The murine allantois: a model system for the study of blood vessel formation

Abstract

The allantois is the embryonic precursor of the umbilical cord in mammals and is one of several embryonic regions, including the yolk sac and dorsal aorta, that undergoes vasculogenesis, the de novo formation of blood vessels. Despite its importance in establishing the chorioallantoic placenta and umbilical circulation, the allantois frequently is overlooked in embryologic studies. Nonetheless, recent studies demonstrate that vasculogenesis, vascular remodeling, and angiogenesis are essential allantois functions in the establishment of the chorioallantoic placenta. Here, we review blood vessel formation in the murine allantois, highlighting the expression of genes and involvement of pathways common to vasculogenesis or angiogenesis in other parts of the embryo. We discuss experimental techniques available for manipulation of the allantois that are unavailable for yolk sac or dorsal aorta, and review how this system has been used as a model system to discover new genes and mechanisms involved in vessel formation. Finally, we discuss the potential of the allantois as a model system to provide insights into disease and therapeutics.

Figure 1

Diagrammatic representation of the developing mouse embryo from the neural plate (allantoic bud) stage to the 22-26 somite (s) stage highlighting the development of the allantois and its vasculature. The age in embryonic days (E), and the stages of development are indicated across the top, as are the developmental processes taking place in the allantois. The allantois first appears as a mesodermal bud emerging from the posterior end of the primitive streak. The bud grows and expands across the exocoelomic cavity and fuses with the chorion to form the chorioallantoic placenta. The processes in allantois vessel development at each stage are indicated across the bottom of the figure. ECs differentiate and migrate to form a primary vascular plexus. The plexus connects with the dorsal aortae of the embryo before chorio-allantoic fusion. The primary plexus remodels to form the central vessel; later, the umbilical artery and the umbilical vein form at E9.5. After chorio-allantoic fusion, the allantoic vessels invade the chroion and form the fetal vascular component of the labyrinthine layer of the placenta. C indicates allantois core; CAP, chorio-allantoic plate; DA, dorsal aorta; epc, ectoplacental cone; exc, exocoelomic cavity; lab, placental labyrinth; m, mesothelium; NP, neural plate; UA, umbilical artery; UV, umbilical vein; and ys, yolk sac.

Figure 2

Three methods for allantois culture. Allantoises can be dissected from embryos from the HF to 4s stage and allowed to attach to glass, plastic, or filters for adherent cultures (top row), or they can be kept in suspension in rolling cultures (middle row) or hanging drops (bottom row). Asterisk indicates the proximal end of the allantois closest to the embryo. In adherent cultures the distal tip of the allantois attaches to the dish within 6 hours and a vascular plexus forms after 24 hours as indicated by staining with Flk1 (brown). Rolling cultures result in development of spheroids with 3 dimensional (3D) morphology, which can be stained whole as shown for Flk1 (blue) and VCAM (brown). Hanging drop cultures of allantoises also form 3D structures with a vascular plexus after 18 hours of culture. Image shows fluorescent staining of a whole mount allantois spheroid with α-SMA (red) and Pecam (green). After treatment with VEGF, the spheroid remodels into a single layer of Pecam-positive ECs surrounded by α-SMA–positive SMCs as shown in an optical section. Images a, b, and c are reprinted with permission from Downs et al, Arora et al, and Gentile et al, respectively.