Position title: Professor
Phone: Phone: (608) 265-5411
RESEARCH INTERESTS - Development of the fetal-placental connection
B.S., 1978, University of Illinois, Chicago, IL, Biology
M.S., Ph.D., 1980, 1984, University of Illinois, Chicago, IL, Genetics
Post-Doc., 1984-1988, University of California-San Francisco, CA, Molecular Biology
Post-Doc., 1989-1993, University of Oxford, England, Mouse Embryology
The embryonic-extraembryonic interface
The conceptus of placental mammals is composed of embryonic and extraembryonic cells and tissues all of which are established by the fertilized egg. It is little appreciated that in utero, these tissues play mutual roles to ensure survival and correct development of the fetus.
Study of the biology of the allantois, or pre-umbilical cord, at the posterior embryonic-extraembryonic interface has resulted in our discovery of a number of novel architectural features here. Through Brachyury and Hedgehog signaling, these unexpected elements build and organize the placental vasculature and its connection to the fetus as well as posterior fetal organs and they shape the allantois into the umbilical cord, thereby establishing vital exchange between the fetus and its mother.
Importantly, the existence and characterization of these features have challenged long-standing dogma concerning the limits of the primitive streak and potency of visceral endoderm, and especially, they have called into question the existence of a transitory population of segregated “primordial germ cells” (PGCs) within the allantois and associated hindgut. Their properties pave the way for understanding holistically how extraembryonic tissues serve the fetus within its mother during gestation.
(A) Current Status of the architecture of the posterior embryonic-extraembryonic interface
Allantoic Core Domain, ACD – the extraembryonic component of the primitive streak. This component of the streak (i) designates the placement of the vessel of confluence, aligning it with the fetal body axis and cardiovascular system (below); (ii) induces adjacent allantois-associated extraembryonic visceral endoderm (AX) to undergo an EMT (below); (iii) forms a core of stem cells that contribute abundantly to the fetal-placental interface; and (iv) organizes the arterial vasculature of the placenta with that of the fetus (Inman and Downs, 2006a, b; Downs et al., 2009; Mikedis and Downs, 2012; Rodriguez and Downs, 2017).
Allantois-associated extraembryonic visceral endoderm, AX – Under influence of the primitive streak, the AX becomes bi-potently mesendodermal, undergoing an epithelial-to-mesenchymal transition to contribute mesoderm to the allantois and placental blood vessels and definitive endoderm to the hindgut (Downs and Rodriguez, 2017).
Vessel of Confluence, * – conserved amongst placental mammals, the VOC (*) is the site of convergence of the conceptus’s major arterial blood vessels: fetal dorsal aorta, umbilical artery, and omphalomesenteric artery. Defects in its formation and/or placement, result in birth defects and/or fetal morbidity (Downs et al., 1998; Rodriguez et al., 2017).
Rod – A stiff allantoic rod-like structure consisting of Collagen Type IV is created from the bi-potential AX, and plays major roles in organizing formation and placement of the umblical artery and ensuring allantoic elongation to the chorion (Rodriguez and Downs, 2017).
A single distinct CASPASE3+ cell separates the bi-potential AX from the visceral endoderm of the yolk sac blood islands (Rodriguez and Downs, 2017).
Brachyury ensures an axially-aligned arterial fetal-placental vasculature, whist Hedgehog signaling morphologically shapes the allantois into its characteristic projectile form and ultimately, the free umbilical cord (Rodriguez and Downs, 2017).
Together, the ACD and AX are sufficient to create the fetal-placental interface (Rodriguez and Downs, 2017).
Other abbreviations: al, allantois; am, amnion; bi, yolk sac blood island; DCM, dorsal cuboidal mesoderm; ips, intraembryonic primitive streak; vys, visceral yolk sac; VCM, ventral cuboidal mesoderm; xve, extraembryonic visceral endoderm. See reviews below, in particular, Downs, 2022, for the most up-to-date consideration of this interface.
(B) Modification of the standard fate map of the zygote incorporating definitive endoderm (Kwon et al., 2008; Rodriguez and Downs, 2017) and extraembryonic mesoderm (red arrows), both derived from visceral endoderm (Rodriguez and Downs, 2017) into the lineage map previously published by Gardner (1983).
