Birth defects associated with perturbations in preimplantation, gastrulation, and axis extension: from conjoined twinning to caudal dysgenesis

Classification of sirenomelia by the presence or absence of bones in the lower limb according to Stocker and Heifetz. (Reprinted with permission from Ref . Copyright 1987 S. Karger AG)

Signals that regulate axial elongation in the mouse embryo. (a) Molecular pathways acting at the posterior growth region and contributing to the antagonistic gradients of FGF and retinoic acid (RA). (b) Mouse embryo at embryonic day 9. The chordoneural hinge (CNH) is located caudally and lies contiguous with the notochord and neural tube. (c) Noggin expression in the ventral ectodermal ridge (VER) controls the level of BMP signaling required for caudal ventral mesoderm morphogenesis.

Mesoderm formation in the mammalian embryo. (a) Cells in the epiblast undergo an epithelial to mesenchymal transition at the primitive streak and colonize the space between the epiblast and hypoblast. (b) Ingressing cells migrate in all directions. Cells ingressing through the node migrate anteriorly forming the notochord. Cells ingressing through the streak and migrating cranially contribute to the head and cardiac mesoderm. Epiblast cells migrating laterally form the mesoderm at each side of the midline. (c) The different mesodermal lineages are established along the mediolateral axis of the embryo.

Classification of conjoined twins according to Spencer.

Uterine disposition of monozygotic twins. (a) Dichorionic diamniotic twins develop from a single zygote divided before the inner cell mass (ICM) forms. (b) Monochorionic diamniotic twins form when the ICM divides between the 3rd and 8th day of development. (c) Monochorionic monoamniotic twins develop from the division of the bilaminar disc.

Events occurring during the first 2 weeks of human development. (a) Zygote. (b) Two‐cell stage embryo. (c) Morula before compaction. (d) Blastocyst forms after compaction and formation of the blastocoel. The embryo segregates into epiblast and hypoblast. (e) Implantation starts at the end of the 1st week. The syncytiotrophoblast begins to invade the uterine wall. (f) The amniotic cavity appears. (g) Hypoblast cells invade the lining of the primary yolk sac. Extraembryonic reticulum forms between the hypoblast and cytotrophoblast. Extraembryonic mesoderm colonizes the reticulum. (h) The reticulum cavitates leaving the chorionic cavity. The extraembryonic mesoderm and underlying cytotrophoblast form the chorion.