The Allantoic Core Domain: New insights into development of the murine allantois and its relation to the primitive streak

Abstract

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.

The hypoblast (visceral endoderm): an evo-devo perspective

“…2D,E) that is not displaced, at least by the early neural plate (E8.0) stage, by endoderm recruited from the epiblast (Tam and Beddington, 1992). At neural plate stages, the PVE might be part of the transitional visceral endoderm (TVE) that overlies the extra-embryonic/embryonic junction (Bonnevie, 1950), where it might establish a posterior node and the allantois (Downs, 2009;Downs et al, 2009) (Fig. 2E).…”

“…Characterization of 'allantoisassociated extra-embryonic visceral endoderm' (AX) is in its early stages and thus, its relation to the PVE and other types of VE is not yet clear. Its initial morphology is that of TVE (Bonnevie, 1950;Downs et al, 2009), which gradually transforms into the flattened and molecular profile of EVE. However, the AX might induce the formation of a novel vessel, provisionally referred to as the 'vessel of confluence' (VOC) (Downs, 2009).…”

On the origin of amniotic stem cells: of mice and men

“…Recent evidence suggests that the posterior primitive streak extends in the extraembryonic region. The extraembryonic compartment of the primitive streak creates a node-like cells reservoir from which the allantois emerges (Downs et al, 2009). (D) Sagittal section of an E7.5 mouse embryo.…”

Mouse Primordial Germ Cells

“…(B 1-3 ) In the current model, in which PGCs are thought to be lineage restricted, the posterior end of the (Continued) Part of the problem in studying the extragonadal germ line is that its relationship to the allantois has been largely ignored. Recent evidence has unexpectedly revealed that the mouse allantois is not a mass of extraembryonic mesoderm but, rather, an architecturally complex structure Downs et al, 2009) whose proximal region contains a stem/progenitor cell pool, the Allantoic Core Domain (ACD), defined by Brachyury (T) (Fig. 1.2A 1 , B 1 ).…”