasts is just beginning. In the second stage of implantation the decidualized fibroblasts form a cup around the luminal epithelium. Evidence of adhesion of the cell membranes of trophoblast cells with the cell membranes of luminal epithelial cells can be seen at this time. The layer of fibroblasts immediately surrounding the luminal epithelium becomes epithelioid, resulting in a relative isolation of the luminal epithelium from its vascularization. This stage is well-developed by the afternoon of day 6. By the morning of day 7, the luminal epithelium has disappeared from the region of the forming ectoplacental cone down to the level of the abembryonic trophoblast. The trophoblast cells on the lateral aspect of the blastocyst are directly in contact with the residual basement membrane of the luminal epithelial cells, and are separated by this structure and a small connective tissue cleft from the stromal cells. The importance of the relative isolation of the epithelium by the stromal reaction and the adhesion of the cell membrane of the trophoblast cells to the cell membranes of epithelial cells with regard to removal and phagocytosis of epithelial elements are discussed, and many of the cytological features observed during the process are described.The ready availability of the rat and mouse have made these animals favorite objects for the study of implantation. Duval (1891) took advantage of the abundance of rats in the slaughterhouses of Paris for his study materials. However, he used Lataste's collection of mouse reproductive tracts for the comparison of stages and the timing of these stages. His studies, plus those that followed in the next two decades dealing with both the mouse (Burckhard, '01; Sobotta, '03) and the rat and mouse (Melissinos, '07), established many of the essential features of implantation in these myomorph rodents. The mouse was the major subject of these studies, and little distinction was made between the mouse and rat, which were considered to be in the same genus at that time. Although these studies predated a precise knowledge of the reproductive cycle, timed implantation stages were obtained by using Sobotta's method of placing female mice that had given birth to litters 21 days previously with males, then AM. J. ANAT., 120: 185-226.examining them subsequently for copulation plugs.The most comprehensive study of implantation in the rat is that of Huber ('15). He reviewed not only the literature on implantation in the mouse mentioned above, but also the less distinguished literature on th...