PUBLICATIONS CITED (above)
- Downs, K. M., Gifford, S., Blahnik, M. and Gardner, R. L. (1998). Vascularization in the murine allantois occurs by vasculogenesis without accompanying erythropoiesis. Development 125, 4507-4520.
- Vascularization of the allantois occurs de novo and with distinct polarity. The vessel of confluence, hitherto undescribed, was first identified as the site where the conceptus’s major arterial vessels become patent. Unlike the yolk sac, the allantois does not create primitive red blood cells as it vascularizes de novo. However, as shown in later studies, the allantois has definitive erythro-myeloid potential.
- Inman, K. E. and Downs, K. M. (2006a). Spatiotemporal localization of Brachyury (T) in the embryonic and extraembryonic tissues of the mouse gastrula. Gene Exp. Patterns, 6, 783-793.
- As an established “marker” of the primitive streak, T was characterized in the base of the allantois, codifying previous anecdotal results, and providing clues that the streak extends into the exocoelom.
- Inman, K. E. and Downs, K. M. (2006b). Brachyury is required for elongation and vasculogenesis in the murine allantois. Development, 133, 2947-2959.
- Allantoic elongation in Brachyury mutants (Glucksohn-Schonheimer, 1944) is not due to defects in the intraembryonic primitive streak, as previously thought, but rather to T’s intrinsic presence within the allantois itself, a finding that paved the way to full characterization of the extraembryonic/allantoic component of the streak and its functions.
- Brachyury was further shown to be involved in vascularization, an activity later refined by defining its key role in organizing the placental vasculature with respect to the antero-posterior axis and fetal cardiovasculature (Rodriguez et al., 2017, below).
- Downs, K. M., Inman, K. E., Jin, D. X., and Enders, A. C. (2009). The Allantoic Core Domain (ACD): New insights into development of the murine allantois and its relation to the primitive streak. Dev. Dyn. 238, 532-553.
- The presence of the primitive streak within the exocoelom and base of the allantois was verified by further immunohistochemistry, electron microscopy, and functional and genetic analyses. It was concluded that the streak’s extension plays a major role at the embryonic-extraembryonic interface, distinct from that in the embryo. We provisionally named this extension the “allantoic core domain”, or ACD.
- Mikedis, M. M., and Downs, K. M. (2012). STELLA-positive subregions of the primitive streak cooperate to build the posterior region of the murine conceptus. Dev. Biol. 363, 201-218.
- Fate mapping the entire length of the allantois and intraembryonic primitive streak revealed that the ACD contains two compartments, one which contributes only to the allantois, and another which contains a pool of stem cells that contribute widely to the embryonic-extraembryonic interface and distinct from those of the intraembryonic streak.
- Moreover, the allantois’s pool of STELLA cells, claimed by others to identify primordial germ cells (PGCs), the antecedents to the germ line, also contributed widely to this interface, initiating a further series of experiments and investigations into the literature which questioned the presence of a segregated germ line within the allantois.
- Rodriguez, A. M., Jin, D. X., Mikedis, M. M., Wolfe, A. D., Hashmi, M., Wierenga, L., Viebahn, C., and Downs, K. M. (2017). Brachyury drives formation of a distinct vascular branchpoint critical for fetal-placental arterial union and patterning in the mouse. Dev. Biol. 425, 208-222.
- Streak elongation into the exocoelom is regulated by Brachyury (T), which designates the correct position of the vessel of confluence, whose own formation is dependent upon FGFR1. The vessel of confluence and its relation to the primitive streak are conserved amongst placental mammals.
- Rodriguez, A. M. and Downs, K. M. (2017) Visceral endoderm and the primitive streak interact to build the fetal-placental interface of the mouse gastrula. Dev. Biol. 432, 98-124.
- The discovery and function of bi-potential visceral endoderm and its role, with the primitive streak in creating the placental vasculature are described.
- In addition, an allantoic rod is characterized, leading to the conclusion that the primitive streak is capped at both ends by node- and rod-like structures that organize the arterial vasculature and bring it into intimate association with that of the mother.
