The formation of mesodermal tissues in the mouse embryo during gastrulation and early organogenesis

TLDR

Orthotopic grafts of 3H‑thymidine‑labelled cells have been used to map how tissues adjacent to different regions of the primitive streak in 8‑day mouse embryos develop in vitro. The study employed orthotopic grafts and wheat‑germ‑agglutinin‑conjugated colloidal gold labeling of ectodermal cells to track their recruitment into the paraxial mesoderm of 8‑ and 9‑day embryos. The posterior streak mainly generates extraembryonic mesoderm, the middle region produces lateral mesoderm, and the anterior region yields paraxial mesoderm, gut, and notochord, with adjacent ectoderm contributing paraxial mesoderm and neurectoderm; this pattern mirrors chick fate maps, and the older streak continues to produce embryonic mesoderm and endoderm but not extraembryonic mesoderm, while primitive‑streak‑derived cells are continuously added to the paraxial mesoderm with evidence of cell mixing in the presomitic mesoderm.

Abstract

ABSTRACT Orthotopic grafts of [3H]thymidine-labelled cells have been used to demonstrate differences in the normal fate of tissue located adjacent to and in different regions of the primitive streak of 8th day mouse embryos developing in vitro. The posterior streak produces predominantly extraembryonic mesoderm, while the middle portion gives rise to lateral mesoderm and the anterior region generates mostly paraxial mesoderm, gut and notochord. Embryonic ectoderm adjacent to the anterior part of the streak contributes mainly to paraxial mesoderm and neurectoderm. This pattern of colonization is similar to the fate map constructed in primitive-streak-stage chick embryos. Similar grafts between early-somite-stage (9th day) embryos have established that the older primitive streak continues to generate embryonic mesoderm and endoderm, but ceases to make a substantial contribution to extraembryonic mesoderm. Orthotopic grafts and specific labelling of ectodermal cells with wheat germ agglutinin conjugated to colloidal gold (WGA-Au) have been used to analyse the recruitment of cells into the paraxial mesoderm of 8th and 9th day embryos. The continuous addition of primitive-streak-derived cells to the paraxial mesoderm is confirmed and the distribution of labelled cells along the craniocaudal sequence of somites is consistent with some cell mixing occurring within the presomitic mesoderm.