Primate amnion development

Abstract

Formation of the amnion in humans is crucial for fetal development and a healthy pregnancy. In addition to providing a protective layer to the developing fetus as a component of the amniochorionic fetal membranes, the amnion serves as a signaling center for patterning early embryonic tissues. However, because the amnion is first specified in the human epiblast during implantation, the molecular and cellular events of this early amniogenic process in humans cannot be studied in utero. Recent developments using new human stem cell-derived model systems, as well as single-cell and spatial transcriptomic analyses of early human and monkey embryos, have uncovered new insights into the underpinnings of primate amnion specification. Here, we highlight recent findings from human and monkey models with an emphasis on current understandings of morphogenesis, BMP-driven transcriptional signatures and key players associated with primate amniotic ectoderm specification.

Keywords: Amnion; Early human development; Human pluripotent stem cells.

Fig. 1.. Post-implantation primate embryonic development.

(A) As the primate embryo implants (CS4), an initially unpolarized group of pluripotent epiblast cells begin radial organization and lumen formation to form a cyst (CS5a). Cells proximal to the endometrial pole then differentiate to amniotic ectoderm, giving rise to an asymmetric amnion (orange)-epiblast (blue) structure, called the amniotic sac by CS5b (hypoblast (dark green), trophectoderm (yellow), and amnion-epiblast/embryo boundary cells (bright green). By CS6, the embryonic disc displays a convex structure, and primitive streak-derived cells (cyan) are seen posteriorly. Clear definitive endoderm (Glossary, Box 1) and mesoderm cells (a layer of cyan colored cells ventral to the epiblast) as well as the amniotic mesenchyme (dark magenta) are seen at CS7. Several pink cells are shown to indicate heterogeneity of the primitive streak-derived cell population. Uterus and trophectoderm are omitted after the CS4 and CS6 stages, respectively, for simplification. AmL, amniotic lumen; Am-Emb boundary, amnion–embryo boundary. Anterior-posterior orientation is indicated in the CS7 schematic. (B,C) Schematic representations of CS8 (B) and CS9/10 (C) cynomolgus macaque embryos outlining the amniotic ectoderm (orange), amniotic mesenchyme (dark magenta), embryo (blue) and body stalk (see Glossary, Box 1, composed of extra-embryonic mesoderm cells, teal, shown only in CS8), based on images shown in (Sekulovski et al., 2025). def. endo/mes, definitive endoderm/mesoderm; HF, head fold neuroectoderm; HM, head mesenchyme; HT, heart tube; FB, forebrain neuroepithelium; PS, primitive streak. Amnion-embryo boundary is shown in bright green. Arrowhead and arrow indicate foregut and hindgut pockets, respectively. Note that, in addition to tissues at the anterior and posterior ends, amnion–embryo connections are also seen laterally at the amnion–surface ectoderm boundaries in human embryos at these stages (see http://virtualhumanembryo.lsuhsc.edu and (Muller and O’Rahilly, 2004)). Several extraembryonic mesoderm cell types are not included for simplification.

Fig. 2.. Selected stem cell-derived models to investigate amnion fate specification.

(A-C) Schematic representations of selected stem cell-derived 2D (A), as well as 3D single- (B) and multi- (C) tissue layer models with amniotic cells (orange). Epiblast-like pluripotent cells, blue; mesodermal (embryonic/extra-embryonic) cells, light purple; primordial germ cell-like cells (PGC-LC), dark purple; trophectoderm-like cells, yellow; extra-embryonic endoderm (e.g., hypoblast, yolk sac)-like cells, green; unknown, gray (Ai et al., 2023; Gharibi et al., 2025; Hislop et al., 2024; Kagawa et al., 2022; Liu et al., 2023; Minn et al., 2020; Munger et al., 2022; Nasr Esfahani et al., 2019; Okubo et al., 2024; Oldak et al., 2023; Pedroza et al., 2023; Sekulovski et al., 2024; Shao et al., 2017a; Shao et al., 2017b; Warmflash et al., 2014; Weatherbee et al., 2023; Zheng et al., 2019).

Fig. 3.. Transcriptional overview of human amniogenesis.

A flow chart outlining five distinct amniogenic transcriptional steps: immediate, early, two intermediate (int) and late stages, with a representative marker. Although additional investigation is needed, we described five lineage progressing amniogenic cell states (pluripotency-exiting, early progenitor, late progenitor, specified and maturing) that accompany these transcriptional steps (Sekulovski et al., 2025).