RELEVANT REVIEWS AND METHODOLOGIES
- Downs, K.M. and Davies, T. (1993). Staging of gastrulation in mouse embryos by morphological landmarks in the dissection microscope. Development 118, 1255-1266.
- A universal staging system was developed by which any given litter of mouse embryos, whose morphological features vary greatly within that litter, allowed investigators within and between labs across the world to standardize individual embryos and their activities according to stage.
- Downs, K. M. (2005) Invited Review: In vitro culture model for studying allantoic vasculogenesis and fusion with the chorion. In Placental and Trophoblast Methods and Protocols for Methods in Molecular Medicine Series, M. J. Soares and J. Hunt, eds. (Totowa, JH: Humana Press, Inc.), 121, 241-272.
- Lalit, P. A., Rodriguez, A. M., Downs, K. M., and Kamp, T. J. Generation of expandable, multipotent induced cardiac progenitor cells from mouse fibroblasts and potency testing in ex vivo embryos. Nature Protocols 12, 1029-1054.
- Together, these two reviews provide detailed methodologies and video footage for allantois engraftment, injection of cardiac stem cells into mouse gastrulae, and whole embryo culture of mouse gastrulae.
- Downs, K. M. (2009). Invited Peer-Reviewed Hypothesis: The enigmatic primitive streak: Prevailing notions and challenges concerning the embryonic body axis. BioEssays 31, 892-902.
- This review examines why the primitive streak has historically been claimed to be limited to the embryo only, rather than exploring anecdotal evidence which suggested its extension into the allantois, a role that would make good biological sense for organizing the vital vasculature of the placenta with respect to that of the fetus.
- Mikedis, M. M. and Downs, K. M. (2014). Invited Review: Mouse primordial germ cells (PGCs): a reappraisal. Int. Rev. Mol. Cell Biol. 309, 1-57.
- A century of claims for the whereabouts of the antecedents to the mammalian germ line are examined, highlighting not only non-equivalence between cell lineage and gene expression, but especially calling into question the so-called PGC trajectory, and lack of evidence for migration from allantois/hindgut PGCs to the gonads.
- Together with the discovery that the ACD, where the so-called PGCs are claimed to segregate from the soma, harbors stem cells that build the fetal-placental interface, we considered that the origin of the mammalian germ line is currently unknown.
- Downs, K. M. (2018). Extragonadal primordial germ cells or placental progenitor cells? Reproductive Biomedicine Online 36, 6-11.
- With new claims that the amnion is the site of segregation of PGCs in non-human primates added to those that the extraembryonic allantois and yolk sac are the sites of segregation In rodents and humans, respectively, an extragonadal lineage for mammalian “primordial germ cells” (PGCs) is again examined in light of new understanding the embryonic-extraembryonic interface.
- Downs, K. M. and Rodriguez, A. M. (2020). Invited Review: The fetal-placental connection: A paradigm involving the primitive streak and visceral endoderm with implications for human development. WIRE Dev. Biol. 9, e362.
- Summarizes the role of Brachyury and Hedgehog signaling in establishing the fetal-placental connection.
- Downs, K. M. (2020) Invited Review: Is extraembryonic endoderm a source of placental blood cells? Exp. Hematol. 89, 37-42.
- The little-cited observation that blood cells are present within the visceral endoderm of humans, non-human primates, and rodents is highlighted, and linked to studies of visceral endoderm in isolation which suggest hematopoietic potential.
- Downs, K. M. (2022). Invited review: The mouse allantois: New paradigms at the embryonic-extraembryonic interface. Phil. Trans. Royal Soc. B., in press.
- The embryonic-extraembryonic interface is the theme of this volume of Phil Trans Roy Soc, thereby increasing awareness of its importance in fetal development. In addition to summarizing events leading to formation and function of the allantois, this review article re-interprets a number of previous studies and their conclusions published before discovery of new elements at this interface. In particular, the role of BMP4 at this interface may not be in so-called PGC development, but rather, in development of the ACD stem cell pool which builds the fetal-placental connection. As each event is traced at the embryonic-extraembryonic interface, new avenues for future exploration of this junction are provided.